GMT Map Projections
===================

GMT implements more than 30 different projections. They all project
the input coordinates longitude and latitude to positions on a map. In
general, *x' = f(x,y,z)* and *y' = g(x,y,z)*, where
*z* is implicitly given as the radial vector length to the
*(x,y)* point on the chosen ellipsoid. The functions *f* and
*g* can be quite nasty and we will refrain from presenting details
in this document. The interested read is referred to *Snyder*
[1987] [20]_. We will mostly be using the
:doc:`/coast` command to demonstrate each of
the projections. GMT map projections are grouped into four categories
depending on the nature of the projection. The groups are

#. Conic map projections

#. Azimuthal map projections

#. Cylindrical map projections

#. Miscellaneous projections

Because *x* and *y* are coupled we can only specify one
plot-dimensional scale, typically a map *scale* (for lower-case map
projection code) or a map *width* (for upper-case map projection code).
However, in some cases it would be more practical to specify map
*height* instead of *width*, while in other situations it would be nice
to set either the *shortest* or *longest* map dimension. Users may
select these alternatives by appending a character code to their map
dimension. To specify map *height*, append **+dh** to the given dimension;
to select the minimum map dimension, append **+dl**, whereas you may
append **+du** to select the maximum map dimension. Without the modifier
the map width is selected by default [**+dw**].  All dimensions are called
*plot-units* below and are numbers followed by units **c**, **i**, or **p**.

In GMT version 4.3.0 we noticed we ran out of the alphabet for
1-letter (and sometimes 2-letter) projection codes. To allow more
flexibility, and to make it easier to remember the codes, we implemented
the option to use the abbreviations used by the `PROJ <https://proj.org/>`_ mapping
package. Since some of the GMT projections are not in **PROJ**, we
invented some of our own as well. For a full list of both the old 1- and
2-letter codes, as well as the **PROJ**-equivalents see the quick
reference cards in :doc:`/proj-codes`. For example, **-JM**\ 15c and
**-JMerc**\ /15c have the same meaning.

Conic projections
-----------------

Albers conic equal-area projection (**-Jb** **-JB**) :ref:`... <-Jb_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This projection, developed by Heinrich C. Albers in 1805, is predominantly used to
map regions of large east-west extent, in particular the United States.
It is a conic, equal-area projection, in which parallels are unequally
spaced arcs of concentric circles, more closely spaced at the north and
south edges of the map. Meridians, on the other hand, are equally spaced
radii about a common center, and cut the parallels at right angles.
Distortion in scale and shape vanishes along the two standard parallels.
Between them, the scale along parallels is too small; beyond them it is
too large. The opposite is true for the scale along meridians. To define
the projection in GMT you need to provide the following information:

-  Longitude and latitude of the projection center.

-  Two standard parallels.

-  Map scale in plot-units/degree or 1:xxxxx notation (**-Jb**), or map width (**-JB**).

Note that you must include the "1:" if you choose to specify the scale
that way. E.g., you can say 0.5c which means 0.5 cm/degree or 1:200000
which means 1 unit on the map equals 200,000 units along the standard
parallels. The projection center defines the origin of the rectangular
map coordinates. As an example we will make a map of the region near
Taiwan. We choose the center of the projection to be at 125°E/20°N and
25°N and 45°N as our two standard parallels. We desire a map that is 12
cm wide. The complete command needed to generate the map below is
therefore given by:

.. literalinclude:: /_verbatim/GMT_albers.txt

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

   Albers equal-area conic map projection.


Equidistant conic projection (**-Jd** **-JD**) :ref:`... <-Jd_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The equidistant conic projection was described by the Greek philosopher
Claudius Ptolemy about A.D. 150. It is neither conformal or equal-area,
but serves as a compromise between them. The scale is true along all
meridians and the standard parallels. To select this projection in
GMT you must provide the same information as for the other conic
projection, i.e.,

-  Longitude and latitude of the projection center.

-  Two standard parallels.

-  Map scale in plot-unts/degree or 1:xxxxx notation (**-Jd**), or map width (**-JD**).

The equidistant conic projection is often used for atlases with maps of
small countries. As an example, we generate a map of Cuba:

.. literalinclude:: /_verbatim/GMT_equidistant_conic.txt

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

   Equidistant conic map projection.


Lambert conic conformal projection (**-Jl** **-JL**) :ref:`... <-Jl_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This conic projection was designed by the Alsatian mathematician Johann
Heinrich Lambert (1772) and has been used extensively for mapping of
regions with predominantly east-west orientation, just like the Albers
projection. Unlike the Albers projection, Lambert's conformal projection
is not equal-area. The parallels are arcs of circles with a common
origin, and meridians are the equally spaced radii of these circles. As
with Albers projection, it is only the two standard parallels that are
distortion-free. To select this projection in GMT you must provide the
same information as for the Albers projection, i.e.,

-  Longitude and latitude of the projection center.

-  Two standard parallels.

-  Map scale in plot-untes/degree or 1:xxxxx notation (**-Jl**), or map width (**-JL**).

The Lambert conformal projection has been used for basemaps for all the
48 contiguous States with the two fixed standard parallels 33°N and 45°N.
We will generate a map of the continental USA using these parameters.
Note that with all the projections you have the option of selecting a
rectangular border rather than one defined by meridians and parallels.
Here, we choose the regular WESN region, a "fancy" basemap frame, and
use degrees west for longitudes. The generating commands used were

.. literalinclude:: /_verbatim/GMT_lambert_conic.txt

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

   Lambert conformal conic map projection.


