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## 5.1.5 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)
  Optionally, insert a after pP to indicate CW azimuths rather than CCW directions
  Optionally, append / in degrees to indicate an angular offset [0]

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.

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

    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: 5.2 Conic Projections Up: 5.1 Non-map Projections Previous: 5.1.4 Geographical linear projection Contents Index