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:

- Longitude and latitude of the projection center.
- The horizon, i.e., the number of degrees from
the center to the edge. This must be 90.
- Scale as 1:xxxxx or as radius/latitude where
radius is distance on map in inches from projection center to
a particular [possibly oblique] latitude (
**-Jf**), or map width (**-JF**).

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