The choice for projection center does not affect the projection but it
indicates which meridian (here 100°W) will be vertical on the map. The
standard parallels were originally selected by Adams to provide a
maximum scale error between latitudes 30.5°N and 47.5°N of 0.5–1%. Some
areas, like Florida, experience scale errors of up to 2.5%.

(American) polyconic projection (**-Jpoly** **-JPoly**) :ref:`... <-Jpoly_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The polyconic projection, in Europe usually referred to as the American
polyconic projection, was introduced shortly before 1820 by the
Swiss-American cartographer Ferdinand Rodulph Hassler (1770–1843). As
head of the Survey of the Coast, he was looking for a projection that
would give the least distortion for mapping the coast of the United
States. The projection acquired its name from the construction of each
parallel, which is achieved by projecting the parallel onto the cone
while it is rolled around the globe, along the central meridian, tangent
to that parallel. As a consequence, the projection involves many cones
rather than a single one used in regular conic projections.

The polyconic projection is neither equal-area, nor conformal. It is
true to scale without distortion along the central meridian. Each
parallel is true to scale as well, but the meridians are not as they get
further away from the central meridian. As a consequence, no parallel is
standard because conformity is lost with the lengthening of the meridians.

Below we reproduce the illustration by *Snyder* [1987], with a gridline
every 10 and annotations only every 30° in longitude:

.. literalinclude:: /_verbatim/GMT_polyconic.txt

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

   (American) polyconic projection.


Azimuthal projections
---------------------

Lambert Azimuthal Equal-Area (**-Ja** **-JA**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This projection was developed by Johann Heinrich Lambert in 1772 and is typically used
for mapping large regions like continents and hemispheres. It is an
azimuthal, equal-area projection, but is not perspective. Distortion is
zero at the center of the projection, and increases radially away from
this point. To define this projection in GMT you must provide the
following information:

-  Longitude and latitude of the projection center.

-  Optionally, the horizon, i.e., the number of degrees from the center
   to the edge (<= 180, default is 90).

-  Scale as 1:xxxxx or as radius/latitude where radius is the projected
   distance on the map from projection center to an oblique latitude where 0
   would be the oblique Equator
   (**-Ja**), or map width (**-JA**).

Two different types of maps can be made with this projection depending
on how the region is specified. We will give examples of both types.

Rectangular map
^^^^^^^^^^^^^^^

In this mode we define our region by specifying the longitude/latitude
of the lower left and upper right corners instead of the usual *west,
east, south, north* boundaries. The reason for specifying our area this
way is that for this and many other projections, lines of equal
longitude and latitude are not straight lines and are thus poor choices
for map boundaries. Instead we require that the map boundaries be
rectangular by defining the corners of a rectangular map boundary. Using
0°E/40°S (lower left) and 60°E/10°S (upper right) as our corners we try

.. literalinclude:: /_verbatim/GMT_lambert_az_rect.txt

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

   Rectangular map using the Lambert azimuthal equal-area projection.


Note that an **+r** is appended to the **-R** option to inform GMT that
the region has been selected using the rectangle technique, otherwise it
would try to decode the values as *west, east, south, north* and report
an error since *'east'* < *'west'*.

Hemisphere map
^^^^^^^^^^^^^^

Here, you must specify the world as your region (**-Rg** or
**-Rd**). E.g., to obtain a hemisphere view that shows the Americas, try

.. literalinclude:: /_verbatim/GMT_lambert_az_hemi.txt

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

   Hemisphere map using the Lambert azimuthal equal-area projection.


To geologists, the Lambert azimuthal equal-area projection (with origin
at 0/0) is known as the *equal-area* (Schmidt) stereonet and used for
plotting fold axes, fault planes, and the like. An *equal-angle* (Wulff)
stereonet can be obtained by using the stereographic projection
(discussed later). The stereonets produced by these two projections appear below.

.. _GMT_stereonets:

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

   Equal-Area (Schmidt) and Equal-Angle (Wulff) stereo nets.


Stereographic Equal-Angle projection (**-Js** **-JS**) :ref:`... <-Js_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This is a conformal, azimuthal projection that dates back to the Greeks.
Its main use is for mapping the polar regions. In the polar aspect all
meridians are straight lines and parallels are arcs of circles. While
this is the most common use it is possible to select any point as the
center of projection. The requirements are

-  Longitude and latitude of the projection center.

-  Optionally, the horizon, i.e., the number of degrees from the center
   to the edge (< 180, default is 90).

-  Scale as 1:xxxxx (true scale at pole), slat/1:xxxxx (true scale at
   standard parallel slat), or radius/latitude where radius is distance
   on map in plot-units from projection center to a particular
   oblique latitude (**-Js**), or simply map width (**-JS**).

A map scale factor of 0.9996 will be applied by default
(although you may change this with :term:`PROJ_SCALE_FACTOR`). However,
the setting is ignored when a standard parallel has been specified since
the scale is then implicitly given. We will look at two different types
of maps.

Polar Stereographic Map :ref:`... <-Js_full>`
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In our first example we will let the projection center be at the north
pole. This means we have a polar stereographic projection and the map
boundaries will coincide with lines of constant longitude and latitude.
An example is given by

.. literalinclude:: /_verbatim/GMT_stereographic_polar.txt

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

   Polar stereographic conformal projection.


