.. _GMT_General_Features:

General Features
================

This section explains features common to all the programs in GMT and
summarizes the philosophy behind the system. Some of the features
described here may make more sense once you reach the cook-book section
where we present actual examples of their use.

GMT Modern Mode Hierarchical Levels
-----------------------------------

As you read below of how we handle default settings, command-line history, and
color tables, it is important to understand that under GMT **modern mode** we
maintain several *levels* of these parameters.  As you will see later, this affects
*three* aspects of GMT: The chosen default settings, the current history of
previous common option arguments, and the current color table.  All three items
are given a consistent treatment in GMT modern mode (in classic mode there is
only a single level and no concept of a current color table). Below, *item* refers
to any of those three aspects.

#. The top level is the *session*.  Any item set here is accessible to all other
   levels.

#. The next level is the *figure* level.  A session may create numerous figures
   and items determined at this level are only accessible to that figure and
   plot constructs below it (like subplots).

#. A figure may include a *subplot*.  Before any panels are started, any
   items determined at this level apply to *all* the panels in the subplot.
   For instance, setting a new color table will apply to all the panels that
   need it.

#. Once you start a specific *panel* in a subplot, any items determined at this
   level only apply to that panel.  For instance, changing the font used for
   frame annotations for this panel is not affecting any other panels.

#. Figures or panels may include a map *inset*.  Any items determined in an
   inset is private to that inset and does not affect the higher levels.

There is a distinction between *setting* an item (e.g., a font choice, an option
like plot region, or a color table) and *getting* that item.  When we *specify*
a particular item it is recorded at that level.  When we need to *access*
that item, there may or may not be an item at the current hierarchical level.
If there is not, we look at the level above the current level to see if it has
the required item, and this search may go all the way back to the session level.
In other words, we always give preference to items set at or just above the
current hierarchical level as possible.  If no such item is found anywhere then
we use the GMT defaults or color table, or we must terminate with an error if a
required setting such as a region cannot be determined from your options or data sets.

Discussions below on GMT defaults and history are presented as they apply to
classic mode, but under modern mode these files are maintained at the levels we
just discussed.

GMT units
---------

While GMT has default units for both actual Earth distances and plot
lengths (i.e., dimensions) of maps, it is recommended that you explicitly
indicate the units of your arguments by appending the unit character, as
discussed below. This will aid you in debugging, let others understand your
scripts, and remove any uncertainty as to what unit you thought you wanted.

.. _plt-units:

Dimension units
~~~~~~~~~~~~~~~

GMT programs accept plot dimensional quantities (widths, offsets, etc.) in
**c**\ m, **i**\ nch, or **p**\ oint (1/72 of an inch) [8]_. There are
two ways to ensure that GMT understands which unit you intend to use:

#. Append the desired unit to the dimension you supply. This way is
   explicit and clearly communicates what you intend, e.g.,
   **-JM**\ 10\ **c** means the map width being passed to the **-JM** switch
   is 10 cm, and modifier **+o**\ 24p means we are offsetting a feature
   by 24 points from its initial location.

#. Set the parameter :term:`PROJ_LENGTH_UNIT` to the desired unit. Then,
   all dimensions without explicit units will be interpreted accordingly.
   By default, GMT always initializes :term:`PROJ_LENGTH_UNIT` to cm and
   interprets unitless dimensional values as cm, except for fonts and pen
   thicknesses which are by default interpreted as points.

The latter method is less robust as other users may have a different
default unit set and then your script may not work as intended. For portability,
we therefore recommend you always append the desired unit explicitly.

.. _dist-units:

Distance units
~~~~~~~~~~~~~~

.. _tbl-distunits:

+---------+-------------------+---------+------------------+
| **d**   | Degree of arc     | **M**   | Statute mile     |
+---------+-------------------+---------+------------------+
| **e**   | Meter [Default]   | **n**   | Nautical mile    |
+---------+-------------------+---------+------------------+
| **f**   | Foot              | **s**   | Second of arc    |
+---------+-------------------+---------+------------------+
| **k**   | Kilometer         | **u**   | US Survey foot   |
+---------+-------------------+---------+------------------+
| **m**   | Minute of arc     |         |                  |
+---------+-------------------+---------+------------------+

For Cartesian data the data units do not normally matter
(they could be kg or Lumens for all we know) and are never entered.
Geographic data are different, as distances can be specified in a variety
of ways. GMT programs that accept actual Earth length scales like
search radii or distances can therefore handle a variety of units. These
choices are listed in the Table :ref:`Distance Units <tbl-distunits>`;
simply append the desired unit to the distance value you supply. A value
without a unit suffix will be considered to be in meters. For example, a distance
of 30 nautical miles should be given as 30\ **n**.

Distance calculations
~~~~~~~~~~~~~~~~~~~~~

The calculation of distances on Earth (or other planetary bodies)
depends on the ellipsoidal parameters of the body (via
:term:`PROJ_ELLIPSOID`) and the method of computation. GMT offers three
alternatives that trade off accuracy and computation time.

Flat Earth distances
^^^^^^^^^^^^^^^^^^^^

Quick, but approximate "Flat Earth" calculations make a first-order
correction for the spherical nature of a planetary body by computing the
distance between two points A and B as

.. math::

	 d_f = R \sqrt{(\theta_A - \theta_B)^2 + (\cos \left [ \frac{\theta_A +
	 \theta_B}{2} \right ] \Delta \lambda)^2}, \label{eq:flatearth}

where *R* is the representative (or spherical) radius of the
planet, :math:`\theta` is latitude, and the difference in longitudes,
:math:`\Delta \lambda = \lambda_A - \lambda_B`, is adjusted for any
jumps that might occur across Greenwich or the Dateline. As written, the
geographic coordinates are given in radians. This approach is suitable
when the points you use to compute :math:`d_f` do not greatly differ in
latitude and computation speed is paramount. You can select this mode
of computation by specifying the common GMT option **-j** and appending the directive
**f** (for Flat Earth).  For instance, a search radius of 50 statute miles
using this mode of computation might be specified via **-S**\ 50\ **M** **-jf**.

Great circle distances
^^^^^^^^^^^^^^^^^^^^^^

This is the default distance calculation, which will also approximate
the planetary body by a sphere of mean radius *R*. However, we
compute an exact distance between two points A and B on such a sphere
via the Haversine equation

.. math::

	 d_g = 2R \sin^{-1}  {\sqrt{\sin^2\frac{\theta_A - \theta_B}{2} + \cos
	 \theta_A \cos \theta_B \sin^2 \frac{\lambda_A - \lambda_B}{2}} },
	 \label{eq:greatcircle}

This approach is suitable for most situations unless exact calculations
for an ellipsoid is required (typically for a limited surface area). For
instance, a search radius of 5000 feet using this mode of computation
would be specified as **-S**\ 5000\ **f**.

**Note**: There are two additional GMT defaults that control how
great circle (and Flat Earth) distances are computed. One concerns the
selection of the "mean radius". This is selected by
:term:`PROJ_MEAN_RADIUS`, which selects one of several possible
representative radii. The second is :term:`PROJ_AUX_LATITUDE`, which
converts geodetic latitudes into one of several possible auxiliary
latitudes that are better suited for the spherical approximation. While
both settings have default values to best approximate geodesic distances
(*authalic* mean radius and latitudes), expert users can choose from a
range of options as detailed in the :doc:`/gmt.conf` man page.  Note that
these last two settings are only used if the :term:`PROJ_ELLIPSOID`
is not set to "sphere".

Geodesic distances
^^^^^^^^^^^^^^^^^^

For the most accurate calculations we use a full ellipsoidal
formulation. Currently, we are using Vincenty's [1975] formula [7]_
which is accurate to 0.5 mm. You
select this mode of computation by using the common GMT option **-j**
and appending the directive **e** (for ellipsoidal).
For instance, a search radius of 20 km using this mode of
computation would be set by **-S**\ 20\ **k** **-je**.  You may use the
setting :term:`PROJ_GEODESIC` which defaults to
*Vincenty* but may also be set to *Rudoe* for old GMT4-style calculations
or *Andoyer* for an approximate geodesic (within a few tens of meters)
that is much faster to compute.

GMT defaults
------------

Overview and the gmt.conf file
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There are almost 150 parameters which can be adjusted individually to
modify the appearance of plots or affect the manipulation of data. When
a new session starts (unless **-C** is given), it initializes all parameters to the
GMT defaults [9]_, then tries to open the file ``gmt.conf`` in the current
directory [10]_. If not found, it will look for that file in a
sub-directory ``.gmt`` of your home directory, and finally in your home directory
itself. If successful, the session will read the contents and set the
default values to those provided in the file. By editing this file you
can affect features such as pen thicknesses used for maps, fonts and
font sizes used for annotations and labels, color of the pens,
dots-per-inch resolution of the hardcopy device, what type of spline
interpolant to use, and many other choices. A complete list of all the
parameters and their default values can be found in the
:doc:`/gmt.conf` manual pages. Figures
:ref:`GMT Parameters a <gmt_defaults_a>`,
:ref:`b <gmt_defaults_b>`, and
:ref:`c <gmt_defaults_c>` show the parameters that affect
plots. You may create your own ``gmt.conf`` files by running
:doc:`/gmtdefaults` and then modify those
parameters you want to change. If you want to use the parameter settings
in another file you can do so by copying that file to the current
directory and call it gmt.conf. This makes it easy to maintain several distinct parameter
settings, corresponding perhaps to the unique styles required by
different journals or simply reflecting font changes necessary to make
readable overheads and slides.  At the end of such scripts you should then
delete the (temporary) gmt.conf file.  Note that any arguments given on the
command line (see below) will take precedent over the default values.
E.g., if your ``gmt.conf`` file has *x* offset = 3\ **c** as default, the
**-X**\ 5\ **c** option will override the default and set the offset to 5 cm.

.. _gmt_defaults_a:

.. figure:: /_images/GMT_Defaults_1a.*
   :width: 500 px
   :align: center

   Some GMT parameters that affect plot appearance.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_Defaults_1a.txt


.. _gmt_defaults_b:

.. figure:: /_images/GMT_Defaults_1b.*
   :width: 500 px
   :align: center

   More GMT parameters that affect plot appearance.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_Defaults_1b.txt

.. _gmt_defaults_c:

.. figure:: /_images/GMT_Defaults_1c.*
   :width: 500 px
   :align: center

   Even more GMT parameters that affect plot appearance.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_Defaults_1c.txt

There are at least two good reasons why the GMT default options are
placed in a separate parameter file:

#. It would not be practical to allow for command-line syntax covering
   so many options, many of which are rarely or never changed (such as
   the ellipsoid used for map projections).

#. It is convenient to keep separate ``gmt.conf`` files for specific projects, so
   that one may achieve a special effect simply by running
   GMT commands in the directory whose ``gmt.conf`` file has the desired settings.
   For example, when making final illustrations for a journal article
   one must often standardize on font sizes and font types, etc. Keeping
   all those settings in a separate ``gmt.conf`` file simplifies this process and
   will allow you to generate those illustrations with the same settings
   later on. Likewise, GMT scripts that make figures for PowerPoint
   presentations often use a different color scheme and font size than
   output intended for laser printers. Organizing these various
   scenarios into separate ``gmt.conf`` files will minimize headaches associated with
   micro-editing of illustrations.


.. _auto-scaling:

Automatic GMT settings
~~~~~~~~~~~~~~~~~~~~~~

The **auto** flag for :doc:`GMT parameters </gmt.conf>` signals that suitable
dimensions or settings will be automatically computed when the plot dimensions
are known. The **auto** flag is supported for the following parameters:

================================== ===============================================
:term:`FONT_ANNOT_PRIMARY`         Primary annotation font [11.00p]
:term:`FONT_ANNOT_SECONDARY`       Secondary annotation font [13.20p]
:term:`FONT_HEADING`               Subplot heading font [30.80p]
:term:`FONT_LABEL`                 Axis label font [15.40p]
:term:`FONT_SUBTITLE`              Plot subtitle font [19.80p]
:term:`FONT_TAG`                   Tag/labeling font [17.60p]
:term:`FONT_TITLE`                 Plot title font [24.20p]
:term:`MAP_ANNOT_MIN_SPACING`      Minimum space between annotations [11.00p]
:term:`MAP_ANNOT_OFFSET_PRIMARY`   Primary annotation offset from axis [3.30p]
:term:`MAP_ANNOT_OFFSET_SECONDARY` Secondary annotation offset from axis [3.30p]
:term:`MAP_EMBELLISHMENT_MODE`     Scales attributes relative to feature size
:term:`MAP_FRAME_AXES`             Axes that are drawn and annotated
:term:`MAP_FRAME_PEN`              Pen width of plain frame [1.65p]
:term:`MAP_FRAME_WIDTH`            Width of fancy frame [3.30p]
:term:`MAP_GRID_PEN_PRIMARY`       Pen width of primary gridline [0.28p]
:term:`MAP_GRID_PEN_SECONDARY`     Pen width of secondary gridline [0.55p]
:term:`MAP_HEADING_OFFSET`         Heading offset from subplot [17.60p]
:term:`MAP_LABEL_OFFSET`           Label offset from annotations [6.60p]
:term:`MAP_POLAR_CAP`              Appearance of gridlines near the poles
:term:`MAP_TICK_LENGTH_PRIMARY`    Length of primary tick marks [2.2p/1.1p]
:term:`MAP_TICK_LENGTH_SECONDARY`  Length of secondary tick marks [6.60p/1.65p]
:term:`MAP_TICK_PEN_PRIMARY`       Pen width of primary tick marks [0.55p]
:term:`MAP_TICK_PEN_SECONDARY`     Pen width of secondary tick marks [0.28p]
:term:`MAP_TITLE_OFFSET`           Title offset from plot [13.20p]
================================== ===============================================

The reference dimensions listed in brackets are the values for a plot
with a height and width of 25 cm.  Larger and smaller illustrations
will see a linear magnification or attenuation of these dimensions. The primary
annotation font size will be computed as::

    size = (2/15) * (map_size_in_cm - 10) + 9 [in points]

where :math:`map\_size\_in\_cm = \sqrt(map\_height  \times  map\_width)`.  All other
items will have their reference sizes scaled by :math:`scale = size / 10`. In
modern mode, if you do nothing then all of the above dimensions will be
automatically set based on your plot dimensions.  However, you are free to
override any of them using the methods described in the next section. **Note**:
Selecting **auto** for font sizes and dimensions requires GMT to know the plot
dimensions. If the plot dimensions are not available (e.g., :doc:`/pslegend`
with **-Dx** and no **-R -J**), the settings will be updated using the nominal
font sizes and dimensions for a 10 x 1 cm plot. **Note**: The particular scaling
relationship is experimental in 6.2 and we reserve the right to adjust it
pending further experimentation and user feedback.

For :term:`MAP_POLAR_CAP`, **auto** will determine a suitable *pc_lat* for your
region for all azimuthal projections and a few others in which the geographic
poles are plotted as points (Lambert Conic, Oblique Mercator, Hammer, Mollweide,
Sinusoidal, and van der Grinten).

For :term:`MAP_FRAME_AXES`, **auto** will determine a suitable setting based on the
projection, type of plot, perspective, etc. For example, GMT will determine the
position of different quadrants for perspective and polar plots and select the
equivalent of **WrStZ**. The default for the Gnomonic and general perspective
projections is **WESNZ**. The default for non-perspective, non-Gnomonic, and
non-polar plots using **MAP_FRAME_AXES**\ =\ **auto** is **WrStZ**.

For :term:`MAP_LABEL_OFFSET`, **auto** will scale the offset based on figure size if
:term:`MAP_LABEL_MODE` is set to **annot**, but will default to **32p** if
:term:MAP_LABEL_MODE` is set to **axis**.

For :term:`MAP_EMBELLISHMENT_MODE`, **auto** means we uses the given size of the
embellishment to set relative sizes of ticks, texts and labels, and offsets.
These are otherwise controlled by numerous default settings; see discussion
under :ref:`Embellishments <GMT_Embellishments>`.

