GMT implements 25 different map 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 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
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.