Rectangular stereographic map
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

As with Lambert's azimuthal equal-area projection we have the option to
use rectangular boundaries rather than the wedge-shape typically
associated with polar projections. This choice is defined by selecting
two points as corners in the rectangle and appending an "r" to the
**-R** option. This command produces a map as presented in
Figure :ref:`Polar stereographic <GMT_stereographic_rect>`:

.. literalinclude:: /_verbatim/GMT_stereographic_rect.txt

.. _GMT_stereographic_rect:

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

   Polar stereographic conformal projection with rectangular borders.


General stereographic map
^^^^^^^^^^^^^^^^^^^^^^^^^

In terms of usage this projection is identical to the Lambert azimuthal
equal-area projection. Thus, one can make both rectangular and
hemispheric maps. Our example shows Australia using a projection pole at
130°E/30°S. The command used was

.. literalinclude:: /_verbatim/GMT_stereographic_general.txt

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

   General stereographic conformal projection with rectangular borders.


By choosing 0/0 as the pole, we obtain the conformal stereonet presented
next to its equal-area cousin in the Section `Lambert Azimuthal Equal-Area (-Ja -JA)`_ on the Lambert azimuthal equal-area projection (Figure :ref:`Stereonets <GMT_stereonets>`).

Perspective projection (**-Jg** **-JG**) :ref:`... <-Jg_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The perspective projection imitates in 2 dimensions the 3-dimensional
view of the earth from space. The implementation in GMT is very
flexible, and thus requires many input variables. Those are listed and
explained below, with the values used in
Figure :ref:`Perspective projection <GMT_perspective>` between brackets.

-  Longitude and latitude of the projection center (4°E/52°N).

-  Altitude of the viewer above sea level in kilometers (230 km). If
   this value is less than 10, it is assumed to be the distance of the
   viewer from the center of the earth in earth radii. If an "r" is
   appended, it is the distance from the center of the earth in
   kilometers.

-  Azimuth in degrees (90, due east). This is the direction in which you
   are looking, measured clockwise from north.

-  Tilt in degrees (60). This is the viewing angle relative to zenith.
   So a tilt of 0° is looking straight down, 60° is looking from 30° above
   the horizon.

-  Twist in degrees (180). This is the boresight rotation (clockwise) of
   the image. The twist of 180° in the example mimics the fact that the
   Space Shuttle flies upside down.

-  Width and height of the viewpoint in degrees (60). This number
   depends on whether you are looking with the naked eye (in which case
   you view is about 60° wide), or with binoculars, for example.

-  Scale as 1:xxxxx or as radius/latitude where radius is distance on
   map in plot-units from projection center to a particular
   oblique latitude (**-Jg**), or map width (**-JG**) (e.g., 12 cm).

The imagined view of northwest Europe from a Space Shuttle at 230 km
looking due east is thus accomplished by the following
:doc:`/coast` command:

.. literalinclude:: /_verbatim/GMT_perspective.txt

.. _GMT_perspective:

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

   View from the Space Shuttle in Perspective projection.


Orthographic projection (**-Jg** **-JG**) :ref:`... <-Jg_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The orthographic azimuthal projection is a perspective projection from
infinite distance. It is therefore often used to give the appearance of
a globe viewed from outer space. As with Lambert's equal-area and the
stereographic projection, only one hemisphere can be viewed at any time.
The projection is neither equal-area nor conformal, and much distortion
is introduced near the edge of the hemisphere. The directions from the
center of projection are true. The projection was known to the Egyptians
and Greeks more than 2,000 years ago. Because it is mainly used for
pictorial views at a small scale, only the spherical form is necessary.

To specify the orthographic projection the same options **-Jg** or
**-JG** as the perspective projection are used, but with fewer variables to supply:

-  Longitude and latitude of the projection center.

-  Optionally, the horizon, i.e., the number of degrees from the center
   to the edge (<= 90, default is 90).

-  Scale as 1:xxxxx or as radius/latitude where radius is distance on
   map in plot-units from projection center to a particular
   oblique latitude (**-Jg**), or map width (**-JG**).

Our example of a perspective view centered on 75°W/40°N can therefore be
generated by the following :doc:`/coast` command:

.. literalinclude:: /_verbatim/GMT_orthographic.txt

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

   Hemisphere map using the Orthographic projection.


Azimuthal Equidistant projection (**-Je** **-JE**) :ref:`... <-Je_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The most noticeable feature of this azimuthal projection is the fact
that distances measured from the center are true. Therefore, a circle
about the projection center defines the locus of points that are equally
far away from the plot origin. Furthermore, directions from the center
are also true. The projection, in the polar aspect, is at least several
centuries old. It is a useful projection for a global view of locations
at various or identical distance from a given point (the map center).

To specify the azimuthal equidistant projection you must supply:

-  Longitude and latitude of the projection center.

-  Optionally, the horizon, i.e., the number of degrees from the center
   to the edge (<= 180, default is 180).

-  Scale as 1:xxxxx or as radius/latitude where radius is distance on
   map in plot-units from projection center to a particular
   oblique latitude (**-Je**), or map width (**-JE**).

Our example of a global view centered on 100°W/40°N can therefore be
generated by the following :doc:`/coast`
command. Note that the antipodal point is 180° away from the center, but
in this projection this point plots as the entire map perimeter:

.. literalinclude:: /_verbatim/GMT_az_equidistant.txt

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

   World map using the equidistant azimuthal projection.


