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## Linear Projection with Polar () Coordinates (-–Jp -–JP)

Figure 5.5: Polar (Cylindrical) transformation of () coordinates

In many applications the data is better described in polar or cylindrical (, r) coordinates rather than the usual Cartesian coordinates (x, y). The relationship between the Cartesian and polar coordinates are described by . The polar transformation is simply defined by providing

scale in inches/unit (-Jp) or full width of plot in inches (-JP).

As an example of this projection we will create a gridded data set in polar coordinates using grdmath , a RPN calculator that operates on or creates grdfiles.

    grdmath -R0/360/2/4 -I6/0.1 X 4 MUL PI MUL 180 DIV COS Y 2 POW MUL = test.grd
    grdcontour test.grd -JP3i -B30Ns -P -C2 -S4 >! GMT_polar.ps

We used grdcontour to make a contour map of this data. Because the data file only contains values with , a donut shaped plot appears in Figure 5.5.

Next: Conic Projections Up: Non-map Projections Previous: Geographical linear projection