SUBROUTINE POLARMAP

 POLARMAP plots the earth land map using a polar projection. The latitude is plotted as a linear radius. This routine will plot the entire northern or southern hemisphere. The transformation from a visible point (in the appropriate hemisphere) latitude/longitude pair (a,b) in degrees to a plotted (x,y) pair is:

        a = b * sgn(r) + a
        x = cos( a * pi / 180) * r / 90 + x0
        y = sin( a * pi / 180) * r / 90 + y0
where
        pi = 3.141592654

CALL POLARMAP(x0,y0,r,a)

x0,y0  (R): pole location (in plot units)
r      (R): radius of equator (in plot units)
            > northern hemisphere
            < southern hemisphere
a      (R): angle of prime meridian from horizontal (deg CCW)