For example, here is figure showing the Antarctic and Greenland ice
sheets (the GMT
bash script to generate this figure is
# ... some set up not shown # draw the world pscoast -R0/360/-90/90 -JG-75/41/4.5i -B45/30 -Dc -A10000 \ -G100/100/100 -X7c -Y10c -K -O >>$out gunzip -c greenland.dat.gz | \ psxy -JG -R -M -G0/100/240 -K -O >>$out
GMT uses a terse set of command line flags. In this case, we have:
-R0/360/-90/90, which sets the region to the whole globe
-JG-75/41/4.5iwhich sets the projection to Azimuthal Orthographic with a projection center of 75W, 41N (approximately in Pennsylvania)and the width of the map is 4.5 inches.
-B45/30sets the drawing of gridlines to every 45 degrees in longitude, and every 30 degrees in latitude
-Dcsays to only draw coarse features
-A10000says to limit water features to those larger than 10,000 km square
-Gspecifies the color as gray
- and the
-Yflags offset the map (units are 'c'=cm and 'i'=inches). In GMT, the origin is at the bottom left of the page, and the positive x- and y-axes are to the right and upwards, respectively.
The 2nd line uncompresses
greenland.dat.gz and draws that outline in blue (using the same
-G parameters as before---if you give bare options, GMT will use the the most recent settings...).
#... draw the southern hemisphere and Antarctica in gray pscoast -R0/360/-90/90 -JG-75/-41/4.5i -B45/30 -Dc \ -A10000 -G100/100/100 -X-5c -Y-8c -K -O >>$out # ... and add my outline on top in blue gunzip -c ADDant.txt.gz | psxy -JG -R -M \ -G0/100/240 -K -O >> $out
The GMT default Antarctic outline (drawn by the first command) includes the ice shelves. Here I use a projection center with the same longitude, but with a latitude of -41S (in the eastern Pacific off the coast of S. America...).
The 2nd line uncompresses my outline (which excludes the Ross, Filchner-Ronne, Amery, and Larsen ice shelves) and draws that over the first outline.
Note that the offset of this map is relative to the previous drawing location (which was, if you recall, 7c/10c from the origin at the bottom left). Thus the negative offsets moves the 2nd globe down and to the left.