Changing GMT defaults
~~~~~~~~~~~~~~~~~~~~~

As mentioned, GMT programs will attempt to open a file named  ``gmt.conf``. At
times it may be desirable to override that default. There are several
ways in which this can be accomplished.

*  One method is to start each script by saving a copy of the current  ``gmt.conf``,
   then copying the desired ``gmt.conf`` file to the current directory, and finally
   reverting the changes at the end of the script. Possible side effects
   include premature ending of the script due to user error or bugs
   which means the final resetting does not take place (unless you write
   your script very carefully.)

*  To permanently change some of the GMT parameters on the fly
   inside a script the :doc:`/gmtset` utility
   can be used. E.g., to change the primary annotation font to 12 point
   Times-Bold in red we run

   ::

    gmt set FONT_ANNOT_PRIMARY 12p,Times-Bold,red

   These changes will remain in effect until they are overridden.

*  If all you want to achieve is to change a few parameters during the
   execution of a single command but otherwise leave the environment
   intact, consider passing the parameter changes on the command line
   via the **-**\ **-**\ *PAR=value* mechanism. For instance, to temporarily
   set the output format for floating points to have lots of decimals,
   say, for map projection coordinate output, append
   **-**\ **-**\ :term:`FORMAT_FLOAT_OUT`\ =%.16lg to the command in question.

In addition to those parameters that directly affect the plot there are
numerous parameters than modify units, scales, etc. For a complete
listing, see the :doc:`/gmt.conf` man pages.
We suggest that you go through all the available parameters at least
once so that you know what is available to change via one of the
described mechanisms.  The gmt.conf file can be cleared by running
**gmt clear settings**.

Command line arguments
----------------------

Each program requires certain arguments specific to its operation. These
are explained in the manual pages and in the usage messages.
We have tried to choose letters of the alphabet which
stand for the argument so that they will be easy to remember. Each
argument specification begins with a hyphen (except input file names;
see below), followed by a letter, and sometimes a number or character
string immediately after the letter. *Do not* space between the hyphen,
letter, and number or string. *Do* space between options. Example:

   ::

    gmt coast -R0/20/0/20 -Ggray -JM15c -Wthin -Baf -V -pdf map

Command line history
--------------------

GMT programs "remember" the standardized command line options (See
Chapter :doc:`options`) given during their first invocations in a modern
mode session, and afterwards we do not need to repeat them any further.
For example, if a map was created with an Cartesian linear projection,
then any subsequent :doc:`/plot` commands to plot symbols on the same map
do not need to repeat the region and projection information, as shown here::

     gmt begin map
       gmt basemap -R0/6.5/0/7 -Jx2c -B
       gmt plot @Table_5_11.txt -Sc0.3c -Gred
     gmt end show

Thus, the chosen options remain in effect until you provide new option
arguments on the command line.  **Note**: We keep track of two types of regions,
One is the domain used for a map and one is the domain used for processing,
which often are the same.  When a plot is specified without providing
a region then we look for a previous plot region in the history first, and
if it is not found then we look for the processing domain to use instead.  However,
if a data-processing module is not given a region then we only look
for a previous processing domain; we never substitute a plot domain in that case.

Usage messages, syntax- and general error messages
--------------------------------------------------

Each program carries a usage message. If you enter the program name
without any arguments, the program will write the complete usage message
to standard error (your screen, unless you redirect it). This message
explains in detail what all the valid arguments are. If you enter the
program name followed by a *hyphen* (-) only you will get a shorter
version which only shows the command line syntax and no detailed
explanations. If you incorrectly specify an option or omit a required
option, the program will produce syntax errors and explain what the
correct syntax for these options should be. If an error occurs during
the running of a program, the program will in some cases recognize this
and give you an error message. Usually this will also terminate the run.
The error messages generally begin with the name of the program in which
the error occurred; if you have several programs piped together this
tells you where the trouble is.

Standard input or file, header records
--------------------------------------

Most of the programs which expect table data input can read either
standard input or input in one or several files. These programs will try
to read standard input unless you type the filename(s) on the command line
without the above hyphens. (If the program sees a hyphen, it reads the
next character as an instruction; if an argument begins without a
hyphen, it tries to open this argument as a filename).  This feature
allows you to connect programs with pipes if you like.
To give numerous input files you can either list them all (file1.txt file2.txt ...),
use UNIX wild cards (file*.txt), or make a simple *listfile* with the
names of all your datafiles (one per line) and then use the special
=\ *filelist* mechanism to specify the input files to a module.
This allows GMT modules to obtain the input file names from *filelist*.
If your input is
ASCII and has one or more header records that do not begin with #, you
must use the **-h** option (see Section :ref:`option_-h`).
ASCII files may in many cases also contain segment-headers
separating data segments. These are called "multi-segment files". For
binary table data the **-h** option may specify how many bytes should be
skipped before the data section is reached. Binary files may also
contain segment-headers separating data segments. These segment-headers
are simply data records whose fields are all set to NaN; see Chapter
:doc:`file-formats` for complete documentation.

If filenames are given for reading, GMT programs will first look for
them in the current directory. If the file is not found, the programs
will look in other directories pointed to by the
:ref:`directory parameters <DIR Parameters>` :term:`DIR_DATA` and :term:`DIR_CACHE`
or by the environmental parameters **$GMT_USERDIR**, **$GMT_CACHEDIR** and
**$GMT_DATADIR** (if set). They may be set by the user to point to
directories that contain data sets of general use, thus eliminating the
need to specify a full path to these files. Usually, the :term:`DIR_DATA`
directory will hold data sets of a general nature (tables, grids),
whereas the **$GMT_USERDIR** directory (its default value is $HOME/.gmt)
may hold miscellaneous data sets more specific to the user; this directory
also stores GMT defaults, other configuration files and modern session directories as well as the
directory *server* which olds downloaded data sets from the GMT data server
The :term:`DIR_CACHE` will typically contain other data files
downloaded when running tutorial or example scripts.  See :ref:`directory parameters <DIR Parameters>`
for details. Program output is always written to the current directory
unless a full path has been specified.

URLs and remote files
---------------------

Three classes of files are given special treatment in GMT.

#. Some data sets are ubiquitous and used by nearly all GMT users.
   At the moment this collection is limited to Earth relief grids.  If you specify
   a grid input named **@earth_relief_**\ *res* on a command line then
   such a grid will automatically be downloaded from the GMT Data Server and placed
   in the *server* directory under **$GMT_USERDIR** [~/.gmt].  The resolution *res* allows a choice among
   15 common grid spacings: 01d, 30m, 20m, 15m, 10m, 06m, 05m, 04m, 03m, 02m, 01m,
   30s, and 15s (with file sizes 111 kb, 376 kb, 782 kb, 1.3 Mb, 2.8 Mb, 7.5 Mb,
   11 Mb, 16 Mb, 27 Mb, 58 Mb, 214 Mb, 778 Mb, and 2.6 Gb respectively) as well
   as the SRTM tile resolutions 03s and 01s (6.8 Gb and 41 Gb for the whole set, respectively). Once
   one of these grids have been downloaded any future reference will simply obtain the
   file from **$GMT_USERDIR** (except if explicitly removed by the user).
   **Note**: The 15 arc-sec data comes from the original dataset SRTM15+.
   Lower resolutions are spherically Gaussian-filtered versions of SRTM15+.
   The SRTM (version 3) 1 and 3 arc-sec tiles are only available over land
   between 60 degrees south and north latitude and are stored as highly compressed JPEG2000
   tiles on the GMT server.  These are individually downloaded as requested, converted to netCDF
   grids and stored in subdirectories srtm1 and srtm3 under the server directory, and assembled
   into a seamless grid using :doc:`/grdblend`. A tile is only downloaded and converted
   once (unless the user cleans the data directories).
#. If a file is given as a full URL, starting with **http://**, **https://**,
   or **ftp://**, then the file will be downloaded to the current directory and subsequently
   read from there (until removed by the user).  If the URL is actually a CGI Get
   command (i.e., ends in ?par=val1&par2=val2...) then we download the file
   each time we encounter the URL.
#. Demonstration files used in online documentation, example scripts, or even the
   large test suite may be given in the format @\ *filename*.  When such a file is
   encountered on the command line it is understood to be a short-hand representation
   of the full URL to *filename* on the GMT Cache Data site.
   Since this address may change over time we use the leading
   @ to simplify access to these files.  Such files will also be downloaded
   to :term:`DIR_CACHE` and subsequently read from there (until removed by the user).
#. By default, remote files are downloaded from the SOEST data server.  However, you
   can override that selection by setting the environmental parameter **$GMT_DATA_SERVER** or
   the default setting for :term:`GMT_DATA_SERVER`.  Alternatively, configure the CMake
   parameter GMT_DATA_SERVER at compile time.
#. If your Internet connection is slow or nonexistent (e.g., on a plane) you can also
   limit the size of the largest datafile to download via :term:`GMT_DATA_SERVER_LIMIT` or
   you can temporarily turn off such downloads by setting :term:`GMT_DATA_UPDATE_INTERVAL` to "off".

The user cache (:term:`DIR_CACHE`) and all its contents can be cleared any time
via the command **gmt clear cache**, while the server directory with downloaded data
can be cleared via the command **gmt clear data**.  Finally, when a remote file is requested
we also check if that file has changed at the server and re-download the updated file;
this check is only performed no more often than once a day.

.. figure:: /_images/GMT_SRTM.*
   :width: 700 px
   :align: center

   The 14297 1x1 degree tiles (red) for which SRTM 1 and 3 arc second data are available.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_SRTM.txt

As a short example, we can make a quick map of Easter Island using the SRTM 1x1 arc second
grid via

::

 gmt grdimage -R109:30W/109:12W/27:14S/27:02S -JM15c -B @earth_relief_01s -png easter

Verbose operation
-----------------

Most of the programs take an optional **-V** argument which will run the
program in the "verbose" mode (see Section :ref:`option_-V`).
Verbose will write to standard error information about the
progress of the operation you are running. Verbose reports things such
as counts of points read, names of data files processed, convergence of
iterative solutions, and the like. Since these messages are written to
*stderr*, the verbose talk remains separate from your data output. You
may optionally choose among six models of *verbosity*; each mode adds
more messages with an increasing level of details. The modes are

  - **q** - Quiet, not even fatal error messages are produced.
  - **e** - Error messages only.
  - **w** - Warnings (same as running without **-V**)
  - **t** - Timings (report runtimes for time-intensive algorithms).
  - **i** - Informational messages (same as **-V** only).
  - **c** - Compatibility warnings (if compiled with backward-compatibility).
  - **d** - Debugging messages (mostly of interest to developers).

The verbosity is cumulative, i.e., mode **w** means all messages of mode
**e** as well will be reported.

Program output
--------------

Most programs write their results, including PostScript plots, to
standard output. The exceptions are those which may create binary netCDF
grid files such as :doc:`/surface` (due to the
design of netCDF a filename must be provided; however, alternative
binary output formats allowing piping are available; see Section
:ref:`grid-file-format`).
Most operating systems let you can redirect
standard output to a file or pipe it into another process. Error
messages, usage messages, and verbose comments are written to standard
error in all cases. You can usually redirect standard error as well, if
you want to create a log file of what you are doing. The syntax for
redirection differ among the main shells (Bash and C-shell) and is a bit
limited in DOS.

.. _input-data-formats:

Input data formats
------------------

Most of the time, GMT will know what kind of *x* and *y*
coordinates it is reading because you have selected a particular
coordinate transformation or map projection. However, there may be times
when you must explicitly specify what you are providing as input using
the **-f** switch. When binary input data are expected (**-bi**) you
must specify exactly the format of the records. However, for ASCII input
there are numerous ways to encode data coordinates (which may be
separated by white-space or commas). Valid input data are generally of
the same form as the arguments to the **-R** option (see
Section :ref:`option_-R`), with additional flexibility for calendar data.
Geographical coordinates, for example, can be given in decimal degrees
(e.g., -123.45417) or in the
[±]\ *ddd*\ [:*mm*\ [:*ss*\ [*.xxx*]]][**W**\|\ **E**\|\ **S**\|\ **N**]
format (e.g., 123:27:15W). With **-fp** you may even supply projected
data like UTM coordinates.

Because of the widespread use of incompatible and ambiguous formats, the
processing of input date components is guided by the template
:term:`FORMAT_DATE_IN` in your :doc:`/gmt.conf` file; it is by default set to *yyyy-mm-dd*.
Y2K-challenged input data such as 29/05/89 can be processed by setting
:term:`FORMAT_DATE_IN` to dd/mm/yy. A complete description of possible
formats is given in the :doc:`/gmt.conf` man
page. The *clock* string is more standardized but issues like 12- or
24-hour clocks complicate matters as well as the presence or absence of
delimiters between fields. Thus, the processing of input clock
coordinates is guided by the template :term:`FORMAT_CLOCK_IN` which
defaults to *hh:mm:ss.xxx*.

GMT programs that require a map projection argument will implicitly
know what kind of data to expect, and the input processing is done
accordingly. However, some programs that simply report on minimum and
maximum values or just do a reformatting of the data will in general not
know what to expect, and furthermore there is no way for the programs to
know what kind of data other columns (beyond the leading *x* and
*y* columns) contain. In such instances we must explicitly tell
GMT that we are feeding it data in the specific geographic or calendar
formats (floating point data are assumed by default). We specify the
data type via the **-f** option (which sets both input and output
formats; use **-fi** and **-fo** to set input and output separately).
For instance, to specify that the first two columns are longitude
and latitude, and that the third column (e.g., *z*) is absolute
calendar time, we add **-fi**\ 0x,1y,2T to the command line. For more
details, see the man page for the program you need to use.

.. _output-data-formats:

Output data formats
-------------------

The numerical output from GMT programs can be binary (when **-bo** is
used) or ASCII [Default]. In the latter case the issue of formatting
becomes important. GMT provides extensive machinery for allowing just
about any imaginable format to be used on output. Analogous to the
processing of input data, several templates guide the formatting
process. These are :term:`FORMAT_DATE_OUT` and :term:`FORMAT_CLOCK_OUT` for
calendar-time coordinates, :term:`FORMAT_GEO_OUT` for geographical
coordinates, and :term:`FORMAT_FLOAT_OUT` for generic floating point data.
In addition, the user have control over how columns are separated via
the :term:`IO_COL_SEPARATOR` parameter. Thus, as an example, it is possible
to create limited FORTRAN-style card records by setting
:term:`FORMAT_FLOAT_OUT` to %7.3lf and :term:`IO_COL_SEPARATOR` to none
[Default is tab].

PostScript features
---------------------

PostScript is a command language for driving graphics devices such as
laser printers. It is ASCII text which you can read and edit as you wish
(assuming you have some knowledge of the syntax). We prefer this to
binary metafile plot systems since such files cannot easily be modified
after they have been created. GMT programs also write many comments to
the plot file which make it easier for users to orient themselves should
they need to edit the file (e.g., % Start of x-axis) [16]_. All
GMT programs create PostScript code by calling the :doc:`PSL </devdocs/postscriptlight>` plot
library (The user may call these functions from his/her own C or FORTRAN
plot programs. See the manual pages for :doc:`PSL </devdocs/postscriptlight>` syntax). Although
GMT programs can create very individualized plot code, there will
always be cases not covered by these programs. Some knowledge of
PostScript will enable the user to add such features directly into the
plot file. By default, GMT will produce freeform PostScript output
with embedded printer directives. To produce Encapsulated
PostScript (EPS) that can be imported into graphics programs such as
**CorelDraw**, **Illustrator** or **InkScape** for further
embellishment, simply run gmt :doc:`/psconvert`
**-Te**. See Chapter :doc:`include-figures` for an extensive discussion of converting
PostScript to other formats.

.. _-Wpen_attrib:

Specifying pen attributes
-------------------------

A pen in GMT has three attributes: *width*, *color*, and
*style*. Most programs will accept pen attributes in the form of an
option argument, with commas separating the given attributes, e.g.,

**-W**\ [*width*\ [**c**\|\ **i**\|\ **p**]],[*color*],[*style*\ [**c**\|\ **i**\|\ **p**]]

    *Width* is by default measured in points (1/72 of an inch). Append
    **c**, **i**, or **p** to specify pen width in cm, inch, or points,
    respectively. Minimum-thickness pens can be achieved by giving zero
    width. The result is device-dependent but typically means that as
    you zoom in on the feature in a display, the line thickness stays
    at the minimum. Finally, a few predefined
    pen names can be used: default, faint, and {thin, thick,
    fat}[er\|\ est], and wide. Table :ref:`pennames <tbl-pennames>` shows this
    list and the corresponding pen widths.

.. _tbl-pennames:

    +------------+---------+------------+--------+
    | faint      | 0       | thicker    | 1.5p   |
    +------------+---------+------------+--------+
    | default    | 0.25p   | thickest   | 2p     |
    +------------+---------+------------+--------+
    | thinnest   | 0.25p   | fat        | 3p     |
    +------------+---------+------------+--------+
    | thinner    | 0.50p   | fatter     | 6p     |
    +------------+---------+------------+--------+
    | thin       | 0.75p   | fattest    | 10p    |
    +------------+---------+------------+--------+
    | thick      | 1.0p    | wide       | 18p    |
    +------------+---------+------------+--------+

.. _color_attrib:

    The *color* can be specified in five different ways:

    #. Gray. Specify a *gray* shade in the range 0–255 (linearly going
       from black [0] to white [255]).

    #. RGB. Specify *r*/*g*/*b*, each ranging from 0–255. Here 0/0/0 is
       black, 255/255/255 is white, 255/0/0 is red, etc. Alternatively,
       you can give RGB in hexadecimal using the *#rrggbb* format.

    #. HSV. Specify *hue*-*saturation*-*value*, with the former in the
       0–360 degree range while the latter two take on the range 0–1 [17]_.

    #. CMYK. Specify *cyan*/*magenta*/*yellow*/*black*, each ranging
       from 0–100%.

    #. Name. Specify one of 663 valid color names. See :doc:`/gmtcolors` for
       a list of all valid names. A very small yet versatile
       subset consists of the 29 choices *white*, *black*, and
       [light\|\ dark]{*red, orange, yellow, green, cyan, blue,
       magenta, gray\|\ grey, brown*\ }. The color names are
       case-insensitive, so mixed upper and lower case can be used (like
       *DarkGreen*).

    The *style* attribute controls the appearance of the line. Giving "dotted" or "."
    yields a dotted line, whereas a dashed pen is requested with "dashed" or "-".
    Also combinations of dots and dashes, like ".-" for a dot-dashed
    line, are allowed. To override a default style and secure a solid line you can
    specify "solid" for style.  The lengths of dots and dashes are scaled
    relative to the pen width (dots has a length that equals the pen
    width while dashes are 8 times as long; gaps between segments are 4
    times the pen width). For more detailed attributes including exact
    dimensions you may specify *string*\ [:*offset*], where *string* is a
