5.1.1.2 Geographic coordinates

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5.1.1.2 Geographic coordinates

**Figure 5.2:** Linear transformation of map coordinates.
$\begin{figure}\centering\epsfig{figure=eps/GMT_linear_d.eps}\end{figure}$

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 $^{o}$ periodicity. GMT therefore needs to be informed that it has been given geographical data although a linear transformation 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

gmtset GRID_CROSS_SIZE_PRIMARY 0.1i BASEMAP_TYPE FANCY PLOT_DEGREE_FORMAT ddd:mm:ssF
pscoast -R-55/305/-90/90 -Jx0.014id -B60g30f15/30g30f15WSen -Dc -A1000 -Glightgray -W0.25p -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

Paul Wessel 2004-10-01