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6. 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, and , where is implicitly given as the radial vector length to the point on the chosen ellipsoid. The functions and can be quite nasty and we will refrain from presenting details in this document. The interested read is referred to Snyder [1987]6.1. We will mostly be using the pscoast 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

1. Conic map projections
2. Azimuthal map projections
3. Cylindrical map projections
4. Miscellaneous projections

Because and 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 h to the given dimension; to select the minimum map dimension, append -, whereas you may append + to select the maximum map dimension. Without the modifier the map width is selected by default.

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 Proj4 mapping package. Since some of the GMT projections are not in Proj4, 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 Proj4-equivalents see the quick reference cards in Section 3.2. For example, -JM15c and -JMerc/15c have the same meaning.

Subsections

- 6.1 Conic projections
  - 6.1.1 Albers conic equal-area projection (-Jb -JB)
  - 6.1.2 Equidistant conic projection (-Jd -JD)
  - 6.1.3 Lambert conic conformal projection (-Jl -JL)
  - 6.1.4 (American) polyconic projection (-Jpoly -JPoly
- 6.2 Azimuthal projections
  - 6.2.1 Lambert Azimuthal Equal-Area (-Ja -JA)
    - 6.2.1.1 Rectangular map
    - 6.2.1.2 Hemisphere map
  - 6.2.2 Stereographic Equal-Angle projection (-Js -JS)
    - 6.2.2.1 Polar Stereographic Map
    - 6.2.2.2 Rectangular stereographic map
    - 6.2.2.3 General stereographic map
  - 6.2.3 Perspective projection (-Jg -JG)
  - 6.2.4 Orthographic projection (-Jg -JG)
  - 6.2.5 Azimuthal Equidistant projection (-Je -JE)
  - 6.2.6 Gnomonic projection (-Jf -JF)
- 6.3 Cylindrical projections
  - 6.3.1 Mercator projection (-Jm -JM)
  - 6.3.2 Transverse Mercator projection (-Jt -JT)
  - 6.3.3 Universal Transverse Mercator (UTM) projection (-Ju -JU)
  - 6.3.4 Oblique Mercator projection (-Jo -JO)
  - 6.3.5 Cassini cylindrical projection (-Jc -JC)
  - 6.3.6 Cylindrical equidistant projection (-Jq -JQ)
  - 6.3.7 Cylindrical equal-area projections (-Jy -JY)
  - 6.3.8 Miller Cylindrical projection (-Jj -JJ)
  - 6.3.9 Cylindrical stereographic projections (-Jcyl_stere -JCyl_stere)
- 6.4 Miscellaneous projections
  - 6.4.1 Hammer projection (-Jh -JH)
  - 6.4.2 Mollweide projection (-Jw -JW)
  - 6.4.3 Winkel Tripel projection (-Jr -JR)
  - 6.4.4 Robinson projection (-Jn -JN)
  - 6.4.5 Eckert IV and VI projection (-Jk -JK)
  - 6.4.6 Sinusoidal projection (-Ji -JI)
  - 6.4.7 Van der Grinten projection (-Jv -JV)

Next: 6.1 Conic projections Up: GMT - Technical Reference Previous: 5.2 Linear projection with Contents Index