    series of numbers separated by underscores. These numbers represent
    a pattern by indicating the length of line segments and the gap
    between segments. The optional *offset* phase-shifts the pattern from the
    beginning the line [0]. For example, if you want a yellow line of width
    0.1 cm that alternates between long dashes (4 points), an 8 point
    gap, then a 5 point dash, then another 8 point gap, with pattern
    offset by 2 points from the origin, specify
    **-W**\ 0.1c,yellow,4_8_5_8:2p. Just as with pen width, the
    default style units are points, but can also be explicitly specified
    in cm, inch, or points (see *width* discussion above).

Table :ref:`penex <tbl-penex>` contains additional examples of pen specifications
suitable for, say, :doc:`/plot`.

.. _tbl-penex:

+-------------------------------+-----------------------------------------------------+
| **-W**\ 0.5p                  | 0.5 point wide line of default color and style      |
+-------------------------------+-----------------------------------------------------+
| **-W**\ green                 | Green line with default width and style             |
+-------------------------------+-----------------------------------------------------+
| **-W**\ thin,red,-            | Dashed, thin red line                               |
+-------------------------------+-----------------------------------------------------+
| **-W**\ fat,.                 | Fat dotted line with default color                  |
+-------------------------------+-----------------------------------------------------+
| **-W**\ 0.1c,120-1-1          | Green (in h-s-v) pen, 1 mm thick                    |
+-------------------------------+-----------------------------------------------------+
| **-W**\ faint,100/0/0/0,..-   | Very thin, cyan (in c/m/y/k), dot-dot-dashed line   |
+-------------------------------+-----------------------------------------------------+

In addition to these pen settings there are several
PostScript settings that can affect the appearance of lines. These are
controlled via the GMT defaults settings :term:`PS_LINE_CAP`,
:term:`PS_LINE_JOIN`, and :term:`PS_MITER_LIMIT`. They determine how a line
segment ending is rendered, be it at the termination of a solid line or
at the end of all dashed line segments making up a line, and how a
straight lines of finite thickness should behave when joined at a common
point, as shown in Figures :ref:`Cap <Cap_settings>` and :ref:`Miter <Miter_settings>`.

.. _Cap_settings:

.. figure:: /_images/GMT_cap.*
   :width: 400 px
   :align: center

   Line appearance can be varied by using :term:`PS_LINE_CAP`, choosing from **SQUARE** [Default],
   **ROUND**, or **BUTT**.  The circles and thin lines indicate the coordinates.  All lines
   where plotted with the same width and dash-spacing (-W10p,20_20:0).

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_cap.txt

.. _Miter_settings:

.. figure:: /_images/GMT_joint.*
   :width: 550 px
   :align: center

   Given lines have finite thickness, there are three types of joints where line-segments
   meet that can be adjusted with :term:`PS_LINE_JOIN`.  There is **BEVEL**, **ROUND**, and
   **MITER**.  The last setting also depends on :term:`PS_MITER_LIMIT` which sets a limit on
   the angle at the mitered joint below which we apply a bevel.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_joint.txt

By default, line segments have rectangular ends, but this can
change to give rounded ends. When :term:`PS_LINE_CAP` is set to round then
a segment length of zero will appear as a circle. This can be used to
create circular dotted lines, and by manipulating the *phase* shift in
the *style* attribute and plotting the same line twice one can even
alternate the color of adjacent items.
Figure :ref:`Line appearance <Line_appearance>` shows various lines made in this
fashion by adjusting the joint and cap settings as well as plotting lines twice with
different phase *offset* and color. See the :doc:`/gmt.conf` man page for more information.

.. _Line_appearance:

.. figure:: /_images/GMT_linecap.*
   :width: 500 px
   :align: center

   Line appearance can be varied by using :term:`PS_LINE_CAP`.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_linecap.txt

Experience has shown that the rendering of lines that are short relative to the pen thickness
can sometimes appear wrong or downright ugly.  This is a feature of PostScript interpreters, such as
Ghostscript.  By default, lines are rendered using a fast algorithm which is susceptible to
errors for thick lines.  The solution is to select a more accurate algorithm to render the lines
exactly as intended.  This can be accomplished by using the GMT Defaults :term:`PS_LINE_CAP`
and :term:`PS_LINE_JOIN` by setting both to *round*.  Figure :ref:`Line appearance <Line_badrender>`
displays the difference in results.

.. _Line_badrender:

.. figure:: /_images/GMT_fatline.*
   :width: 500 px
   :align: center

   Very thick line appearance using the default (left) and round line cap and join (right).  The
   red line (1p width) illustrates the extent of the input coordinates.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_fatline.txt

Specifying line attributes
--------------------------

A line is drawn with the texture provided by the chosen pen (`Specifying pen attributes`_).
However, depending on the module, a line also may have other attributes that can be changed in some modules.
Given as modifiers to a pen specification, one or more modifiers may be appended to a pen
specification. The line attribute modifiers are:


* **+o**\ *offset*
    Lines are normally drawn from the beginning to the end point. You can modify this behavior
    by requesting a gap between these terminal points and the start and end of the
    visible line.  Do this by specifying the desired offset between the terminal point and the
    start of the visible line.  Unless you are giving distances in Cartesian data units,
    please append the distance unit, **u**.  Depending on your desired effect, you can append
    plot distance units (i.e., **c**\ m, **i**\ nch, **p**\ oint; Section `Dimension units`_) or map distance units,
    such as **k**\ m, **d**\ egrees, and many other standard distance units listed in
    Section `GMT units`_.  If only one offset is given then it applies equally to both ends of
    the line.  Give two slash-separated distances to indicate different offsets at the
    beginning and end of the line (and use 0 to indicate no offset at one end).

.. _Line_offset:

.. figure:: /_images/GMT_lineoffset.*
   :width: 500 px
   :align: center

   The thin red line shows an original line segment, whereas the 2-point thick pen illustrates the effect
   of plotting the same line while requesting offsets of 1 cm at the beginning and 500 km
   at the end, via **-W**\ 2p\ **+o**\ 1c/500k.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_lineoffset.txt

* **+s**
    Normally, all PostScript line drawing is implemented as a linear spline, i.e., we simply
    draw straight line-segments between the map-projected data points.  Use this modifier to render the
    line using Bezier splines for a smoother curve. **Note**: The spline is fit to the projected
    2-D coordinates, not the raw user coordinates (i.e., it is not a spherical surface spline).

.. _Line_bezier:

.. figure:: /_images/GMT_bezier.*
   :width: 500 px
   :align: center

   (left) Normal plotting of line given input points (red circles) via **-W**\ 2p. (right) Letting
   the projected points be interpolated by a Bezier cubic spline via **-W**\ 2p\ **+s**.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_bezier.txt

* **+v**\ [**b**\|\ **e**]\ *vspecs*
    By default, lines are normally drawn from start to end.  Using the **+v** modifier you can
    place arrow-heads pointing outward at one (or both) ends of the line.  Use **+v** if you
    want the same vector attributes for both ends, or use **+vb** and **+ve** to specify a vector
    only at the beginning or end of the line, respectively.  Finally, these two modifiers may both be given
    to specify different attributes for the two vectors.  The vector specification is very rich
    and you may place other symbols, such as circle, square, or a terminal cross-line, in lieu of the
    vector head (see :doc:`/plot` for more details).

.. _Line_vector:

.. figure:: /_images/GMT_linearrow.*
   :width: 500 px
   :align: center

   Same line as above but now we have requested a blue vector head at the end of the line and a
   red circle at the beginning of the line with **-W**\ 2p\ **+o**\ 1c/500k\ **+vb**\ 0.2i\ **+g**\ red\ **+p**\ faint\ **+b**\ c\ **+ve**\ 0.3i\ **+g**\ blue.
   Note that we also prescribed the line offsets in addition to the symbol endings.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_linearrow.txt

.. _-Gfill_attrib:

Specifying area fill attributes
-------------------------------

Many plotting programs will allow the user to draw filled polygons or
symbols. The fill specification may take two forms (note: not all modules
use **-G** for this task and some have several options specifying different fills):

**-G**\ *fill*
    In the first case we may specify a *gray* shade (0–255), RGB color
    (*r*/*g*/*b* all in the 0–255 range or in hexadecimal *#rrggbb*),
    HSV color (*hue*-*saturation*-*value* in the 0–360, 0–1, 0–1 range),
    CMYK color (*cyan*/*magenta*/*yellow*/*black*, each ranging from
    0–100%), or a valid color *name*; in that respect it is similar to
    specifying the pen color settings (see pen color discussion under
    Section `Specifying pen attributes`_).

**-GP**\|\ **p**\ *pattern*\ [**+b**\ *color*][**+f**\ *color*][**+r**\ *dpi*]
    The second form allows us to use a predefined bit-image pattern.
    *pattern* can either be a number in the range 1–90 or the name of a
    1-, 8-, or 24-bit image raster file. The former will result in one of
    the 90 predefined 64 x 64 bit-patterns provided with GMT and
    reproduced in Chapter :doc:`predefined-patterns`.
    The latter allows the user to create
    customized, repeating images using image raster files.
    The optional **+r**\ *dpi* modifier sets the resolution of this image on the page;
    the area fill is thus made up of a series of these "tiles".  The
    default resolution is 1200.  By specifying upper case **-GP**
    instead of **-Gp** the image will be bit-reversed, i.e., white and
    black areas will be interchanged (only applies to 1-bit images or
    predefined bit-image patterns). For these patterns and other 1-bit
    images one may specify alternative background and foreground colors
    (by appending **+b**\ *color* and/or **+f**\ *color*) that will replace
    the default white and black pixels, respectively. Excluding *color* from
    a fore- or background specification yields a *transparent* image where
    only the back- *or* foreground pixels will be painted.

Due to PostScript implementation limitations the raster images used
with **-G** must be less than 146 x 146 pixels in size; for larger
images see :doc:`/image`. The format of Sun raster files [18]_ is
outlined in Chapter :doc:`file-formats`; other image formats can be
used as well. Note that under
PostScript Level 1 the patterns are filled by using the polygon as a
*clip path*. Complex clip paths may require more memory than the
PostScript interpreter has been assigned. There is therefore the
possibility that some PostScript interpreters (especially those
supplied with older laserwriters) will run out of memory and abort.
Should that occur we recommend that you use a regular gray-shade fill
instead of the patterns. Installing more memory in your printer *may or
may not* solve the problem!

Table :ref:`fillex <tbl-fillex>` contains a few examples of fill specifications.

.. _tbl-fillex:

+-------------------------------------------------+-----------------------------------------------------+
| **-G**\ 128                                     | Solid gray                                          |
+-------------------------------------------------+-----------------------------------------------------+
| **-G**\ 127/255/0                               | Chartreuse, R/G/B-style                             |
+-------------------------------------------------+-----------------------------------------------------+
| **-G**\ #00ff00                                 | Green, hexadecimal RGB code                         |
+-------------------------------------------------+-----------------------------------------------------+
| **-G**\ 25-0.86-0.82                            | Chocolate, h-s-v-style                              |
+-------------------------------------------------+-----------------------------------------------------+
| **-G**\ DarkOliveGreen1                         | One of the named colors                             |
+-------------------------------------------------+-----------------------------------------------------+
| **-Gp**\ 7\ **+r**\ 300                         | Simple diagonal hachure pattern in b/w at 300 dpi   |
+-------------------------------------------------+-----------------------------------------------------+
| **-Gp**\ 7\ **+b**\ red\ **+r**\ 300            | Same, but with red lines on white                   |
+-------------------------------------------------+-----------------------------------------------------+
| **-Gp**\ 7\ **+b**\ red\ **+f**\ -\ **+r**\ 300 | Now the gaps between red lines are transparent      |
+-------------------------------------------------+-----------------------------------------------------+
| **-Gp**\ marble.ras\ **+r**\ 100                | Using user image of marble as the fill at 100 dpi   |
+-------------------------------------------------+-----------------------------------------------------+

Specifying Fonts
----------------

The fonts used by GMT are typically set indirectly via the
GMT defaults parameters. However, some programs, like
:doc:`/text` may wish to have this
information passed directly. A font is specified by a comma-delimited
attribute list of *size*, *fonttype* and *fill*, each of which is
optional. The *size* is the font size (usually in points) but **c**,
**i** or **p** can be added to indicate a specific unit. The *fonttype*
is the name (case sensitive!) of the font or its equivalent numerical ID
(e.g., Helvetica-Bold or 1). The *fill* specifies the gray shade, color or
pattern of the text (see section `Specifying area fill attributes`_ above).
Optionally, you may append **=**\ *pen* to the *fill* value in order to draw a text
outline. If you want to avoid that the outline partially obscures the text,
append **=~**\ *pen* instead; in that case only half the linewidth is plotted
on the outside of the font only.  If an outline is requested, you may optionally
skip the text *fill* by setting it to **-**, in which case the full pen width
is always used. If any of the font attributes is omitted their default or
previous setting will be retained. See Chapter :doc:`postscript-fonts`
for a list of all fonts recognized by GMT.

Stroke, Fill and Font Transparency
----------------------------------

The PostScript language has no built-in mechanism for transparency.
However, PostScript extensions make it possible to request
transparency, and tools that can render such extensions will produce
transparency effects. We specify transparency in percent: 0 is opaque
[Default] while 100 is fully transparent (i.e., the feature will be invisible). As
noted in section :ref:`option_-t`, we can control transparency on a
layer-by-layer basis using the **-t** option. However, we may also set
transparency as an attribute of stroke or fill (including for fonts)
settings. Here, transparency is requested by appending @\ *transparency*
to colors or pattern fills. The transparency *mode* can be changed by
using the GMT default parameter :term:`PS_TRANSPARENCY`; the default is
Normal but you can choose among Color, ColorBurn, ColorDodge, Darken,
Difference, Exclusion, HardLight, Hue, Lighten, Luminosity, Multiply,
Normal, Overlay, Saturation, SoftLight, and Screen. For more
information, see for instance (search online for) the Adobe pdfmark
Reference Manual. Most printers and many PostScript viewers can
neither print nor show transparency. They will simply ignore your
attempt to create transparency and will plot any material as opaque.
Ghostscript and its derivatives such as GMT's
:doc:`/psconvert` support transparency (if
compiled with the correct build option). **Note**: If you use **Acrobat
Distiller** to create a PDF file you must first change some settings to
make transparency effective: change the parameter /AllowTransparency to
true in your \*.joboptions file.

Placement of text
-----------------

Many text labels placed on maps are part of the standard basemap
machinery (e.g., annotations, axis labels, plot titles) and GMT
automatically takes care of where these are placed and how they
are justified.  However, when you wish to add extra text to a plot
in locations of your choice you will need to understand how we
reference text to locations on the map.  Figure :ref:`Text justification <Text_justify>`
discusses the various ways to do this.

.. _Text_justify:

.. figure:: /_images/GMT_pstext_justify.*
   :width: 400 px
   :align: center

   Text strings are placed on maps by associating an *anchor* point on
   the string with a *reference* point on the map.  Nine anchor points
   relative to any text string may be specified by combining any of
   three letter codes for horizontal (**L**\ eft, **C**\ enter, **R**\ ight)
   and vertical (**T**\ op, **M**\ iddle, **B**\ ottom) alignments.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_pstext_justify.txt

Notice how the anchor points refers to the text baseline and do not change
for text whose letters extend below the baseline.

The concept of anchor points extends to entire text paragraphs that you
may want to typeset with :doc:`/text`.

A related point involves the
footprint of the text and any background panel on the map.  We determine
the bounding box for any text string, but very often we wish to extend this
box outwards to allow for some *clearance* between the text and the space
surrounding it.  Programs that allows for such clearance will let you
specify offsets *dx* and *dy* that is used to enlarge the bounding box,
as illustrated in Figure :ref:`Text clearance <Text_clearance>`.

.. _Text_clearance:

.. figure:: /_images/GMT_pstext_clearance.*
   :width: 300 px
   :align: center

   The bounding box of any text string can be enlarged by specifying the
   adjustments *dx* and *dy* in the horizontal and vertical dimension.  The shape of the
   bounding box can be modified as well, including rounded or convex
   rectangles.  Here we have chosen a rounded rectangle, requiring the
   additional specification of a corner radius, *r*.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_pstext_clearance.txt

.. _CPT_section:

Color palette tables
--------------------

Several programs need to relate user data to colors, shades, or even patterns.
For instance, programs that read 2-D gridded data sets and
create colored images or shaded reliefs  need to be told what colors to
use and over what *z*-range each color applies. Other programs may need
to associate a user value with a color to be applied to a symbol, line,
or polygon.  This is the purpose of the color palette table (CPT).  For
most applications, you will simply create a CPT using the tool
:doc:`/makecpt` which will take an existing *dynamic* master
color table and stretch it to fit your chosen data range, or use
:doc:`/grd2cpt` to build a CPT based on
the data distribution in one or more given grid files. However, in rare
situations you may need to make a CPT by hand or using text tools
like **awk** or **perl**. Finally, if you have your own preferred color
table you can convert it into a dynamic CPT and place it in your GMT
user directory and it will be found and behave like other GMT master CPTs.

Color palette tables (CPT) comes in two flavors: (1) Those designed to
work with categorical data (e.g., data where interpolation of values is
undefined) and (2) those designed for regular, continuously-varying
data. In both cases the *fill* information follows the format given in
Section `Specifying area fill attributes`_. The z-values in CPTs can
be scaled by using the **+u**\|\ **U**\ *unit* mechanism.  Append these
modifiers to your CPT names when used in GMT commands.  The **+u**\ *unit*
modifier will scale z *from unit to* meters, while **+U**\ *unit* does
the inverse (scale z *from meters to unit*).

**Note**: Users are allowed to name their CPT files anything they want, but
we recommend the use of the file extension ".cpt".  This allows us to prevent
any confusion when parsing filenames that may have sequences that otherwise
might look like a file *modifier* (e.g., data.my+u5.cpt). Since valid modifiers
are *appended* to a file name, finding such an extension simplifies parsing.

Since GMT supports several coordinate systems for color specification,
many master (or user) CPTs will contain the special comment

| ``# COLOR_MODEL = model``

where *model* specifies how the color-values in the CPT should be interpreted.
By default we assume colors are given as red/green/blue triplets (each in the
0-255 range) separated by
slashes (model = *rgb*), but alternative representations are the HSV system
of specifying hue-saturation-value triplets (with hue in 0-360 range and
saturation and value ranging from 0-1) separated by hyphens (model = *hsv*),
or the CMYK system of specifying cyan/magenta/yellow/black quadruples in percent,
separated by slashes (model = *cmyk*).

Categorical CPTs
~~~~~~~~~~~~~~~~

