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5.1.1.2 Geographic coordinates

Figure 5.2: Linear transformation of map coordinates.

While the Cartesian linear projection is primarily designed for regular floating point x,y data, it is sometimes necessary to plot geographical data in a linear projection. This poses a problem since longitudes have a 360° periodicity. GMT therefore needs to be informed that it has been given geographical coordinates even though a linear transformation has been chosen. We do so by adding a g (for geographical) or d (for degrees) directly after -R or by appending a g or d to the end of the -Jx (or -JX) option. As an example, we want to plot a crude world map centered on 125°E. Our command will be

    gmtset GRID_CROSS_SIZE_PRIMARY 0.1i BASEMAP_TYPE FANCY PLOT_DEGREE_FORMAT ddd:mm:ssF
    pscoast -Rg-55/305/-90/90 -Jx0.014i -B60g30f15/30g30f15WSen -Dc -A1000 -Glightgray -Wthinnest -P \
        > GMT_linear_d.ps
    gmtset GRID_CROSS_SIZE_PRIMARY 0

with the result reproduced in Figure 5.2.

Next: 5.1.1.3 Calendar time coordinates Up: 5.1.1 Cartesian Linear Transformation Previous: 5.1.1.1 Regular floating point Contents Index