Gnomonic projection (**-Jf** **-JF**) :ref:`... <-Jf_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The Gnomonic azimuthal projection is a perspective projection from the
center onto a plane tangent to the surface. Its origin goes back to the
old Greeks who used it for star maps almost 2500 years ago. The
projection is neither equal-area nor conformal, and much distortion is
introduced near the edge of the hemisphere; in fact, less than a
hemisphere may be shown around a given center. The directions from the
center of projection are true. Great circles project onto straight
lines. Because it is mainly used for pictorial views at a small scale,
only the spherical form is necessary.

To specify the Gnomonic projection you must supply:

-  Longitude and latitude of the projection center.

-  Optionally, the horizon, i.e., the number of degrees from the center
   to the edge (< 90, default is 60).

-  Scale as 1:xxxxx or as radius/latitude where radius is distance on
   map in plot-units from projection center to a particular
   oblique latitude (**-Jf**), or map width (**-JF**).

Using a horizon of 60, our example of this projection centered on
120°W/35°N can therefore be generated by the following :doc:`/coast` command:

.. literalinclude:: /_verbatim/GMT_gnomonic.txt

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

   Gnomonic azimuthal projection.


Cylindrical projections
-----------------------

Cylindrical projections are easily recognized for its shape: maps are
rectangular and meridians and parallels are straight lines crossing at
right angles. But that is where similarities between the cylindrical
projections supported by GMT (Mercator, transverse Mercator, universal
transverse Mercator, oblique Mercator, Cassini, cylindrical equidistant,
cylindrical equal-area, Miller, and cylindrical stereographic
projections) stops. Each have a different way of spacing the meridians
and parallels to obtain certain desirable cartographic properties.

Mercator projection (**-Jm** **-JM**) :ref:`... <-Jm_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Probably the most famous of the various map projections, the Mercator
projection takes its name from the Flemish cartographer Gheert Cremer,
better known as Gerardus Mercator, who presented it in 1569. The
projection is a cylindrical and conformal, with no distortion along the
equator. A major navigational feature of the projection is that a line
of constant azimuth is straight. Such a line is called a rhumb line or
*loxodrome*. Thus, to sail from one point to another one only had to
connect the points with a straight line, determine the azimuth of the
line, and keep this constant course for the entire voyage [21]_. The
Mercator projection has been used extensively for world maps in which
the distortion towards the polar regions grows rather large, thus
incorrectly giving the impression that, for example, Greenland is larger
than South America. In reality, the latter is about eight times the size
of Greenland. Also, the Former Soviet Union looks much bigger than
Africa or South America. One may wonder whether this illusion has had
any influence on U.S. foreign policy.

In the regular Mercator projection, the cylinder touches the globe along
the equator. Other orientations like vertical and oblique give rise to
the Transverse and Oblique Mercator projections, respectively. We will
discuss these generalizations following the regular Mercator projection.

The regular Mercator projection requires a minimum of parameters. To use
it in GMT programs you supply this information (the first two items
are optional and have defaults):

-  Central meridian [Middle of your map].

-  Standard parallel for true scale [Equator]. When supplied, central
   meridian must be supplied as well.

-  Scale along the equator in plot-units/degree or 1:xxxxx (**-Jm**), or map
   width (**-JM**).

Our example presents a world map at a scale of 0.03 cm per degree
which will give a map 10.8-cm wide. It was created with the command:

.. literalinclude:: /_verbatim/GMT_mercator.txt

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

   Simple Mercator map.


While this example is centered on the Dateline, one can easily choose
another configuration with the **-R** option. A map centered on
Greenwich would specify the region with **-R**-180/180/-70/70.

Transverse Mercator projection (**-Jt** **-JT**) :ref:`... <-Jt_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The transverse Mercator was invented by Johann Heinrich Lambert in 1772. In this
projection the cylinder touches a meridian along which there is no
distortion. The distortion increases away from the central meridian and
goes to infinity at 90° from center. The central meridian, each meridian
90° away from the center, and equator are straight lines; other parallels
and meridians are complex curves. The projection is defined by
specifying:

-  The central meridian.

-  Optionally, the latitude of origin (default is the equator).

-  Scale along the equator in plot-units/degree or 1:xxxxx (**-Jt**), or map
   width (**-JT**).

The optional latitude of origin defaults to Equator if not specified.
Although defaulting to 1, you can change the map scale factor via the
:term:`PROJ_SCALE_FACTOR` parameter. Our example shows a transverse
Mercator map of south-east Europe and the Middle East with 35°E as the
central meridian:

.. literalinclude:: /_verbatim/GMT_transverse_merc.txt

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

   Rectangular Transverse Mercator map.


The transverse Mercator can also be used to generate a global map - the
equivalent of the 360° Mercator map. Using the command

.. literalinclude:: /_verbatim/GMT_TM.txt

we made the map illustrated in Figure :ref:`Global transverse Mercator
<GMT_TM>`. Note that
when a world map is given (indicated by **-R**\ *0/360/s/n*), the
arguments are interpreted to mean oblique degrees, i.e., the 360° range
is understood to mean the extent of the plot along the central meridian,
while the "south" and "north" values represent how far from the central
longitude we want the plot to extend. These values correspond to
latitudes in the regular Mercator projection and must therefore be less
than 90.

.. _GMT_TM:

.. figure:: /_images/GMT_TM.*
   :width: 450 px
   :align: center

   A global transverse Mercator map.