Categorical data are information on which normal numerical operations
are not defined. As an example, consider various land classifications
(desert, forest, glacier, etc.) and it is clear that even if we assigned
a numerical value to these categories (e.g., desert = 1, forest = 2,
etc) it would be meaningless to compute average values (what would 1.5
mean?). For such data a special format of the CPTs are provided.
Here, each category is assigned a unique key, a color or pattern, and an
optional label (usually the category name) marked by a leading
semi-colon. Keys (if numerical) must be monotonically increasing but do
not need to be consecutive. The format is

+-----------------+--------+--------------+
| key\ :sub:`1`   | *Fill* | [;\ *label*] |
+-----------------+--------+--------------+
| ...             |        |              |
+-----------------+--------+--------------+
| key\ :sub:`n`   | *Fill* | [;\ *label*] |
+-----------------+--------+--------------+

For usage with points, lines, and polygons, the keys may be text (single words),
and then GMT will use strings to find the corresponding *Fill* value. Strings
may be supplied as trailing text in data files (for points) or via the **-Z**\ *category*
option in multiple segment headers (or set via **-a**\ *Z*\ =\ *aspatialname*).
If any of your keys are called B, F, or N you must escape them with a leading backslash
to avoid confusion with the flags for background, foreground and NaN colors.
The *Fill* information follows the format given in Section `Specifying area fill attributes`_.
For categorical data, background color or foreground color do not apply. The not-a-number (NaN)
color (for *key*-values not found or blank) is defined in the :doc:`/gmt.conf` file, but it can be
overridden by the statement

+-----+---------------------+
| N   | Fill\ :sub:`nan`    |
+-----+---------------------+

While you can make such categorical CPTs by hand, both :doc:`/makecpt` and :doc:`/grd2cpt` have options to
simplify adding string keys and labels from comma-separated arguments.

Regular CPTs
~~~~~~~~~~~~

Suitable for continuous data types and allowing for color
interpolations, the format of the regular CPTs is:

+---------------+-------------------+---------------+-------------------+----------+------------------------------+
| z\ :sub:`0`   | Color\ :sub:`min` | z\ :sub:`1`   | Color\ :sub:`max` | [**A**]  | [;\ *label*]                 |
+---------------+-------------------+---------------+-------------------+----------+------------------------------+
| ...                                                                                                             |
+---------------+-------------------+---------------+-------------------+----------+------------------------------+
| z\ :sub:`n-2` | Color\ :sub:`min` | z\ :sub:`n-1` | Color\ :sub:`max` | [**A**]  | [;\ *labell*\ [;\ *labelu*]] |
+---------------+-------------------+---------------+-------------------+----------+------------------------------+


Thus, for each "*z*-slice", defined as the interval between two
boundaries (e.g., :math:`z_0` to :math:`z_1`), the color can be
constant (by letting Color\ :math:`_{max}` = Color\ :math:`_{min}` or -)
or a continuous, linear function of *z*. If patterns are used then the
second (max) pattern must be set to -. The optional flag **A** is used
to indicate annotation of the color scale when plotted using
:doc:`/colorbar`. The optional flag **A** may
be **L**, **U**, or **B** to select annotation of the lower, upper, or
both limits of the particular *z*-slice, respectively. However,
the standard **-B** option can be used by
:doc:`/colorbar` to affect annotation and
ticking of color scales. Just as other GMT programs, the *stride* can
be omitted to determine the annotation and tick interval automatically
(e.g., **-Baf**). The optional semicolon followed by a text label will
make :doc:`/colorbar`, when used with the
**-L** option, place the supplied label instead of formatted *z*-values.
**Note**: If the last slice should have both lower and upper
custom labels then you must supply *two* semicolon-separated labels and set the
annotation code to **B**.


The background color (for *z*-values < :math:`z_0`), foreground color (for *z*-values >
:math:`z_{n-1}`), and not-a-number (NaN) color (for *z*-values =
NaN) are all defined in the :doc:`/gmt.conf` file, but can be overridden by the
statements

+-----+---------------------+
| B   | Fill\ :sub:`back`   |
+-----+---------------------+
| F   | Fill\ :sub:`fore`   |
+-----+---------------------+
| N   | Fill\ :sub:`nan`    |
+-----+---------------------+

which can be inserted into the beginning or end of the CPT. If you
prefer the HSV system, set the :doc:`/gmt.conf` parameter accordingly and replace red,
green, blue with hue, saturation, value. Color palette tables that
contain gray-shades only may replace the *r/g/b* triplets with a single
gray-shade in the 0–255 range. For CMYK, give *c/m/y/k* values in the
0–100 range.

A few programs (i.e., those that plot polygons such as
:doc:`/grdview`, :doc:`/colorbar`,
:doc:`/plot` and
:doc:`/plot3d`) can accept pattern fills instead
of gray-shades. You must specify the pattern as in Section `Specifying area fill attributes`_
(no leading **-G** of course), and only the first pattern (for low
*z*) is used (we cannot interpolate between patterns). Finally,
some programs let you skip features whose *z*-slice in the CPT
file has gray-shades set to -. As an example, consider

+-----+----------+------+-----------+
| 30  | p16+r200 | 80   | \-        |
+-----+----------+------+-----------+
| 80  | \-       | 100  | \-        |
+-----+----------+------+-----------+
| 100 | 200/0/0  | 200  | 255/255/0 |
+-----+----------+------+-----------+
| 200 | yellow   | 300  | green     |
+-----+----------+------+-----------+

where slice 30 < z < 80 is painted with pattern # 16 at 200 dpi,
slice 80 < z < 100 is skipped, slice 100 < z < 200 is
painted in a range of dark red to yellow, whereas the slice
200 < z < 300 will linearly yield colors from yellow to green,
depending on the actual value of *z*.

Some programs like :doc:`/grdimage` and
:doc:`/grdview` apply artificial illumination
to achieve shaded relief maps. This is typically done by finding the
directional gradient in the direction of the artificial light source and
scaling the gradients to have approximately a normal distribution on the
interval [-1,+1]. These intensities are used to add "white" or "black"
to the color as defined by the *z*-values and the CPT. An intensity
of zero leaves the color unchanged. Higher values will brighten the
color, lower values will darken it, all without changing the original
hue of the color (see Chapter :doc:`colorspace` for more details). The
illumination is decoupled from the data grid file in that a separate
grid file holding intensities in the ±1 range must be provided.
Such intensity files can be derived from the data grid using
:doc:`/grdgradient` and modified with
:doc:`/grdhisteq`, but could equally well be
a separate data set. E.g., some side-scan sonar systems collect both
bathymetry and backscatter intensities, and one may want to use the
latter information to specify the illumination of the colors defined by
the former. Similarly, one could portray magnetic anomalies superimposed
on topography by using the former for colors and the latter for shading.

Master (dynamic) CPTs
~~~~~~~~~~~~~~~~~~~~~

The CPTs distributed with GMT are *dynamic*.  This means they have several
special properties that modify the behavior of programs that use them.
Dynamic CPTs comes in a few different flavors: Some CPTs were designed
to behave differently across a *hinge* value (e.g., a CPT designed specifically
for topographic relief may include a discontinuity in color across the
coastline at *z = 0*), and when users select these CPTs they will be stretched
to fit the user's desired data range separately for each side of this *hard* hinge.
Basically, a *hard* hinge CPT is the juxtaposition of two different CPTs joined
at the hinge and these sections are stretched independently. Such CPT files
are identified as such via the special comment

| ``# HARD_HINGE``

and all hard hinges must occur at data value *z = 0* (but you can change this value by
adding **+h**\ *value* to the name of the CPT).
Other CPTs may instead have a *soft* hinge which indicates a natural hinge or transition
point in the CPT itself, unrelated to any natural data set *per se*. These CPTs
are flagged by the special comment

| ``# SOFT_HINGE``

CPTs with soft hinges behave as regular (non-hinge) CPTs *unless* the user activates then by
appending **+h**\ [*hinge*] to the CPT name.  This modifier will convert the soft
hinge into a hard hinge at the user-specified data value *hinge* [which defaults to 0].
As for hard hinges, soft hinges must occur at data value *z = 0* in the CPT.
Note that if your specified data range *excludes* an activated soft or hard hinge then we
only perform color sampling from the *half* of the CPT that pertains to the data range.
All dynamic CPTs will need to be stretched to the user's preferred range, and there
are two modes of such scaling: Some CPTs designed for a specific application
(again, the topographic relief is a good example) have a *default range*
specified in the master table via the special comment


| ``# RANGE = <zmin/zmax>``

and when used by applications the CPT may be automatically stretched to reflect
this natural range.  In contrast, dynamic CPTs *without* a natural range are instead
stretched to fit the range of the data in question (e.g., a grid's range).
Exceptions to these rules are implemented in the two *CPT-producing* modules
:doc:`/makecpt` and :doc:`/grd2cpt`, both of which can read dynamic CPTs
and produce *static* CPTs satisfying a user's specific range needs.  These
tools can also read static CPTs for which a new range must be specified (or computed
from data), reversing the order of colors, and even isolating a section
of an incoming CPT.  Here, :doc:`/makecpt` can be told the data range or compute
it from data tables while :doc:`/grd2cpt` can derive the range from one or more grids.

.. figure:: /_images/GMT_hinge.*
   :width: 500 px
   :align: center

   The top color bar is a dynamic master CPT (here, globe) with a hard hinge at sea level and
   a natural range from -10,000 to +10,000 meters. However, our data range
   is asymmetrical, going from -8,000 meter depths up to +3,000 meter elevations.
   Because of the hinge, the two sides of the CPT will be stretched separately
   to honor the desired range while utilizing the full color range.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_hinge.txt

All CPT master tables can be found in Chapter :ref:`Of Colors and Color Legends`
where those with hard or soft hinges are identified by triangles at their hinges.

CPTs from color lists
~~~~~~~~~~~~~~~~~~~~~

GMT can build color tables "on the fly" from a comma-separated list of colors
and a range of *z*-values to go with them.  As illustrated below, there are
four different ways to create such CPTs. In this example, we will operate with
a list of three colors: red,yellow and purple, given to modules with the option **-C**\ red,yellow,purple,
and utilize a fixed data range of *z = 0-6*.
Four different CPTs result because we either select a *continuous* or *discrete table*, and because the *z*-intervals are
either *equidistant* or *arbitrary*.  The top continuous color table with equidistant spacing (a) is selected
with the range **-T**\ 0/6, meaning the colors will continuously change from red (at *z = 0*) via
yellow (at *z = 3*) to purple (at *z = 6*). Next, a discrete table with the same range (b)
is obtained with **-T**\ 0/6/2, yielding colors that are either constant red (*z = 0-2*), yellow (*z = 2-4*)
or purple (*z = 4-6*). The next discrete table (c) illustrates how to specify arbitrary
node points in the CPT by providing a comma-separated list of values (**-T**\ 0,4,5.5,6). Now, the constant
color intervals have unequal ranges, illustrated by red (*z = 0-4*), yellow (*z = 4-5.5*) and purple (*z = 5.5-6*).  Finally, we
create a continuous color table (d) with arbitrary nodes by giving **-T**\ 0,2,6 and adding **-Z**;
the latter option forces a continuous CPT pinned to a given list of node values.  Now, the colors
continuously change from red (at *z = 0*) via yellow (at *z = 2*) to purple (at *z = 6*).
Modules that obtain the *z*-range indirectly (e.g., :doc:`/grdimage`) may use the exact data range
to set the quivalent of a **-T**\ *min/max* option.  You may append **+i**\ *dz* to the
color list to have the *min* and *max* values rounded down and up to nearest multiple of *dz*, respectively.

.. figure:: /_images/GMT_colorlist.*
   :width: 500 px
   :align: center

   Lists of colors (here red,yellow,purple) can be turned into discrete or continuous CPT tables on the fly.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_colorlist.txt

Cyclic (wrapped) CPTs
~~~~~~~~~~~~~~~~~~~~~

Any color table you produce can be turned into a cyclic or *wrapped* color table.
This is performed by adding the **-Ww** option when running :doc:`/makecpt` or
:doc:`/grd2cpt`.  This option simply adds the special comment

| ``# CYCLIC``

to the color table and then GMT knows that when looking up a color from a *z*
value it will remove an integer multiple of the *z*-range represented by the
color table so that we are always inside the range of the color table.  This
means that the fore- and back-ground colors can never be activated.  Wrapped
color tables are useful for highlighting small changes.

