5.1.4 Geographical linear projection

Figure 5.4: Linear transformation of map coordinates

While these linear projections are primarily designed for generic x,y data, it is sometimes necessary to plot geographical data in a linear projection. This poses a problem since longitudes have a 360$^{o}$ periodicity. GMT therefore needs to be informed that it has been given geographical data although a linear projection has been chosen. We do so by appending a d (for degrees) to the end of the -–Jx (or -–JX) option. As an example, we want to plot a crude world map centered on 125$^{o}$E. Our command will be

#!/bin/sh
#    $Id: GMT_linear_d.sh,v 1.1 2001/03/21 04:10:21 pwessel Exp $
#

gmtset GRID_CROSS_SIZE 0.1i BASEMAP_TYPE FANCY DEGREE_FORMAT 3
pscoast -R-55/305/-90/90 -Jx0.014id -B60g30f15/30g30f15WSen -Dc -A1000 -G200 -W0.25p -P \
    > GMT_linear_d.ps
gmtset GRID_CROSS_SIZE 0

with the result reproduced in Figure 5.4.