Universal Transverse Mercator (UTM) projection (**-Ju** **-JU**) :ref:`... <-Ju_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A particular subset of the transverse Mercator is the Universal
Transverse Mercator (UTM) which was adopted by the US Army for
large-scale military maps. Here, the globe is divided into 60 zones
between 84°S and 84°N, most of which are 6 wide. Each of these UTM zones
have their unique central meridian. Furthermore, each zone is divided
into latitude bands but these are not needed to specify the projection
for most cases. See Figure :ref:`Universal Transverse Mercator
<GMT_utm_zones>` for all zone designations.

.. _GMT_utm_zones:

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

   Universal Transverse Mercator zone layout.


GMT implements both the transverse Mercator and the UTM projection.
When selecting UTM you must specify:

-  UTM zone (A, B, 1–60, Y, Z). Use negative values for numerical zones
   in the southern hemisphere or append the latitude modifiers C–H, J–N,
   P–X) to specify an exact UTM grid zone.

-  Scale along the equator in plot-units/degree or 1:xxxxx (**-Ju**), or map
   width (**-JU**).

In order to minimize the distortion in any given zone, a scale factor of
0.9996 has been factored into the formulae. (although a standard, you
can change this with :term:`PROJ_SCALE_FACTOR`). This makes the UTM
projection a *secant* projection and not a *tangent* projection like the
transverse Mercator above. The scale only varies by 1 part in 1,000 from
true scale at equator. The ellipsoidal projection expressions are
accurate for map areas that extend less than 10 away from the central
meridian. For larger regions we use the conformal latitude in the
general spherical formulae instead.

Oblique Mercator projection (**-Jo** **-JO**) :ref:`... <-Jo_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Oblique configurations of the cylinder give rise to the oblique Mercator
projection. It is particularly useful when mapping regions of large
lateral extent in an oblique direction. Both parallels and meridians are
complex curves. The projection was developed in the early 1900s by
several workers. Several parameters must be provided to define the
projection. GMT offers three different definitions:

#. Option **-Jo**\ [**a**\|\ **A**] or **-JO**\ [**a**\|\ **A**]:

   -  Longitude and latitude of projection center.

   -  Azimuth of the oblique equator.

   -  Scale in plot-units/degree or 1:xxxxx along oblique equator (**-Jo**),
      or map width (**-JO**).

#. Option **-Jo**\ [**b**\|\ **B**] or **-JO**\ [**b**\|\ **B**]:

   -  Longitude and latitude of projection center.

   -  Longitude and latitude of second point on oblique equator.

   -  Scale in plot-units/degree or 1:xxxxx along oblique equator (**-Jo**),
      or map width (**-JO**).

#. Option **-Joc**\|\ **C** or **-JOc**\|\ **C**:

   -  Longitude and latitude of projection center.

   -  Longitude and latitude of projection pole.

   -  Scale in plot-units/degree or 1:xxxxx along oblique equator (**-Jo**),
      or map width (**-JO**).

For all three definitions, the upper case **A**\|\ **B**\|\ **C** means we
will allow projection poles in the southern hemisphere [By default we map any such
poles to their antipodes in the north hemisphere].  Our example was produced by the command

.. literalinclude:: /_verbatim/GMT_obl_merc.txt

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

   Oblique Mercator map using **-Joc**. We make it clear which direction is North by
   adding a star rose with the **-Td** option.


It uses definition 3 for an oblique view of some Caribbean islands. Note
that we define our region using the rectangular system described
earlier. If we do not append **+r** to the **-R** string then the
information provided with the **-R** option is assumed to be oblique
degrees about the projection center rather than the usual geographic
coordinates. This interpretation is chosen since in general the
parallels and meridians are not very suitable as map boundaries.

Normally, the oblique Equator becomes the horizontal (*x*) axis in the projected units.
You can select the vertical (*y*) axis by appending **+v**.

When working with oblique projections such as here, it is often much more convenient
to specify the map domain in the projected coordinates relative to the map center.
The figure below shows two views of New Zealand using the oblique Mercator projection
that in both cases specifies the region using **-R**\ -1000/1000/-500/500\ **+uk**.  The
unit **k** means the following bounds are in projected km and we let GMT determine the
geographic coordinates of the two diagonal corners internally.

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

   (left) Oblique view of New Zealand centered on its geographical center (Nelson)
   indicated by the white circle for an oblique Equator with azimuth 35.  This
   resulted in the argument **-JOa**\ 173:17:02E/41:16:15S/35/3i.
   The map is 2000 km by 1000 km and the Cartesian
   coordinate system in the projected units are indicated by the bold axes.  The blue
   circle is the point (40S,180E) and it has projected coordinates (*x* = 426.2, *y* = -399.7).
   (right) Same dimensions but now specifying an azimuth of 215, yielding a projection
   pole in the southern hemisphere (hence we used **-JOA** to override the restriction,
   i.e., **-JOA**\ 173:17:02E/41:16:15S/215/3i.)
   The projected coordinate system is still aligned as before but the Earth has been rotated
   180 degrees.  The blue point now has projected coordinates (*x* = -426.2, *y* = 399.7).

The oblique Mercator projection will by default arrange the output so that the oblique
Equator becomes the new horizontal, positive *x*-axis.  For features with an orientation
more north-south than east-west, it may be preferable to align the oblique Equator with
the vertical, positive *y*-axis instead.  This configuration is selected by appending
**+v** to the **-J** projection option.  The example below shows this behaviour.