.. figure:: /_images/GMT_cyclic.*
   :width: 500 px
   :align: center

   Cyclic color bars are indicated by a cycle symbol on the left side of the bar.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_cyclic.txt

.. _manipulating_CPTs:

Manipulating CPTs
~~~~~~~~~~~~~~~~~

There are many ways to turn a master CPT into a custom CPT that works for your
particular data range.  The tools :doc:`/makecpt` and :doc:`/grd2cpt` allow
several types of transformations to take place:

    #. You can reverse the *z*-direction of the CPT using option **-Iz**.
       This is useful when your data use a different convention for
       positive and negative (e.g., perhaps using positive depths instead of
       negative relief).
    #. You can invert the order of the colors in the CPT using option **-Ic**.
       This is different from the previous option in that only the colors
       are rearranged (it is also possible to issue **-Icz** to combine both effects.)
    #. You can select just a subset of a master CPT with **-G**, in effect creating
       a modified master CPT that can be scaled further.
    #. Finally, you can scale and translate the (modified) master CPT range to
       your actual data range or a sub-range thereof.

The order of these transformations is important.  For instance, if **-Iz** is given
then all other *z*-values need to be referred to the new sign convention. For most
applications only the last transformation is needed.

.. figure:: /_images/GMT_CPTscale.*
   :width: 500 px
   :align: center

   Examples of two user CPTs for the range -0.5 to 3 created from the same master.  One (left) extracted a
   subset of the master before scaling while the other (right) used the entire range.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_CPTscale.txt

Automatic CPTs
~~~~~~~~~~~~~~

A few modules (:doc:`/grdimage`, :doc:`/grdview`) that expects a CPT option will
provide a default CPT if none is provided.  By default, the default CPT is the
*turbo* color table, but this is overridden if the user uses the @earth_relief
(we select *geo*) or @srtm_relief (we select *srtm*) data sets.  After selection,
these CPTs are read and scaled to match the range of the grid values. You may append
**+i**\ *dz* to the CPT to have the exact range rounded to nearest multiple of *dz*.
This is helpful if you plan to place a colorbar and prefer start and stop *z*-values
that are multiples of *dz*.

The Drawing of Vectors
----------------------

GMT supports plotting vectors in various forms. A vector is one of
many symbols that may be plotted by :doc:`/plot`
and :doc:`/plot3d`, is the main feature in
:doc:`/grdvector`, and is indirectly used by
other programs. All vectors plotted by GMT consist of two separate
parts: The vector line (controlled by the chosen pen attributes) and the
optional vector head(s) (controlled by the chosen fill). We distinguish
between three types of vectors:

#. Cartesian vectors are plotted as straight lines. They can be
   specified by a start point and the direction and length (in map
   units) of the vector, or by its beginning and end point. They may
   also be specified giving the azimuth and length (in km) instead.

#. Circular vectors are (as the name implies) drawn as circular arcs and
   can be used to indicate opening angles. It accepts an origin, a
   radius, and the beginning and end angles.

#. Geo-vectors are drawn using great circle arcs. They are specified by
   a beginning point and the azimuth and length (in km) of the vector,
   or by its beginning and end point.

.. figure:: /_images/GMT_arrows.*
   :width: 500 px
   :align: center

   Examples of Cartesian (left), circular (middle), and geo-vectors (right)
   for different attribute specifications. Note that both full and half
   arrow-heads can be specified, as well as no head at all.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_arrows.txt

There are numerous attributes you can modify, including how the vector
should be justified relative to the given point (beginning, center, or
end), where heads (if any) should be placed, if the head should just be
the left or right half, if the vector attributes should shrink for
vectors whose length are less than a given cutoff length, and the size
and shape of the head. These attributes are detailed further in the
relevant manual pages.

.. figure:: /_images/GMT_arrows_types.*
   :width: 500 px
   :align: center

   Examples of different vector heads and attributes.  The default is the standard
   triangular arrow head, which can be modified by adjusting the apex angle [30] or
   changing its shape via the :term:`MAP_VECTOR_SHAPE` setting.
   Other vector heads are the circle (**c**), the terminal line (**t**), the
   arrow fin (**i**) and the plain head (**A**) and tail (**I**); the last two
   are line-drawings only and cannot be filled.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_arrows_types.txt

.. _Char-esc-seq:

Character escape sequences
--------------------------

For annotation labels or text strings plotted with
:doc:`/text`, GMT provides several escape
sequences that allow the user to temporarily switch to the symbol font,
turn on sub- or superscript, etc., within words. These conditions are
toggled on/off by the escape sequence @\ **x**, where **x** can be one
of several types. The escape sequences recognized in GMT are listed in
Table :ref:`escape <tbl-escape>`. Only one level of sub- or superscript is supported.
Note that under Windows the percent symbol indicates a batch variable,
hence you must use two percent-signs for each one required in the escape
sequence for font switching. In bash scripts the brackets have special meaning, hence you must add double quotes.

.. _tbl-escape:

+-------------------+----------------------------------------------------------------+
| @~                | Turns symbol font on or off                                    |
+-------------------+----------------------------------------------------------------+
| @+                | Turns superscript on or off                                    |
+-------------------+----------------------------------------------------------------+
| @-                | Turns subscript on or off                                      |
+-------------------+----------------------------------------------------------------+
| @#                | Turns small caps on or off                                     |
+-------------------+----------------------------------------------------------------+
| @\_               | Turns underline on or off                                      |
+-------------------+----------------------------------------------------------------+
| @%\ *fontno*\ %   | Switches to another font; @%% resets to previous font          |
+-------------------+----------------------------------------------------------------+
| @:\ *size*:       | Switches to another font size; @:: resets to previous size     |
+-------------------+----------------------------------------------------------------+
| @;\ *color*;      | Switches to another font color; @;; resets to previous color   |
+-------------------+----------------------------------------------------------------+
| @!                | Creates one composite character of the next two characters     |
+-------------------+----------------------------------------------------------------+
| @.                | Prints the degree symbol                                       |
+-------------------+----------------------------------------------------------------+
| @@                | Prints the @ sign itself                                       |
+-------------------+----------------------------------------------------------------+

Shorthand notation for a few special European characters has also been added (for others
you must use the full octal code):


.. _tbl-shorthand:

+----------+------------+----------+------------+
| *Code*   | *Effect*   | *Code*   | *Effect*   |
+==========+============+==========+============+
| @E       | Æ          | @e       | æ          |
+----------+------------+----------+------------+
| @O       | Ø          | @o       | ø          |
+----------+------------+----------+------------+
| @A       | Å          | @a       | å          |
+----------+------------+----------+------------+
| @C       | Ç          | @c       | ç          |
+----------+------------+----------+------------+
| @N       | Ñ          | @n       | ñ          |
+----------+------------+----------+------------+
| @U       | Ü          | @u       | ü          |
+----------+------------+----------+------------+
| @s       | ß          | @i       | í          |
+----------+------------+----------+------------+

However, if your input text contains UTF-8 code characters (e.g., ü, Î)
and you select the ISOLatin1+ character encoding then GMT will substitute
the correct PostScript octal codes for you automatically.

PostScript fonts used in GMT may be re-encoded to include several
accented characters used in many European languages. To access these,
you must specify the full octal code \\xxx allowed for
your choice of character encodings determined by the
:term:`PS_CHAR_ENCODING` setting described in the
:doc:`/gmt.conf` man page. Only the special
characters belonging to a particular encoding will be available. Many
characters not directly available by using single octal codes may be
constructed with the composite character mechanism @!.

Some examples of escape sequences and embedded octal codes in
GMT strings using the Standard+ encoding:

| ``2@~p@~r@+2@+h@-0@- E\363tv\363s`` = 2\ :math:`\pi r^2h_0` Eötvös
| ``10@+-3 @Angstr@om`` = 10\ :math:`^{-3}` Ångstrøm
| ``Stresses are @~s@~@+*@+@-xx@- MPa`` = Stresses are :math:`\sigma^{*}_{xx}` MPa
| ``Se@nor Gar@con`` = Señor Garçon
| ``M@!\305anoa Stra@se`` = Mānoa Straße
| ``A@#cceleration@# (ms@+-2@+)`` = ACCELERATION (ms\ :math:`^{-2}`)

The option in :doc:`/text` to draw a
rectangle surrounding the text will not work for strings with escape
sequences. A chart of characters and their octal codes is given in
Chapter :doc:`octal-codes`.

.. _GMT_Embellishments:

Plot embellishments
-------------------

Apart from visualizing your data sets, GMT maps can also be embellished in several ways.
The 9 embellishments currently available are

*  **Map scale** showing the true scale at some location(s) on the map.

*  **Directional rose** showing true north and other cardinal directions.

*  **Magnetic rose** showing magnetic north and declination deviations.

*  **Color bar** relating the colors of your image to the data values.

*  **Map legend** showing the meaning of the symbols on your map.

*  **Image overlay** of raster images or EPS figures (e.g., institutional logos, photos, etc.).

*  **GMT logo** overlay.

*  **Map inset** showing perhaps the location of your detailed area in a regional or global context.

*  **Vertical scale** showing the vertical scale of anomalies on a map.

Each of these features share a common system for specifying the location on the plot where the
feature will be placed.  They also share a common way for specifying the placement of a rectangular
panel behind the feature (to provide a uniform background, for instance).  Thus, before we discuss
the different features in more detail we will first review the "reference point/anchor point"
system used by GMT to specify such locations in relation to the underlying map, and then discuss
the background panel attribute settings.

.. _Reference_Points:

Reference and anchor point specification
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. figure:: /_images/GMT_anchor.*
   :width: 500 px
   :align: center

   The placement of a map feature (here represented by a green rectangle) in relation
   to the underlying map.  The nine named *reference* points (blue circles) on the map perimeter (and center)
   can be used to specify a location.  Using the same system of nine points on the map feature
   (cyan circles) we select one of these as our *anchor* point (here TL, indicated by the orange square).
   The anchor point can optionally be shifted away from the reference point by an amount *dx/dy* in the direction
   implied by the anchor point (in this case to the top and left), yielding the adjusted
   anchor point (red square).
   The feature is then placed such that its adjusted anchor point matches the reference point.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_anchor.txt

Placing a feature on the map means selecting a *reference* point somewhere on the map, an
*anchor* point somewhere on the feature, and then positioning the feature so that the two points overlap.
It may be helpful to consider the analog of a boat dropping an anchor: The boat navigates to the
reference point and then, depending on where on the boat the anchor is located, moves so that the
anchor connection point overlies the reference point, then drops the anchor.
There are four different ways to specify the reference point on a map, allowing for complete freedom
to select any location inside or outside the map.  The reference point syntax is [**g**\|\ **j**\|\ **J**\|\ **n**\|\ **x**]\ *refpoint*;
the five codes **g**\|\ **j**\|\ **J**\|\ **n**\|\ **x** refer to the five ways:

.. _Reference_Points_g:

#. [**g**] Specify *refpoint* using *data* coordinates, e.g., the longitude and latitude of the reference point.
   This mechanism is useful when you want to tie the location of the feature to an actual point
   best described by data coordinates.  An example of such a reference point might
   be **g**\ 135W/20N.

.. _Reference_Points_j:

#. [**j**] Specify *refpoint* using one of the nine *justification codes*, equivalent to the justification
   codes for placing text strings in :doc:`/text`.  This mechanism is illustrated in the figure above and
   is the preferred mechanism when you just want to place the feature **inside** the basemap at
   one of the corners or centered at one of the sides (or even smack in the middle).  Justification codes
   are a combination of a horizontal (**L**, **C**, **R**) and a vertical (**T**, **M**, **B**) code.
   An example of such a reference point might be **jTL**\ . When used, the anchor point on the map feature
   will default to the same justification, i.e., **TL** in this example.

#. [**J**] This is the same as **j** except it implies that the default anchor point is the mirror opposite of the
   justification code. Thus, when using **JTL**\, the anchor point on the map feature will default to **BR**.
   This is practical for features that are drawn **outside** of the basemap (like color bars often are).

.. _Reference_Points_x:

#. [**x**] Specify *refpoint* using *plot* coordinates, i.e., the distances in inches, centimeters, or
   points from the lower left plot origin.  This mechanism is preferred when you wish to lay out
   map features using familiar measurements of distance from origins. An example of such a reference
   point might be **x**\ 2.75i/2c.

.. _Reference_Points_n:

#. [**n**] Specify *refpoint* using *normalized* coordinates, i.e., fractional coordinates between 0
   and 1 in both the *x* and *y* directions.  This mechanism avoids units and is useful if you want to always
   place features at locations best referenced as fractions of the plot dimensions.
   An example of such a reference point might be **n**\ 0.2/0.1.

If no code is specified we default to **x**.

.. _Anchor_Point_j:

With the reference point taken care of, it is time to select the anchor point.
While the reference point selection gives unlimited flexibility to pick
any point inside or outside the map region, the anchor point selection is limited to the nine justification points
discussed for the **j** reference point code above.  Add **+j**\ *anchor* to indicate which justification
point of the map feature should be co-registered with the chosen reference point.  If an anchor point is not
specified then it defaults to the justification point set for the reference point (if **j**\ *code* was
used to set it), or to the mirror opposite of the reference point (if **J**\ *code* was used); with all other
specifications of the reference point, the anchor point takes on the default value of **MC** (for map rose and
map scale) or **BL** (all other map features). Adding **+j**\ *anchor* overrules those defaults.
For instance, **+jTR**\  would select the top right point on the map feature as the anchor.

.. _Anchor_Point_o:

It is likely that you will wish to offset the anchor point away from
your selection by some arbitrary amount, particularly if the reference point is specified with **j**\|\ **J**\ *code*.
Do so with  **+o**\ *dx*\ [/*dy*], where *dy* equals *dx* if it is not provided.
These increments are added to the projected plot coordinates of the anchor point, with
positive values moving the reference point in the same direction as the 2-character code of the anchor point implies.
Finally, the adjusted anchor point is matched with the reference point.

Take for example an anchor point on the top left of the map feature, either by using a reference point **jTL**\ , or **JBR**\ ,
or explicitly setting **+j**\ TL.
Then **+o**\ 2c/1c will move the anchor point 2 cm left and 1 cm above the top left corner of the map feature.
In other words, the top left corner of the map feature will end up 2 cm to the right and 1 cm below the selected reference point.

Similarly, **+jBR** will align the bottom right corner of the map feature, and **+o**\ 2c/1c will offset it 2 cm to the left
and 1 cm up. When using middle (**M**) or center (**C**) justifications, to offset works the same way as bottom (**B**) or left (**L**),
respectively, i.e., moving the map feature up or to the right.

.. _Background-panel:

The background panel
~~~~~~~~~~~~~~~~~~~~

For most maps you will wish to place a background panel of uniform color behind
any of the map features you plan to add.  Because the panel is linked to the map feature
you have selected, the parameters such as location and dimensions are handled automatically.
What remains is to specify the *attributes* of the panel.  Typically, panels settings are
given via a module's **-F** option by appending one or more modifiers.  Here is a list of
the attributes that are under your control:

#. Color or pattern.  You specify the fill you want with **+g**\ *fill* [Default is no fill].
   For instance, paint the panel yellow with **+g**\ yellow.

#. Panel frame pen.  Turn on the frame outline with **+p**, using the pen defined via
   :term:`MAP_FRAME_PEN`.  You may override this choice with **+p**\ *pen*
   [Default is no outline].  A very bold red outline might look like **+p**\ thick,red.

#. Rounded versus straight rectangle.  By specifying a corner radius with **+r**\ *radius*
   you can round the corners [Default is no rounding]. Here is a 0.5-cm radius rounding:
   **+r**\ 0.5c.

#. Inner frame.  A secondary, inner frame outline may be added as well with the modifier
   **+i**\ [[*gap*/]\ *pen*].  The default pen is given by :term:`MAP_DEFAULT_PEN`,
   with a default *gap* between the outer and inner frames of 2 points.  Add arguments to override
   these defaults, such as **+i**\ 0.1c/thin,dashed to get a thin, dashed inner frame offset by
   0.1 cm from the main (outer) frame.

#. Panel clearance.  The panel's dimensions are automatically determined from knowledge of
   its contents.  However, it is sometimes required to add some extra clearance around most or
   all sides, and you can do so with **+c**\ [*clearance*], with a 4-point clearance being
   the default.  Add one (uniform), two (different horizontal and vertical clearances), or
   four (separate for sides west, east, south, and north) clearances, separated by slashes.  For instance, to add
   a 1 cm clearance in x and 5 points in y, use **+c**\ 1c/5p.

#. Drop-down shadow.  Append **+s** to simulate a gray shadow cast toward the southeast.
   You may append [*dx*/*dy*/][*shade*] to change the shade color and the offset of the
   shade [Default is 4p/-4p/gray50].  If happy with the placement but desiring a dark blue
   shadow, add **+s**\ darkblue.

.. figure:: /_images/GMT_panel.*
   :width: 400 px
   :align: center

   A map panel is a rectangular background placed behind any of the map features.  It has
   several attributes that can be changed with panel option modifiers.  The light green rounded
   rectangle was specified with **-F+g**\ lightgreen\ **+r**, while the white panel on the
   lower right was set with **-F+p**\ 1p\ **+i+s+g**\ white\ **+c**\ 0.1i (we added a light
   dashed box to indicate the effect of the clearance setting).

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_panel.txt

.. _Placing-map-scales:

Placing map scales
~~~~~~~~~~~~~~~~~~

Traditionally, a map scale is added to maps for helping the reader understand the particular scale
used for this map, i.e., it portrays the relationship between actual distances on the Earth
(in km, miles, meters, etc.) and distances on the map (in cm, inches, points).  Depending on
the map projection the map scale will vary continuously but may be constant along a line of
latitude (e.g., Mercator projection).  Thus, in placing the map scale on the map there are
two locations involved: (1) The *reference* point where the map scale's *anchor* should be
pinned, and (2) the *projection* point where the scale is computed and thus where the map
scale is true.  Map scales can be plotted by :doc:`/basemap` or :doc:`/coast`, and in
addition to the required *refpoint* and anchor arguments specifying where the scale should be placed there
are both required and optional modifiers.  These are given via these modules' **-L** option.
Here is a list of the attributes that is under your control:

#. Scale bar length.  Required modifier is given with **+w**\ *length*, where
   *unit* is one of the recognized distance units.  An example might be **+w**\ 250n for
   a bar representing 250 nautical miles at the map scale origin.

