The Gnomonic azimuthal projection is a perspective projection from the center onto a plane tangent to the surface. Its origin goes back to the old Greeks who used it for star maps almost 2500 years ago. The projection is neither equal-area nor conformal, and much distortion is introducted near the edge of the hemisphere; in fact, less than a hemisphere may be shown around a given center. The directions from the center of projection are true. Great circles project onto straight lines. Because it is mainly used for pictoral views at a small scale, only the spherical form is necessary.
To specify the Gnomonic projection you must supply:
Using a horizon of 60, our example of this projection centered on 120W/35N can therefore be generated by the following pscoast command:
pscoast -Rg -JF-120/35/60/4.5i -Bg15 -Dc -A10000 -Glightgray -W0.25p -P > GMT_gnomonic.ps