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

   Oblique view of Baja California using the vertical oblique Equator modifier.  This plot
   resulted from the argument **-JOa**\ 120W/25N/-30/6c\ **+v**\ .


Cassini cylindrical projection (**-Jc** **-JC**) :ref:`... <-Jc_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This cylindrical projection was developed in 1745 by César-François
Cassini de Thury for the survey of France. It is occasionally called
Cassini-Soldner since the latter provided the more accurate mathematical
analysis that led to the development of the ellipsoidal formulae. The
projection is neither conformal nor equal-area, and behaves as a
compromise between the two end-members. The distortion is zero along the
central meridian. It is best suited for mapping regions of north-south
extent. The central meridian, each meridian 90° away, and equator are
straight lines; all other meridians and parallels are complex curves.
The requirements to define this projection are:

-  Longitude and latitude of central point.

-  Scale in plot-units/degree or as 1:xxxxx (**-Jc**), or map width (**-JC**).

A detailed map of the island of Sardinia centered on the 8°45'E meridian
using the Cassini projection can be obtained by running the command:

.. literalinclude:: /_verbatim/GMT_cassini.txt

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

   Cassini map over Sardinia.


As with the previous projections, the user can choose between a
rectangular boundary (used here) or a geographical (WESN) boundary.

Cylindrical equidistant projection (**-Jq** **-JQ**) :ref:`... <-Jq_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This simple cylindrical projection is really a linear scaling of
longitudes and latitudes. The most common form is the Plate Carrée
projection, where the scaling of longitudes and latitudes is the same.
All meridians and parallels are straight lines. The projection can be
defined by:

-  The central meridian [Middle of your map].

-  Standard parallel [Equator].

-  Scale in plot-units/degree or as 1:xxxxx (**-Jq**), or map width (**-JQ**).

The first two of these are optional and have defaults. When the standard
parallel is defined, the central meridian must be supplied as well.

A world map centered on the dateline using this projection can be
obtained by running the command:

.. literalinclude:: /_verbatim/GMT_equi_cyl.txt

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

   World map using the Plate Carrée projection.


Different relative scalings of longitudes and latitudes can be obtained
by selecting a standard parallel different from the equator. Some
selections for standard parallels have practical properties as shown in
Table :ref:`JQ <tbl-JQ>`.

.. _tbl-JQ:

+-----------------------------------------------------+--------+
+=====================================================+========+
| Grafarend and Niermann, minimum linear distortion   | 61.7°  |
+-----------------------------------------------------+--------+
| Ronald Miller Equirectangular                       | 50.5°  |
+-----------------------------------------------------+--------+
| Ronald Miller, minimum continental distortion       | 43.5°  |
+-----------------------------------------------------+--------+
| Grafarend and Niermann                              | 42°    |
+-----------------------------------------------------+--------+
| Ronald Miller, minimum overall distortion           | 37.5°  |
+-----------------------------------------------------+--------+
| Plate Carrée, Simple Cylindrical, Plain/Plane       | 0°     |
+-----------------------------------------------------+--------+

Cylindrical equal-area projections (**-Jy** **-JY**) :ref:`... <-Jy_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This cylindrical projection is actually several projections, depending
on what latitude is selected as the standard parallel. However, they are
all equal area and hence non-conformal. All meridians and parallels are
straight lines. The requirements to define this projection are:

-  The central meridian.

-  The standard parallel.

-  Scale in plot-units/degree or as 1:xxxxx (**-Jy**), or map width (**-JY**)

While you may choose any value for the standard parallel and obtain your
own personal projection, there are seven choices of standard parallels
that result in known (or named) projections. These are listed in Table :ref:`JY <tbl-JY>`.

.. _tbl-JY:

+-------------------+---------------------+
+===================+=====================+
| Balthasart        | 50°                 |
+-------------------+---------------------+
| Gall              | 45°                 |
+-------------------+---------------------+
| Hobo-Dyer         | 37°30' (= 37.5°)    |
+-------------------+---------------------+
| Trystan Edwards   | 37°24' (= 37.4°)    |
+-------------------+---------------------+
| Caster            | 37°04' (= 37.0666°) |
+-------------------+---------------------+
| Behrman           | 30°                 |
+-------------------+---------------------+
| Lambert           | 0°                  |
+-------------------+---------------------+

For instance, a world map centered on the 35°E meridian using the Behrman
projection (Figure :ref:`Behrman cylindrical projection <GMT_general_cyl>`)
can be obtained by running the command:

.. literalinclude:: /_verbatim/GMT_general_cyl.txt

.. _GMT_general_cyl:

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

   World map using the Behrman cylindrical equal-area projection.


As one can see there is considerable distortion at high latitudes since
the poles map into lines.

Miller Cylindrical projection (**-Jj** **-JJ**) :ref:`... <-Jj_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This cylindrical projection, presented by Osborn Maitland Miller of the
American Geographic Society in 1942, is neither equal nor conformal. All
meridians and parallels are straight lines. The projection was designed
to be a compromise between Mercator and other cylindrical projections.
Specifically, Miller spaced the parallels by using Mercator's formula
with 0.8 times the actual latitude, thus avoiding the singular poles;
the result was then divided by 0.8. There is only a spherical form for
this projection. Specify the projection by:

-  Optionally, the central meridian (default is the middle of your map).