#. Map scale origin.  Required modifier given with **+c**\ [*slon*/]\ *slat*, where the longitude
   of the scale origin is optional for projections with constant scale along parallels.  For
   a Mercator projection it may look like **+c**\ 30N while an oblique projection may need **+c**\ 100W/23N,
   for instance.

#. Fancy scale bar.  By default a plain-looking scale bar is plotted.  For a free upgrade to a fancier bar,
   append **+f**.  The fancier bar is, well, a bit fancier.

#. Scale label. Turn on scale labels with **+l**.  By default, the scale label is initialized to
   equal the distance unit name.  Use the **+l**\ *label* argument to supply your own scale label,
   such as **+l**\ "Distances at Equator".

#. Scale label alignment.  The default alignment is on top of the bar [**+at**], but you can change
   this by selecting another alignment by appending them to the **+a** modifier, including
   **b**\ ottom, **l**\ eft, or **r**\ ight.  Here, **+ab** would align on the bottom of the scale.

#. Append distance unit.  For the fancy scale, adding **+u** will append the distance unit specified
   with **+w** to all distance annotations along the bar, while for the plain scale it will replace
   the default scale label with the unit abbreviation.

.. figure:: /_images/GMT_mapscale.*
   :width: 500 px
   :align: center

   Example of two map scales for a Mercator projection evaluated at 53 degrees north.
   The left-most scale was placed with **-Lj**\ *ML*\ **+c**\ 53\ **+w**\ 1000k\ **+f+l**\ "Scale at 53@.N"
   while the scale on the right was placed with **-Lj**\ *BR*\ **+c**\ 53\ **+w**\ 1000k\ **+l+f**.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_mapscale.txt

Note that for the purpose of anchor justification (**+j**) the footprint of the map scale is
considered the rectangle that contains the scale and all selected labels and annotations, i.e.,
the map scale's *bounding box*.

.. _Placing-dir-map-roses:

Placing directional map roses
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Map roses showing the cardinal directions of a map help the reader orient themselves, especially
for oblique projections where north-south is not vertically aligned.  However, these roses also
have ornamental value and can be used on any map projection.  As for map scales, a directional
map rose is added with :doc:`/basemap` or :doc:`/coast` and selected by the **-Td** option.
This option accepts the *reference* point where the map rose's *anchor* should be
pinned.  In addition to the required *refpoint* and *anchor* arguments (and their standard
modifiers discussed earlier) there are three optional modifiers:

#. Size of map rose.  Use **+w**\ *size* to specify the full width and height of the rose. E.g., a 3 cm
   rose would require **+w**\ 3c.  Alternatively, append % to set the *size* as a percentage of the
   map width [Default is 10% if **+w** is not given].

#. Cardinal points.  By default only the four cardinal points (W, E, S, N) are included in the rose.
   You can extend that with the **+f**\ *level* modifier, where *level* is 1 [Default], 2, or 3.  Selecting
   2 will include the two intermediate orientations NW-SE and NE-SW, while 3 adds the four additional
   orientations WNW-ESE, NNW-SSE, NNE-SSW, and ENE-WSW.

#. Add labels.  Do so with **+l**,  which places the current one-letter codes for west, east, south,
   and north at the four cardinal points.  These letters depend on the setting of :term:`GMT_LANGUAGE`
   and for the default English we use W, E, S, N, respectively.  You can replace these labels with four custom
   labels via  **+l**\ *w,e,s,n*, i.e., four comma-separated labels in the specified order.  You can exclude any
   of the cardinal points from being labeled by giving no label in the corresponding order.  E.g., **+l**",,Down,Up"
   would write Down and Up at the south and north cardinal point, respectively.  Note that for the plain
   directional rose only the north annotation will be placed.

.. figure:: /_images/GMT_dir_rose.*
   :width: 500 px
   :align: center

   Plain and fancy directional map roses. (left) Bare-bones plain rose showing arrow towards north
   and a cross indicating the cardinal directions, specified by **-Tdg**\ 0/0\ **+w**\ 1i. (middle) Fancy rose
   obtained by adding **+f** and **+l**\ ,,,N to get the north label.  (right) Fancy directional rose
   at level 3 with labels by adding **+f**\ 3\ **+l**.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_dir_rose.txt

.. _Placing-mag-map-roses:

Placing magnetic map roses
~~~~~~~~~~~~~~~~~~~~~~~~~~

Map roses showing the magnetic directions of a map are useful when magnetic data are presented,
or when declinations are significantly nonzero.  However, as for directional roses the magnetic rose
also has ornamental value.  The magnetic rose consists of two concentric angular scales: The first
(outer) ring shows directional angles while the second (inner) ring is optional and portrays the
magnetic directions, which differ for nonzero declination. As for style, the two-ring rose looks a
bit like a standard compass.  As for directional roses, a magnetic
map rose is added with :doc:`/basemap` or :doc:`/coast` and selected by the **-Tm** option.
As for other features, append the required *reference* point where the magnetic map rose's *anchor*
should be pinned.  There are several optional modifiers:

#. Specify size of map rose.  Use **+w**\ *size* to specify the full width of the rose.  E.g., a 3 cm
   rose would imply **+w**\ 3c. Alternatively, append % to set the *size* as a percentage of the map
   width [Default is 15% if **+w** is not given].

#. Specify Declination.  To add the inner angular scale, append **d**\ *dec*\ [/\ *dlabel*], where
   *dec* is the declination value in decimal or ddd:mm:ss format, and *dlabel* is an optional string
   that replaces the default label (which is "d = *dec*", with d being a Greek delta and we format
   the specified declination).  Append **d**\ *dec*/- to indicate you do not want any declination label.
   As an example, consider **d**\ 11/"Honolulu declination".

#. Draw the secondary (outer) ring outline.  Normally it is not drawn, but you can change that by appending
   **+p**\ *pen*.  For instance, adding **+p**\ thin will draw the ring with the selected thin pen.

#. Add labels.  As for directional roses you do so with **+l**, which places the current one-letter codes for west, east, south,
   and north at the four cardinal points.  These letters depend on the setting of :term:`GMT_LANGUAGE`
   and for the default English we use W, E, S, N, respectively.  You can replace these labels with four custom
   labels via  **+l**\ *w,e,s,n*, i.e., four comma-separated labels in the specified order.  You can exclude any
   of the cardinal points from being labeled by giving no label in the corresponding order.  E.g., **+l**",,Down,Up"
   would write Down and Up at the south and north cardinal point, respectively.

#. Draw the primary (inner) ring outline.  It is also not normally drawn; change that by appending
   **+i**\ *pen*.  For instance, adding **+i**\ thin,blue will draw the ring with the selected thin, blue pen.

#. Set annotation, tick and grid intervals.  Each ring has a default annotation [30], tick [5], and grid [1]
   interval (although here "grid interval" is just a finer tick interval drawn at half tickmark length).
   Adjust these three intervals with **+t**\ *intervals*.  If you selected **+d** then you must supply
   two sets of such intervals (i.e., 6 comma-separated values), where the first (primary) set refers to
   the declination-adjusted ring and the second (secondary) set refers to the directional (outer) ring.
   If only three intervals are given then we assume you want the same intervals for both rings.  As an example,
   to annotate every 90 degrees and tick every 15 and 5 degrees, add **+t**\ 90/15/5.

.. figure:: /_images/GMT_mag_rose.*
   :width: 600 px
   :align: center

   Magnetic direction map rose. This symbol is quite complicated and has many items whose attributes are
   in part controlled by GMT defaults parameters and in part by the above modifiers.  The color-coded legend
   indicates which parameters controls the font, pen, or color of the correspond item of the rose.  This rose
   was specified by **-Tmg**\ -2/0.5\ **+w**\ 2.5i\ **+d**\ -14.5\ **+t**\ 45/10/5\ **+i**\ 0.25p,blue\ **+p**\ 0.25p,red\ **+l+j**\ CM.
   See :doc:`/gmt.conf` for more details on the default parameters.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_mag_rose.txt

Placing color scale bars
~~~~~~~~~~~~~~~~~~~~~~~~

Color scale bars are used in conjunction with color-coded surfaces, symbols, lines, or even text, to
relate the chosen color to a data value or category.  For instance, color images of topography
or other gridded data will need a mechanism for users to decode what the colors represent.  Typically, we do this
by adding a color scale bar on the outside (or inside) of the map boundaries.  The module
:doc:`/colorbar` places the color scale bar, with location and size determined by the **-D** attributes.
As for other map features we must specify the reference and anchor points and any adjustments to them, then
supply suitable required and optional modifiers:

#. Give dimensions of color bar.  Use **+w**\ *length*/*width* to specify the full width and height of the bar.
   For instance, a 10 cm long bar of height 0.5 cm would imply **+w**\ 10c/0.5c.

#. Set orientation of color bar.  By default, we place a vertically aligned bar.  Select a horizontal bar by
   adding **+h**.

#. Specify color bar label alignment.  By default we place the chosen annotations, scale (i.e., x-axis) label
   and unit (i.e., y-axis) label on the opposite side of the color scale bar anchor point.  Change this
   with **+m** and append any combination of **a**, **l**, or **u** to flip the annotations or labels
   to the opposite side.  Append **c** to plot vertical labels as column text (this cannot be used with
   **+h**, obviously).

#. Extend the color bar.  You can use the **+e** modifier to add sidebar triangles for displaying the
   current back- and foreground colors.  Append **b** (background) or **f** (foreground) to get the implied side
   only [Default is both].  Optionally, append triangle height [Default is half the bar *width*].

#. Add missing data key.  Append **+n** to draw a rectangle with the current NaN color and label it NaN.
   Optionally, append a replacement *text*.  One example might be **+n**\ "No data".

.. figure:: /_images/GMT_colorbar.*
   :width: 500 px
   :align: center

   Color bar placed beneath a map (here truncated).  We extended the bar to show background and foreground
   colors, and used the frame-annotation machinery to add labels.  The bar was placed with
   **-D**\ *JBC*\ **+e**.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_colorbar.txt

Placing map legends
~~~~~~~~~~~~~~~~~~~

Adding map legends is the standard way to communicate what various symbols placed on your map
represent.  For instance, you may use this mechanism to convey the information that circles are
earthquake locations, triangles are places where you ate Thai food, and dashed lines indicate
some sort of gang-land demarcation line that you should not cross without paying the locals due respect.
Map legends are placed by the module :doc:`/legend`, with location and size determined by the
various **-D** attributes. We must again specify the reference and anchor points and any adjustments to them
first, then supply suitable required and optional modifiers:

#. Give legend dimensions.  You must specify the required legend width, while legend height is optional
   and if not given is computed based on the contents of the legend.  The syntax is therefore
   **+w**\ *width*\ [/*height*] in your desired plot units.  Thus, **+w**\ 12c sets the legend width
   as 12 cm but the height will become whatever is needed to contain the information.

#. Set line-spacing.  You may optionally specify the line-spacing used for the setting of the legend.  The legend will
   typically consist of several lines that may or may not contain text, but the spacing between
   these lines are controlled by the chosen line-spacing factor times the current primary annotation
   font setting, i.e., :term:`FONT_ANNOT_PRIMARY`.  The default line spacing factor
   is 1.1; change this with **+l**\ *linefactor*.

.. figure:: /_images/GMT_legend.*
   :width: 500 px
   :align: center

   Example of a map legend placed with :doc:`/legend`.  Apart from the placement and dimensions discussed
   here, :doc:`/legend` reads macro commands that specifies each item of the legend, including colors,
   widths of columns, the number of columns, and presents a broad selection of items.  Here, we
   simply used **-Dx**\ 0/0\ **+w**\ 14c\ **+j**\ *BL*.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_legend.txt

Placing raster and EPS images on maps
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When preparing posters for meetings one will often need to include the organization's logo,
which may be available to you as an Encapsulated PostScript File (EPS) or as a raster image,
such as PNG or JPG.  At other times, you may wish to place photos or other raster images on
your map.  The module :doc:`/image` can help with this, and like the other map feature
placements it requires a reference point and its optional adjustments via the **-D** option.
In addition, we require one (of two) modifiers to determine the image size.

#. Specify image width.  This is a required modifier and is set via **+w**\ *width*\ [/*height*].
   If *height* is specified as 0 then we compute the height from *width* and the aspect
   ratio of the image, for instance **+w**\ 4c/0.  If *width* is negative the we use its absolute value as width
   but interpolate the image in PostScript to the device resolution.

#. Specify image resolution.  For raster images (not EPS) you may instead specify the size of the
   plotted image by specifying its resolution in dots per inch, via **+r**\ *dpi*.  The
   actual size of the images is then controlled by its number of pixels times the *dpi*.

#. Enable image replication.  For raster images (not EPS) you may optionally append **+n**\ *nx*\ [/*ny*]
   to indicate that you want the source image to be replicated that many times in the two
   directions, resulting in a tiling of the map using the selected image.  This may be useful
   in conjunction with an active clip path set by :doc:`/clip`.

.. figure:: /_images/GMT_images.*
   :width: 500 px
   :align: center

   Placement of EPS and raster images. (left) The US National Science Foundation (NSF) has
   generously funded the development of GMT and their JPG logo is reproduced here via
   **-Dj**\ *ML*\ **+w**\ 1.5i\ **+o**\ 0.1i. (right)
   The School of Ocean and Earth Science and Technology at the University of Hawaii at Manoa
   hosts the gmt server and its EPS logo is shown via **-Dj**\ *MR*\ **+o**\ 0.1i\ **+w**\ 2i.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_images.txt

Placing a GMT logo on maps
~~~~~~~~~~~~~~~~~~~~~~~~~~

It is possible to overlay the GMT logo on maps as well, using the module :doc:`/gmtlogo`.
Like other features it requires reference and anchor points and their optional adjustments via the **-D** option.
In addition, we require one modifier to set the logo's size.

#. Specify logo width.  This is a required modifier and is set via **+w**\ *width*.
   The height is automatically set (it is half the width).  To place a 5 cm wide
   GMT logo, append **+w**\ 5c.

