7.25 Global distribution of antipodes

As promised in Example 23, we will study antipodes. The antipode of a point at
is the point at
. We seek an answer
to the question that has plagued so many for so long: Given the distribution of
land and ocean, how often is the antipode of a point on land also on land? And
what about marine antipodes? We use * grdlandmask* and

#!/bin/csh # # GMT Example 25 $Id: job25.csh,v 1.5 2004/07/01 17:57:13 pwessel Exp $ # # Purpose: Display distribution of antipode types # GMT progs: grdlandmask, grdmath, grd2xyz, gmtmath, grdimage, pscoast, pslegend # Unix progs: cat # # Create D minutes global grid with -1 over oceans and +1 over land set D = 30 grdlandmask -Rg -I${D}m -Dc -A500 -N-1/1/1/1/1 -F -Gwetdry.grd # Manipulate so -1 means ocean/ocean antipode, +1 = land/land, and 0 elsewhere grdmath wetdry.grd DUP 180 ROTX FLIPUD ADD 2 DIV = key.grd # Calculate percentage area of each type of antipode match. grdmath -Rg -I${D}m -F Y COSD 60 $D DIV 360 MUL DUP MUL PI DIV DIV 100 MUL = scale.grd grdmath key.grd -1 EQ 0 NAN scale.grd MUL = tmp.grd grd2xyz tmp.grd -S -ZTLf >! key.b set ocean = `gmtmath -bi1s -Ca -S key.b SUM UPPER RINT =` grdmath key.grd 1 EQ 0 NAN scale.grd MUL = tmp.grd grd2xyz tmp.grd -S -ZTLf >! key.b set land = `gmtmath -bi1s -Ca -S key.b SUM UPPER RINT =` grdmath key.grd 0 EQ 0 NAN scale.grd MUL = tmp.grd grd2xyz tmp.grd -S -ZTLf >! key.b set mixed = `gmtmath -bi1s -Ca -S key.b SUM UPPER RINT =` # Generate corresponding color table cat << EOF >! key.cpt -1 blue 0 blue 0 gray 1 gray 1 red 2 red EOF # Create the final plot and overlay coastlines gmtset ANNOT_FONT_SIZE_PRIMARY +10p PLOT_DEGREE_FORMAT dddF grdimage key.grd -JKs180/9i -B60/30:."Antipodal comparisons":WsNE -K -Ckey.cpt -Y1.2i \ -U/-0.75i/-0.95i/"Example 25 in Cookbook" >! example_25.ps pscoast -R -J -O -K -Wthinnest -Dc -A500 >> example_25.ps # Place an explanatory legend below pslegend -R0/9/0/0.5 -Jx1i/-1i -O -Dx4.5/0/6i/0.3i/TC -Y-0.2i -Fthick << EOF >> example_25.ps N 3 S 0.15i s 0.2i red 0.25p 0.3i Terrestrial Antipodes [$land %] S 0.15i s 0.2i blue 0.25p 0.3i Oceanic Antipodes [$ocean %] S 0.15i s 0.2i gray 0.25p 0.3i Mixed Antipodes [$mixed %] EOF \rm -f *.grd key.* .gmt*

In the end we obtain a funny-looking map depicting the antipodal distribution as well as displaying in legend form the requested percentages (Figure 7.26). Note that the script is set to evaluate a global 30 minute grid for expediency (), hence several smaller landmasses that do have terrestrial antipodes do not show up. If you want a more accurate map you can set the parameter to a smaller increment (try 5 and wait a few minutes).