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, append /origin 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.
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.