.. figure:: /_images/GMT_coverlogo.*
   :width: 300 px
   :align: center

   Placement of the GMT logo. The logo itself only has a size modifier but the :doc:`/gmtlogo`
   module allows additional attributes such as a background map panel.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_coverlogo.txt

Placing map insets
~~~~~~~~~~~~~~~~~~

Our penultimate map embellishment is the map inset.
A map inset may appear to be the easiest feature to add since it only consists of an empty map panel.
What you put in this panel is up to you (and we will show some examples).  However, unlike
the other map features there are two ways to specify the placement of the map inset.
The first is the standard way of specifying the reference and anchor points and the inset dimensions,
while the second specifies a *subregion* in the current plot that should be designated the
map inset area.  Depending on the map projection this may or may not be a rectangular area.
Map insets are produced by the module :doc:`/inset` and located via the **-D** option. Unless you
use the reference point approach you must first append *xmin*/*xmax*/*ymin*/*ymax*\ [**+r**][**+u**\ *unit*],
where the optional *unit* modifier **+u** indicates that the four coordinates to follow are projected
distances (e.g., km, miles).  If the unit modifier is missing then we assume the coordinates are
map coordinates (e.g., geographic *west*, *east*, *south*, and *north*).  For oblique
projections you may wish to specify the domain using the lower-left and upper-right coordinates
instead (similar to how the **-R** option works), by adding **+r**\ .  Some optional modifiers are available:

#. Set inset size.  If you specified a reference point then you must also specify the inset dimensions with the
   **+w**\ *width*\ [/*height*], where *height* defaults to *width* if not given.
   Append the unit of the dimensions, which may be distance units such as km, feet, etc., and
   the map projection will be used to determine inset dimensions on the map.  For instance,
   **+w**\ 300k/200k is a 300x200 km region (which depends on the projection) while **+w**\ 5c
   is a 5cm square box.

#. Save the location and dimensions.  For all but the simplest of map insets you will need to
   know the exact location of the resulting inset and its dimensions.  For instance, if you
   specified the inset using the **TR** anchor point and a width and height of 100 km you will need to
   know what this means in terms of positions on the map in plot units.  In terms of the modifiers
   this would be **jTR**\ **+w**\ 100k.  See the figure caption for an example.

.. figure:: /_images/GMT_inset.*
   :width: 500 px
   :align: center

   Demonstration of how a map inset may be used to place a global overview map as an inset in a
   regional map.  Main map shows the regional area of Australia.  We place an inset in the upper
   right area with **-Dj**\ TR\ **+w**\ 3.8c\ **+o**\ 0.4c/0.25c.
   See Example :ref:`example_44` for more details.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_inset.txt

Placing a vertical scale on maps
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Our final embellishment is reserved for wiggles plotted along track with :doc:`/wiggle` and
is activated as an option within that module.
Like other features, it requires reference and anchor points and their optional adjustments via the **-D** option.
In addition, we offer a few modifier to set the scale bar's remaining attributes:

#. Specify vertical scale bar length.  This is a required modifier and is set via **+l**\ *length*.
   The length is given in the data (*z*) units of your plot.  To indicate that your vertical scale bar
   should reflect 100 nTesla, append **+l**\ 100.  The actual dimension of the scale bar on your map
   depends on the data scale set in :doc:`/wiggle` via **-Z**.

#. Place the label on the left side of the vertical scale bar.  This is an optional modifier and is set via **+m**.
   By default, the scale bar has open ``teeth`` pointing right and a label on that side. The **m** moves the
   label to the left and reverses the teeth direction as well.

#. Add a unit to the vertical scale bar label.  This is an optional modifier and is set via **+u**\ *unit*.
   To append nT (nTesla) to the label you would specify **+u**\ nT.

.. figure:: /_images/GMT_vertscale.*
   :width: 600 px
   :align: center

   Placement of a vertical scale bar. As for other embellishments the :doc:`/wiggle`
   module allows additional attributes such as a background map panel.

.. toggle::

   Here is the source script for the figure above:

   .. literalinclude:: /_verbatim/GMT_vertscale.txt

.. _grid-file-format:

Grid file format specifications
-------------------------------

GMT has the ability to read and write grids using more than one grid file format
(see Table :ref:`grdformats <tbl-grdformats>` for supported format and their IDs).
For reading, GMT will automatically determine the format of grid files, while for
writing you will normally have to append *=ID* to the filename if you want GMT to
use a different format than the default. The automatic reading procedure follows an heuristic
where certain formats are tentatively decoded with GMT internal drivers and if they fail than
we resort to use the GDAL library to do the readings. This normally works pretty well but in case
of failure (e.g. a GMT driver failed to read binary file with a separate header that also could
have been stored in an ASCII file with embed header) the user should explicitly try to force a
reading via GDAL. That is, to append a *=gd* suffix to file name.

By default, GMT will create new grid files using the **nf** format;
however, this behavior can be overridden by setting the
:term:`IO_GRIDFILE_FORMAT` defaults parameter to any of the other
recognized values (or by appending *=ID*).

GMT can also read netCDF grid files produced by other software
packages, provided the grid files satisfy the COARDS and Hadley Centre
conventions for netCDF grids. Thus, products created under those
conventions (provided the grid is 2-, 3-, 4-, or 5-dimensional) can be
read directly by GMT and the netCDF grids written by GMT can be read
by other programs that conform to those conventions. Three such programs are
`ncview <https://cirrus.ucsd.edu/~pierce/software/ncview/index.html>`_, and `Panoply
<http://www.giss.nasa.gov/tools/panoply/>`_; others can be found on the
`netCDF website <http://www.unidata.ucar.edu/software/netcdf/software.html>`_.
Note that although many additional programs can read netCDF files, some are unable
to read netCDF 4 files (if data compression has been applied).

In addition, users with some C-programming experience may add their own
read/write functions and link them with the GMT library to extend the
number of predefined formats. Technical information on this topic can be
found in the source file ``gmt_customio.c``. Users who are considering this
approach should contact the GMT team for guidance.

.. _tbl-grdformats:

+----------+---------------------------------------------------------------+
| **ID**   | **Explanation**                                               |
+==========+===============================================================+
|          | *GMT 4 netCDF standard formats*                               |
+----------+---------------------------------------------------------------+
| nb       | GMT netCDF format (8-bit integer, COARDS, CF-1.7)             |
+----------+---------------------------------------------------------------+
| ns       | GMT netCDF format (16-bit integer, COARDS, CF-1.7)            |
+----------+---------------------------------------------------------------+
| ni       | GMT netCDF format (32-bit integer, COARDS, CF-1.7)            |
+----------+---------------------------------------------------------------+
| nf       | GMT netCDF format (32-bit float, COARDS, CF-1.7)              |
+----------+---------------------------------------------------------------+
| nd       | GMT netCDF format (64-bit float, COARDS, CF-1.7)              |
+----------+---------------------------------------------------------------+
|          | *GMT 3 netCDF legacy formats*                                 |
+----------+---------------------------------------------------------------+
| cb       | GMT netCDF format (8-bit integer, depreciated)                |
+----------+---------------------------------------------------------------+
| cs       | GMT netCDF format (16-bit integer, depreciated)               |
+----------+---------------------------------------------------------------+
| ci       | GMT netCDF format (32-bit integer, depreciated)               |
+----------+---------------------------------------------------------------+
| cf       | GMT netCDF format (32-bit float, depreciated)                 |
+----------+---------------------------------------------------------------+
| cd       | GMT netCDF format (64-bit float, depreciated)                 |
+----------+---------------------------------------------------------------+
|          | *GMT native binary formats*                                   |
+----------+---------------------------------------------------------------+
| bm       | GMT native, C-binary format (bit-mask)                        |
+----------+---------------------------------------------------------------+
| bb       | GMT native, C-binary format (8-bit integer)                   |
+----------+---------------------------------------------------------------+
| bs       | GMT native, C-binary format (16-bit integer)                  |
+----------+---------------------------------------------------------------+
| bi       | GMT native, C-binary format (32-bit integer)                  |
+----------+---------------------------------------------------------------+
| bf       | GMT native, C-binary format (32-bit float)                    |
+----------+---------------------------------------------------------------+
| bd       | GMT native, C-binary format (64-bit float)                    |
+----------+---------------------------------------------------------------+
|          | *Miscellaneous grid formats*                                  |
+----------+---------------------------------------------------------------+
| rb       | SUN raster file format (8-bit standard)                       |
+----------+---------------------------------------------------------------+
| rf       | GEODAS grid format GRD98 (NCEI)                               |
+----------+---------------------------------------------------------------+
| sf       | Golden Software Surfer format 6 (32-bit float)                |
+----------+---------------------------------------------------------------+
| sd       | Golden Software Surfer format 7 (64-bit float)                |
+----------+---------------------------------------------------------------+
| af       | Atlantic Geoscience Center AGC (32-bit float)                 |
+----------+---------------------------------------------------------------+
| ei       | ESRI Arc/Info ASCII Grid Interchange format (ASCII integer)   |
+----------+---------------------------------------------------------------+
| ef       | ESRI Arc/Info ASCII Grid Interchange format (ASCII float)     |
+----------+---------------------------------------------------------------+
| gd       | Import/export via GDAL                                        |
+----------+---------------------------------------------------------------+

Because some formats have limitations on the range of values they can
store it is sometimes necessary to provide more than simply the name of
the file and its ID on the command line. For instance, a native short
integer file may use a unique value to signify an empty node or NaN, and
the data may need translation and scaling prior to use. Therefore, all
GMT programs that read or write grid files will decode the given
filename as follows:

name[=\ *ID*][**+d**\ *divisor*][**+n**\ *invalid*][**+o**\ *offset*][**+s**\ *scale*]

where anything in brackets is optional. If you are reading a grid then
no *ID* is needed: just continue to pass the name of the grid file.
However, if you write another format than the default netCDF you must append
the =\ *ID* string, where *ID* is the format code listed above. In addition,
should you want to (1) multiply the data by a *scale* factor (or alternatively
divide the data by a *divisor*), and (2) add a constant offset you must append
the **+s**\ *scale* (or **+d**\ *divisor*) and **+o**\ *offset* modifiers.
Finally, if you need to indicate that a certain data value should be interpreted
as a NaN (not-a-number) you must append **+n**\ *invalid* modifier to file name.
For output, you may specify scale as *a* for auto-adjusting the scale and/or offset of
packed integer grids (=\ *ID*\ **+s**\ *a* is a shorthand for
=\ *ID*\ **+s**\ *a*\ **+o**\ *a*).

**Note**: Users are allowed to name their grid files anything they want.  However,
if you tend to use what *could* look like modifier-sequences to GMT (e.g., using
filenames like data.grid+o4) you can prevent any confusion by using either the
GMT-recommended ".grd" or ".nc" as grid file extensions (e.g., data.my+o4.grd).
Since valid modifiers are *appended* to a file name, finding such an extension simplifies parsing.

Note that the GMT netCDF and native binary grids store the grid scale and offset
in the file, hence if you specify these attributes when writing a file then upon reading the grid
these settings will automatically take effect.  You can override them by supplying different scales
and offsets, of course.  For the  grid formats that do not store these attributes
you will need to supply them both when reading and writing.

Some of the grid formats allow writing to standard output and reading
from standard input which means you can connect GMT programs that
operate on grid files with pipes, thereby speeding up execution and
eliminating the need for large, intermediate grid files. You specify
standard input/output by leaving out the filename entirely. That means
the "filename" will begin with "=\ *ID*". Note that the netCDF format
does not allow piping.

Everything looks clearer after a few examples:

*  To write a native binary float grid file, specify the name as ``my_file.f4=bf``.

*  To read a native short integer grid file, multiply the data by 10 and
   then add 32000, but first let values that equal 32767 be set to NaN,
   use the filename ``my_file.i2=bs+s10+o32000+n32767``.

*  To read a Golden Software "surfer" format 6 grid file, just pass the
   file name, e.g., ``my_surferfile.grd``.

*  To read a 8-bit standard Sun raster file (with values in the 0–255
   range) and convert it to a 1 range, give the name as ``rasterfile+s7.84313725e-3+o-1``
   (i.e., 1/127.5).

*  To write a native binary short integer grid file to standard output
   after subtracting 32000 and dividing its values by 10, give filename
   as ``=bs+s0.1+o-3200``.

*  To write an 8-bit integer netCDF grid file with an auto-adjusted
   offset, give filename as ``=nb+oa``.

*  To read a short integer *.bil* grid file stored in binary and and force
   the reading via GDAL, add suffix *=gd* as in ``n45_e008_1arc_v3.bil=gd``.

*  To write a lossless, deflate compressed, and tiled GeoTIFF grid (or image) use,
   ``output.tif=gd:GTiff+cTILED=YES+cCOMPRESS=DEFLATE+cPREDICTOR=3``.
   See also :ref:`Writing grids and images <Write-grids-images>` as well as available options
   for each output format from the GDAL driver documentation,
   `for example <https://gdal.org/drivers/raster/gtiff.html>`_.

Programs that both read and/or write more than one grid file may specify
different formats and/or scaling for the files involved. The only
restriction with the embedded grid specification mechanism is that no
grid files may actually use the "=" character as part of their name
(presumably, a small sacrifice).

One can also define special file suffixes to imply a specific file
format; this approach represents a more intuitive and user-friendly way
to specify the various file formats. The user may create a file called
``gmt.io`` in the current directory or home directory, or in the directory
``~/.gmt`` and define any number of custom formats. The following is an example of
a ``gmt.io`` file:

+---------------------------------------------------------------------------+
| # GMT i/o shorthand file                                                  |
|                                                                           |
| # It can have any number of comment lines like this one anywhere          |
| # suffix format_id scale offset NaN Comments                              |
+-------+-----+-----+---+-------+-------------------------------------------+
| grd   | nf  | \-  | \-| \-    | Default format                            |
+-------+-----+-----+---+-------+-------------------------------------------+
| b     | bf  | \-  | \-| \-    | Native binary floats                      |
+-------+-----+-----+---+-------+-------------------------------------------+
| i2    | bs  | \-  | \-| 32767 | 2-byte integers with NaN value            |
+-------+-----+-----+---+-------+-------------------------------------------+
| ras   | rb  | \-  | \-| \-    | Sun raster files                          |
+-------+-----+-----+---+-------+-------------------------------------------+
| byte  | bb  | \-  | \-| 255   | Native binary 1-byte grids                |
+-------+-----+-----+---+-------+-------------------------------------------+
| bit   | bm  | \-  | \-| \-    | Native binary 0 or 1 grids                |
+-------+-----+-----+---+-------+-------------------------------------------+
| mask  | bm  | \-  | \-| 0     | Native binary 1 or NaN masks              |
+-------+-----+-----+---+-------+-------------------------------------------+
| faa   | bs  | 0.1 | \-| 32767 | Native binary gravity in 0.1 mGal         |
+-------+-----+-----+---+-------+-------------------------------------------+
| ns    | ns  | a   | a | \-    | 16-bit integer netCDF grid with           |
|       |     |     |   |       | auto-scale and auto-offset                |
+-------+-----+-----+---+-------+-------------------------------------------+

These suffixes can be anything that makes sense to the user. To activate
this mechanism, set parameter :term:`IO_GRIDFILE_SHORTHAND` to TRUE in
your :doc:`/gmt.conf` file. Then, using the filename ``stuff.i2`` is equivalent to saying ``stuff.i2=bs+n32767``, and the
filename ``wet.mask`` means wet.mask=bm+n0. For a file intended for masking, i.e.,
the nodes are either 1 or NaN, the bit or mask format file may be as
small as 1/32 the size of the corresponding grid float format file.

Modifiers for changing units of grid coordinates
------------------------------------------------

A few GMT tools require that the two horizontal dimensions be
specified in meters. One example is
:doc:`/grdfft` which must compute the 2-D
Fourier transform of a grid and evaluate wave numbers in the proper units
(1/meter). There are two situations where the user may need to change
the coordinates of the grid passed to such programs:

-  You have a geographic grid (i.e., in longitude and latitude). Simply
   supply the **-fg** option and your grid coordinates will
   automatically be converted to meters via a "Flat Earth" approximation
   on the currently selected ellipsoid (**Note**: This is only possible in
   those few programs that require this capability. In general, **-fg**
   is used to specify table coordinates).

-  You have a Cartesian grid but the units are not meters (e.g., they
   may perhaps be in km or miles). In this case you may append the file
   modifier **+u**\ *unit*, where *unit* is one of non-angular units listed
   in Table :ref:`distunits <tbl-distunits>`. For example, reading in the grid (which has
   distance units of km) and converting distances to meters is done by
   specifying the filename as *filename*\ **+u**\ k. On output, any derived grids will revert
   to their original units *unless* you specify another unit modifier to
   the output grid. This may be used, for instance, to save the original
   grid with distances in meters using some other unit.

For convenience, we also support the inverse translation, i.e.,
**+U**\ *unit*. This modifier can be used to convert your grid
coordinates *from* meters *to* the specified unit. Example :ref:`example_28` shows a
case where this is being used to change an UTM grid in meters to km.
These modifiers are only allowed when map projections are not selected
(or are Cartesian).

.. _modifiers-for-CF:

Modifiers for COARDS-compliant netCDF files
-------------------------------------------

When the netCDF grid file contains more than one 2-dimensional variable,
GMT programs will load the first such variable in the file and ignore
all others. Alternatively, the user can select the required variable by
adding the suffix "?\ *varname*" to the grid file name. For example, to
get information on the variable "slp" in file , use:

   ::

    gmt grdinfo "file.nc?slp"

Since COARDS-compliant netCDF files are the default, the additional
suffix "=nf" can be omitted.

If there are no 2-dimensional variables and no specific variable was
selected, we default to the first higher-dimensional matrix and select
the first layer.

In case the named grid is 3-dimensional, GMT will load the first
(bottom) layer. If another layer is required, either add "[*index*]"
or "(*level*)", where *index* is the index of the third (depth) variable
(starting at 0 for the first layer) and *level* is the numerical value
of the third (depth) variable associated with the requested layer. To
indicate the second layer of the 3-D variable "slp" use as file name: ``file.nc?slp[1]``.

When you supply the numerical value for the third variable using
"(*level*)", GMT will pick the layer closest to that value. No
interpolation is performed (for such interpolations, see :doc:`/grdinterpolate`).

Note that the question mark, brackets and parentheses have special
meanings on Unix-based platforms. Therefore, you will need to either
*escape* these characters, by placing a backslash in front of them, or
place the whole file name plus modifiers between single quotes or double
quotes.

A similar approach is followed for loading 4-dimensional grids. Consider
a 4-dimensional grid with the following variables:

   ::

    lat(lat): 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
    lon(lon): 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
    depth(depth): 0, 10, 20, 30, 40, 50, 60, 70, 80, 90
    time(time): 0, 12, 24, 36, 48
    pressure(time,depth,lat,lon): (5000 values)

To get information on the 10x10 grid of pressure at
depth 10 and at time 24, one would use:

   ::

    gmt grdinfo "file.nc?pressure[2,1]"

or (only in case the coordinates increase linearly):

   ::

    gmt grdinfo "file.nc?pressure(24,10)"

Programs that generally deal with columns of one-dimensional data, like
or can use multi-dimensional netCDF files in a very similar way. If a
variable in a netCDF file is one-dimensional, there is nothing more
needed than name the variables on the command line. For example:

   ::

    gmt plot "file.nc?lon/lat" ...
    gmt convert "file.nc?time/lat/lon"

If one or more of the selected variables are two-dimensional, and have
the same leading dimension as the other selected variables they will be
plotted in their entirety. For example, if a netCDF files contains 6
time steps recording temperature at 4 points, and the variable ``temp`` is a 6 by
4 array, then the command ``gmt convert "file.nc?time/temp"`` can result in:

   ::

    2012-06-25T00:00:00 20.1 20.2 20.1 20.3
    2012-06-25T12:00:00 24.2 23.2 24.5 23.5
    2012-06-26T00:00:00 16.1 16.2 16.1 16.3
    2012-06-26T12:00:00 22.1 23.0 23.9 23.5
    2012-06-27T00:00:00 17.5 16.9 17.2 16.8
    2012-06-27T12:00:00 27.2 27.2 27.5 27.5

If, for example, only the second temperature column is needed, use
``gmt convert "file.nc?time/temp[1]"`` (indices start counting at 0).

The COARDS conventions set restrictions on the names that can be used
for the units of the variables and coordinates. For example, the units
of longitude and latitude are "degrees_east" and "degrees_north",
respectively. Here is an example of the header of a COARDS compliant
netCDF file (to be obtained using **ncdump**):

   ::

    netcdf M2_fes2004 {
    dimensions:
            lon = 2881 ;
            lat = 1441 ;
    variables:
            float lon(lon) ;
                    lon:long_name = "longitude" ;
                    lon:units = "degrees_east" ;
                    lon:actual_range = 0., 360. ;
            float lat(lat) ;
                    lat:long_name = "latitude" ;
                    lat:units = "degrees_north" ;
                    lat:actual_range = -90., 90. ;
            short amp(lat, lon) ;
                    amp:long_name = "amplitude" ;
                    amp:unit = "m" ;
                    amp:scale_factor = 0.0001 ;
                    amp:add_offset = 3. ;
                    amp:_FillValue = -32768s ;
            short pha(lat, lon) ;
                    pha:long_name = "phase" ;
                    pha:unit = "degrees" ;
                    pha:scale_factor = 0.01 ;
                    pha:_FillValue = -32768s ;

This file contains two grids, which can be plotted separately using the
names ``M2_fes2004.nc?amp`` and ``M2_fes2004.nc?pha``. The attributes ``long_name`` and ``unit`` for each variable
are combined in GMT to a single unit string. For example, after
reading the grid ``y_unit`` equals ``latitude [degrees_north]``. The
same method can be used in reverse to set the proper variable names and
units when writing a grid. However, when the coordinates are set
properly as geographical or time axes, GMT will take care of this. The
user is, however, still responsible for setting the variable name and
unit of the z-coordinate. The default is simply "z".

Modifiers to read and write grids and images via GDAL
-----------------------------------------------------

GMT can read and write a variety of grid and image formats via GDAL. This
extends the capability of GMT to handle data sets from a variety of
sources.

Reading multi-band images
~~~~~~~~~~~~~~~~~~~~~~~~~

