6.2.5 Gnomonic Projection (-Jf -JF)

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:

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Longitude and latitude of the projection center.

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The horizon, i.e., the number of degrees from the center to the edge. This must be $<$ 90$^{o}$.

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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$^{o}$, our example of this projection centered on 120$^{o}$W/35$^{o}$N 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

Figure 6.12: Gnomonic azimuthal projection.

TM