-  Scale in plot-units/degree or as 1:xxxxx (**-Jj**), or map width (**-JJ**).

For instance, a world map centered on the 90°E meridian at a map scale of
1:400,000,000 (Figure :ref:`Miller projection <GMT_miller>`) can be obtained as
follows:

.. literalinclude:: /_verbatim/GMT_miller.txt

.. _GMT_miller:

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

   World map using the Miller cylindrical projection.


Cylindrical stereographic projections (**-Jcyl_stere** **-JCyl_stere**) :ref:`... <-Jcyl_stere_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The cylindrical stereographic projections are certainly not as notable
as other cylindrical projections, but are still used because of their
relative simplicity and their ability to overcome some of the downsides
of other cylindrical projections, like extreme distortions of the higher
latitudes. The stereographic projections are perspective projections,
projecting the sphere onto a cylinder in the direction of the antipodal
point on the equator. The cylinder crosses the sphere at two standard
parallels, equidistant from the equator. The projections are defined by:

-  The central meridian (uses the middle of the map when omitted).

-  The standard parallel (default is the Equator). When used, central
   meridian needs to be given as well.

-  Scale in plot-units/degree or as 1:xxxxx (**-Jcyl_stere**), or map width
   (**-JCyl_stere**)

Some of the selections of the standard parallel are named for the
cartographer or publication that popularized the projection
(Table :ref:`JCylstere <tbl-JCylstere>`).

.. _tbl-JCylstere:

+---------------------------------------------------------+-------------+
+=========================================================+=============+
| Miller's modified Gall                                  | 66.159467°  |
+---------------------------------------------------------+-------------+
| Kamenetskiy's First                                     | 55°         |
+---------------------------------------------------------+-------------+
| Gall's stereographic                                    | 45°         |
+---------------------------------------------------------+-------------+
| Bolshoi Sovietskii Atlas Mira or Kamenetskiy's Second   | 30°         |
+---------------------------------------------------------+-------------+
| Braun's cylindrical                                     | 0°          |
+---------------------------------------------------------+-------------+

A map of the world, centered on the Greenwich meridian, using the Gall's
stereographic projection (standard parallel is 45°,
Figure :ref:`Gall's stereographic projection <GMT_gall_stereo>`),
is obtained as follows:

.. literalinclude:: /_verbatim/GMT_gall_stereo.txt

.. _GMT_gall_stereo:

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

   World map using Gall's stereographic projection.


Miscellaneous projections
-------------------------

GMT supports eight common projections for global presentation of data or
models. These are the Hammer, Mollweide, Winkel Tripel, Robinson, Eckert
IV and VI, Sinusoidal, and Van der Grinten projections. Due to the small
scale used for global maps these projections all use the spherical
approximation rather than more elaborate elliptical formulae.

In all cases, the specification of the central meridian can be skipped.
The default is the middle of the longitude range of the plot, specified
by the (**-R**) option.

Hammer projection (**-Jh** **-JH**) :ref:`... <-Jh_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The equal-area Hammer projection, first presented by the German
mathematician Ernst von Hammer in 1892, is also known as Hammer-Aitoff
(the Aitoff projection looks similar, but is not equal-area). The border
is an ellipse, equator and central meridian are straight lines, while
other parallels and meridians are complex curves. The projection is
defined by selecting:

-  The central meridian [Middle of your map].

-  Scale along equator in plot-units/degree or 1:xxxxx (**-Jh**), or map width (**-JH**).

A view of the Pacific ocean using the Dateline as central meridian is accomplished thus

.. literalinclude:: /_verbatim/GMT_hammer.txt

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

   World map using the Hammer projection.


Mollweide projection (**-Jw** **-JW**) :ref:`... <-Jw_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This pseudo-cylindrical, equal-area projection was developed by the
German mathematician and astronomer Karl Brandan Mollweide in 1805.
Parallels are unequally spaced straight lines with the meridians being
equally spaced elliptical arcs. The scale is only true along latitudes
40°44' north and south. The projection is used mainly for global maps
showing data distributions. It is occasionally referenced under the name
homalographic projection. Like the Hammer projection, outlined above, we
need to specify only two parameters to completely define the mapping of
longitudes and latitudes into rectangular *x*/*y* coordinates:

-  The central meridian [Middle of your map].

-  Scale along equator in plot-units/degree or 1:xxxxx (**-Jw**), or map width (**-JW**).

An example centered on Greenwich can be generated thus:

.. literalinclude:: /_verbatim/GMT_mollweide.txt

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

   World map using the Mollweide projection.


Winkel Tripel projection (**-Jr** **-JR**) :ref:`... <-Jr_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In 1921, the German mathematician Oswald Winkel a projection that was to
strike a compromise between the properties of three elements (area,
angle and distance). The German word "tripel" refers to this junction of
where each of these elements are least distorted when plotting global
maps. The projection was popularized when Bartholomew and Son started to
use it in its world-renowned "The Times Atlas of the World" in the mid
20th century. In 1998, the National Geographic Society made the Winkel
Tripel as its map projection of choice for global maps.

Naturally, this projection is neither conformal, nor equal-area. Central
meridian and equator are straight lines; other parallels and meridians
are curved. The projection is obtained by averaging the coordinates of
the Equidistant Cylindrical and Aitoff (not Hammer-Aitoff) projections.
The poles map into straight lines 0.4 times the length of equator. To
use it you must enter

-  The central meridian [Middle of your map].

