1.4.3 Mercator projection

Despite the problems of extreme horizontal exaggeration at high latitudes, the conformal Mercator projection (-JM) remains the stalwart of location maps used by scientists. It is one of several cylindrical projections offered by GMT; here we will only have time to focus on one such projection. The complete syntax is simply

-JMwidth

To make coastline maps we use pscoast which automatically will access the GMT coastline data base derived from the GSHHS database^1.1. In addition to the common switches we may need to use some of several pscoast -specific options (see Table 1.2).

Table 1.2: Main options when making coastline plots or overlays.

Option	Purpose
-A	Exclude small features or those of high hierarchical levels (see Appendix K)
-D	Select data resolution (full, high, intermediate, low, or crude)
-G	Set color of dry areas (default does not paint)
-I	Draw rivers (chose features from one or more hierarchical categories)
-L	Plot map scale (length scale can be km, miles, or nautical miles)
-N	Draw political borders (including US state borders)
-S	Set color for wet areas (default does not paint)
-W	Draw coastlines and set pen thickness

One of -W, -G, -S must be selected. Our first coastline example is from Latin America:

pscoast -R-90/-70/0/20 -JM6i -P -B5g5 -G180/120/60 >! map.ps