:doc:`/grdimage` and :doc:`/image` both lets the user select
individual bands in a multi-band image file and treats the result as an
image (that is the values, in the 0–255 range, are treated as colors,
not data). To select individual bands you use the **+b**\ *band-number*
mechanism that must be appended to the image filename. Here,
*band-number* can be the number of one individual band (the counting
starts at zero), or it could be a comma-separated list of bands. For example

   ::

    gmt image jpeg_image_with_three_bands.jpg+b0 -jpg gray

will plot only the first band (i.e., the red band) of the jpeg image as
a gray-scale image, and

   ::

    gmt image jpeg_image_with_three_bands.jpg+b2,1,0 -jpg bgr

will plot the same image in color but where the RGB band order has been reversed.

Instead of treating them as images, all other GMT programs that
process grids can read individual bands from an image but will consider
the values to be regular data. For example, let ``multiband`` be the name of a
multi-band file with a near infrared component in band 4 and red in band
3. We will compute the NDVI (Normalized Difference Vegetation Index),
which is defined as NDVI = (NIR - R) / (NIR + R), as

   ::

    gmt grdmath multiband=gd+b3 multiband=gd+b2 SUB multiband=gd+b3 \
                multiband=gd+b2 ADD DIV = ndvi.nc

The resulting grid ``ndvi.nc`` can then be plotted as usual.

Reading more complex multi-band IMAGES or GRIDS
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It is also possible to access to sub-datasets in a multi-band grid. The
next example shows how we can extract the SST from the MODIS file ``A20030012003365.L3m_YR_NSST_9``
that is stored in the HDF "format". We need to run the GDAL program
*gdalinfo* on the file because we first
must extract the necessary metadata from the file:

.. code-block:: none

    gdalinfo A20030012003365.L3m_YR_NSST_9
    Driver: HDF4/Hierarchical Data Format Release 4
    Files: A20030012003365.L3m_YR_NSST_9
    Size is 512, 512
    Coordinate System is `'
    Metadata:
     Product Name=A20030012003365.L3m_YR_NSST_9
     Sensor Name=MODISA
     Sensor=
     Title=MODISA Level-3 Standard Mapped Image
    ...
     Scaling=linear
     Scaling Equation=(Slope*l3m_data) + Intercept = Parameter value
     Slope=0.000717185
     Intercept=-2
     Scaled Data Minimum=-2
     Scaled Data Maximum=45
     Data Minimum=-1.999999
     Data Maximum=34.76
    Subdatasets:
     SUBDATASET_1_NAME=HDF4_SDS:UNKNOWN:"A20030012003365.L3m_YR_NSST_9":0
     SUBDATASET_1_DESC=[2160x4320] l3m_data (16-bit unsigned integer)
     SUBDATASET_2_NAME=HDF4_SDS:UNKNOWN:"A20030012003365.L3m_YR_NSST_9":1
     SUBDATASET_2_DESC=[2160x4320] l3m_qual (8-bit unsigned integer)

Now, to access this file with GMT we need to use the =gd mechanism and
append the name of the sub-dataset that we want to extract. Here, a
simple example using :doc:`/grdinfo` would be

   ::

    gmt grdinfo A20030012003365.L3m_YR_NSST_9=gd?HDF4_SDS:UNKNOWN:"A20030012003365.L3m_YR_NSST_9":0

    HDF4_SDS:UNKNOWN:A20030012003365.L3m_YR_NSST_9:0: Title: Grid imported via GDAL
    HDF4_SDS:UNKNOWN:A20030012003365.L3m_YR_NSST_9:0: Command:
    HDF4_SDS:UNKNOWN:A20030012003365.L3m_YR_NSST_9:0: Remark:
    HDF4_SDS:UNKNOWN:A20030012003365.L3m_YR_NSST_9:0: Gridline node registration used
    HDF4_SDS:UNKNOWN:A20030012003365.L3m_YR_NSST_9:0: Grid file format: gd = Import through GDAL (convert to float)
    HDF4_SDS:UNKNOWN:A20030012003365.L3m_YR_NSST_9:0: x_min: 0.5 x_max: 4319.5 x_inc: 1 name: x nx: 4320
    HDF4_SDS:UNKNOWN:A20030012003365.L3m_YR_NSST_9:0: y_min: 0.5 y_max: 2159.5 y_inc: 1 name: y ny: 2160
    HDF4_SDS:UNKNOWN:A20030012003365.L3m_YR_NSST_9:0: z_min: 0 z_max: 65535 name: z
    HDF4_SDS:UNKNOWN:A20030012003365.L3m_YR_NSST_9:0: scale_factor: 1 add_offset: 0

Be warned, however, that things are not yet completed because while the
data are scaled according to the equation printed above ("Scaling
Equation=(Slope\*l3m_data) + Intercept = Parameter value"), this
scaling is not applied by GDAL on reading so it cannot be done
automatically by GMT. One solution is to do the reading and scaling
via :doc:`/grdmath` first, i.e.,

   ::

    gmt grdmath A20030012003365.L3m_YR_NSST_9=gd?HDF4_SDS:UNKNOWN:"A20030012003365.L3m_YR_NSST_9":0 \
                0.000717185 MUL -2 ADD = sst.nc

then plot the ``sst.nc`` directly.

.. _Write-grids-images:

Writing grids and images
~~~~~~~~~~~~~~~~~~~~~~~~

Saving images in the common raster formats is possible but, for the time being, only from :doc:`/grdimage` and even
that is restricted to raster type information. That is, vector data (for instance, coast lines) or text will not
be saved. To save an image with :doc:`/grdimage` use the **-A**\ *outimg=driver* mechanism, where *driver*
is the driver code name used by GDAL (e.g. GTiff) (run *gdal_translate --formats* for the full list.)

For all other programs that create grids, it is also possible to save them using GDAL. To do it one need to use
the =gd appended with the necessary information regarding the driver and the data type to use. Generically,
=\ **gd**\ [**+s**\ *scale*][**+o**\ *offset*][**+n**\ *nan*][:<*driver*\ >[/\ *dataType*][**+c**\ *options*]]
where *driver* is the same as explained above and *dataType* is a 2 or 3 chars code from:
u8\|u16\|i16\|u32\|i32\|float32, and where i\|u denotes signed\|unsigned. If not provided the default type
is float32. Both driver names and data types are case insensitive. The *options* is a list of one or more concatenated
number of GDAL *-co* options. For example, to write a lossless JPG2000 grid one would append
**+c**\ QUALITY=100\ **+c**\ REVERSIBLE=YES\ **+c**\ YCBCR420=NO
**Note**: You will have to specify a *nan* value for integer data types unless you wish that all NaN data values
should be replaced by zero.

Consider setting :term:`IO_NC4_DEFLATION_LEVEL` to reduce file size and to further increase read/write performance.
Especially when working with subsets of global grids, masks, and grids with repeating grid values, the improvement is
usually significant.

The NaN data value
------------------

For a variety of data processing and plotting tasks there is a need to
acknowledge that a data point is missing or unassigned. In the "old
days", such information was passed by letting a value like -9999.99 take
on the special meaning of "this is not really a value, it is missing".
The problem with this scheme is that -9999.99 (or any other floating
point value) may be a perfectly reasonable data value and in such a
scenario would be skipped. The solution adopted in GMT is to use the
IEEE concept Not-a-Number (NaN) for this purpose. Mathematically, a NaN
is what you get if you do an undefined mathematical operation like
0/0; in ASCII data files they appear as the textstring NaN. This
value is internally stored with a particular bit pattern defined by IEEE
so that special action can be taken when it is encountered by programs.
In particular, a standard library function called ``isnan`` is used to
test if a floating point is a NaN. GMT uses these tests extensively to
determine if a value is suitable for plotting or processing (if a NaN is
used in a calculation the result would become NaN as well). Data points
whose values equal NaN are not normally plotted (or plotted with the
special NaN color given in :doc:`/gmt.conf`). Several tools such as
:doc:`/xyz2grd`, :doc:`/gmtmath`, and
:doc:`/grdmath` can convert user data to NaN
and vice versa, thus facilitating arbitrary masking and clipping of data
sets. Note that a few computers do not have native IEEE hardware
support. At this point, this applies to some of the older Cray
super-computers. Users on such machines may have to adopt the old
'-9999.99' scheme to achieve the desired results.

Data records that contain NaN values for the *x* or *y* columns (or the
*z* column for cases when 3-D Cartesian data are expected) are usually
skipped during reading. However, the presence of these bad records can
be interpreted in two different ways, and this behavior is controlled by
the :term:`IO_NAN_RECORDS` defaults parameter. The default setting (*gap*)
considers such records to indicate a gap in an otherwise continuous
series of points (e.g., a line), and programs can act upon this
information, e.g., not to draw a line across the gap or to break the
line into separate segments. The alternative setting (*bad*) makes no
such interpretation and simply reports back how many bad records were
skipped during reading; see Section :ref:`option_-g` for details.

.. _Directory parameters:

Directory parameters
--------------------

GMT versions prior to GMT 5 relied solely on several environment variables
(**$GMT_SHAREDIR**, **$GMT_DATADIR**, **$GMT_USERDIR**, and **$GMT_TMPDIR**), pointing
to folders with data files and program settings. Beginning with version
5, some of these locations are now (also or exclusively) configurable
with the :doc:`/gmtset` utility.
When an environment variable has an equivalent parameter in the :doc:`/gmt.conf` file,
then the parameter setting will take precedence over the environment variable.

Variable **$GMT_SHAREDIR**
    was sometimes required in previous GMT versions to locate the GMT
    share directory where all run-time support files such as coastlines,
    custom symbols, PostScript macros, color tables, and much more reside.
    If this parameter is not set (default), GMT will make a reasonable
    guess of the location of its share folder. Setting this variable is
    usually not required and recommended only under special circumstances.

Variable **$GMT_DATADIR** and parameter :term:`DIR_DATA`
    may point to one or more directories where large and/or widely used
    data files can be placed. All GMT programs look in these directories
    when a file is specified on the command line and it is not present in
    the current directory. This allows maintainers to consolidate large
    data files and to simplify scripting that use these files since the
    absolute path need not be specified. Separate multiple directories
    with commas. Any directory
    name that ends in a trailing slash (/) will be searched recursively
    (not under Windows).

Variable **$GMT_USERDIR**
    may point to a directory where the user places custom configuration
    files (e.g., an alternate ``coastline.conf`` file, preferred default
    settings in ``gmt.conf``, custom symbols and color palettes, math
    macros for :doc:`/gmtmath` and :doc:`/grdmath`, and shorthands for
    gridfile extensions via ``gmt.io``). When **$GMT_USERDIR** is not defined,
    then the default value **$HOME**/.gmt will be assumed. Users may also place their own
    data files in this directory as GMT programs will search for files
    given on the command line in both :term:`DIR_DATA` and **$GMT_USERDIR**.

Variable **$GMT_CACHEDIR**
    may point to a directory where the user places cached data files
    downloaded from the GMT data server. When **$GMT_CACHEDIR** is not defined,
    then the default value **$HOME**/.gmt/cache will be assumed. The cache
    directory can be emptied by running gmt **gmt clear cache**.

Variable **$GMT_TMPDIR**
    may indicate the location, where GMT will write its state parameters
    via the two files ``gmt.history`` and ``gmt.conf``. If **$GMT_TMPDIR** is not
    set, these files are written to GMT session directory [for modern mode] or
    the current directory [for classic mode].

Parameter :term:`DIR_DCW`
    specifies where to look for the optional Digital Charts of the World
    database (for country coloring or selections).

Parameter :term:`DIR_GSHHG`
    specifies where to look for the required
    Global Self-consistent Hierarchical High-resolution Geography database.


Note that files whose full path is given will never be searched for in
any of these directories.

Footnotes
---------

.. [7]
   Vicenty, T. (1975), Direct and inverse solutions of geodesics on the
   ellipsoid with application of nested equations, *Surv. Rev.,
   XXII(176)*, 88–93.

.. [8]
   PostScript definition. In the typesetting industry a slightly
   different definition of point (1/72.27 inch) is used, presumably to
   cause needless trouble.

.. [9]
   Choose between SI and US default units by modifying in the
   GMT share directory.

.. [10]
   To remain backwards compatible with GMT 4 we will also look for
   but only if cannot be found.

.. [16]
   To keep PostScript files small, such comments are by default turned
   off; see :term:`PS_COMMENTS` to enable them.

.. [17]
   For an overview of color systems such as HSV, see Chapter :doc:`colorspace`.

.. [18]
   Convert other graphics formats to Sun ras format using GraphicsMagick's or ImageMagick's **convert** program.
