This transformation converts polar coordinates (angle and radius
)
to positions on a plot. Now
and
, hence it is similar
to a regular map projection because
and
are coupled and
(i.e.,
) has a 360
periodicity.
With input and output points both in the plane it is a two-dimensional projection.
The transformation comes in two flavors:
Consequently, the polar transformation is defined by providing
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 --PLOT_DEGREE_FORMAT=+ddd > GMT_polar.ps rm -f test.grd
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 5.6.