-  Scale along equator in plot-units/degree or 1:xxxxx (**-Jr**), or map width (**-JR**).

Centered on Greenwich, the example in Figure :ref:`Winkel Tripel projection
<GMT_winkel>` was created by this command:

.. literalinclude:: /_verbatim/GMT_winkel.txt

.. _GMT_winkel:

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

   World map using the Winkel Tripel projection.


Robinson projection (**-Jn** **-JN**) :ref:`... <-Jn_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The Robinson projection, presented by the American geographer and
cartographer Arthur H. Robinson in 1963, is a modified cylindrical
projection that is neither conformal nor equal-area. Central meridian
and all parallels are straight lines; other meridians are curved. It
uses lookup tables rather than analytic expressions to make the world
map "look" right [22]_. The scale is true along latitudes 38. The
projection was originally developed for use by Rand McNally and is
currently used by the National Geographic Society. To use it you must
enter

-  The central meridian [Middle of your map].

-  Scale along equator in plot-units/degree or 1:xxxxx (**-Jn**), or map width
   (**-JN**).

Again centered on Greenwich, the example below was created by this command:

.. literalinclude:: /_verbatim/GMT_robinson.txt

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

   World map using the Robinson projection.


Eckert IV and VI projection (**-Jk** **-JK**) :ref:`... <-Jk_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The Eckert IV and VI projections, presented by the German cartographer
Max Eckert-Greiffendorff in 1906, are pseudo-cylindrical equal-area
projections. Central meridian and all parallels are straight lines;
other meridians are equally spaced elliptical arcs (IV) or sinusoids
(VI). The scale is true along latitudes 40°30' (IV) and 49°16' (VI). Their
main use is in thematic world maps. To select Eckert IV you must use
**-JKf** (**f** for "four") while Eckert VI is selected with **-JKs**
(**s** for "six"). If no modifier is given it defaults to Eckert VI. In
addition, you must enter

-  The central meridian [Middle of your map].

-  Scale along equator in plot-units/degree or 1:xxxxx (**-Jk**), or map width
   (**-JK**).

Centered on the Dateline, the Eckert IV example below was created by
this command:

.. literalinclude:: /_verbatim/GMT_eckert4.txt

.. figure:: /_images/GMT_eckert4.*
   :width: 500 px
   :align: center

   World map using the Eckert IV projection.


The same script, with **s** instead of **f**, yields the Eckert VI map:

.. figure:: /_images/GMT_eckert6.*
   :width: 500 px
   :align: center

   World map using the Eckert VI projection.


Sinusoidal projection (**-Ji** **-JI**) :ref:`... <-Ji_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The sinusoidal projection is one of the oldest known projections, is
equal-area, and has been used since the mid-16th century. It has also
been called the "Equal-area Mercator" projection. The central meridian
is a straight line; all other meridians are sinusoidal curves. Parallels
are all equally spaced straight lines, with scale being true along all
parallels (and central meridian). To use it, you need to select:

-  The central meridian [Middle of your map].

-  Scale along equator in plot-units/degree or 1:xxxxx (**-Ji**), or map width
   (**-JI**).

A simple world map using the sinusoidal projection is therefore obtained by

.. literalinclude:: /_verbatim/GMT_sinusoidal.txt

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

   World map using the Sinusoidal projection.


To reduce distortion of shape the interrupted sinusoidal projection was
introduced in 1927. Here, three symmetrical segments are used to cover
the entire world. Traditionally, the interruptions are at 160°W, 20°W, and
60°E. To make the interrupted map we must call
:doc:`/coast` for each segment and superpose
the results. To produce an interrupted world map (with the traditional
boundaries just mentioned) that is 14.4 cm wide we use the scale
14.4/360 = 0.04 and offset the subsequent plots horizontally by their
widths (140\ :math:`\cdot`\ 0.04 and 80\ :math:`\cdot`\ 0.04):

.. literalinclude:: /_verbatim/GMT_sinus_int.txt

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

   World map using the Interrupted Sinusoidal projection.


The usefulness of the interrupted sinusoidal projection is basically
limited to display of global, discontinuous data distributions like
hydrocarbon and mineral resources, etc.

Van der Grinten projection (**-Jv** **-JV**) :ref:`... <-Jv_full>`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The Van der Grinten projection, presented by Alphons J. van der Grinten
in 1904, is neither equal-area nor conformal. Central meridian and
Equator are straight lines; other meridians are arcs of circles. The
scale is true along the Equator only. Its main use is to show the entire
world enclosed in a circle. To use it you must enter

-  The central meridian [Middle of your map].

-  Scale along equator in plot-units/degree or 1:xxxxx (**-Jv**), or map width (**-JV**).

Centered on the Dateline, the example below was created by this command:

.. literalinclude:: /_verbatim/GMT_grinten.txt

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

   World map using the Van der Grinten projection.

Footnotes
---------

.. [20]
   Snyder, J. P., 1987, Map Projections A Working Manual, U.S.
   Geological Survey Prof. Paper 1395.

.. [21]
   This is, however, not the shortest distance. It is given by the great
   circle connecting the two points.

.. [22]
   Robinson provided a table of *y*-coordinates for latitudes
   every 5. To project values for intermediate latitudes one must
   interpolate the table. Different interpolants may result in slightly
   different maps. GMT uses the
   interpolant selected by the parameter :term:`GMT_INTERPOLANT` in the
   file.
