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## 6.2.4 Orthographic projection (-Jg -JG)

The orthographic azimuthal projection is a perspective projection from infinite distance. It is therefore often used to give the appearance of a globe viewed from outer space. As with Lambert's equal-area and the stereographic projection, only one hemisphere can be viewed at any time. The projection is neither equal-area nor conformal, and much distortion is introduced near the edge of the hemisphere. The directions from the center of projection are true. The projection was known to the Egyptians and Greeks more than 2,000 years ago. Because it is mainly used for pictorial views at a small scale, only the spherical form is necessary.

To specify the orthographic projection the same options -Jg or -JG as the perspective projection are used, but with fewer variables to supply:

- Longitude and latitude of the projection center.
- Optionally, the horizon, i.e., the number of degrees from the center to the edge (90°, default is 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 (-Jg), or map width (-JG).

Our example of a perspective view centered on 75°W/40°N can therefore be generated by the following pscoast command:

    pscoast -Rg -JG-75/41/4.5i -B15g15 -Dc -A5000 -Gblack -P > GMT_orthographic.ps

Figure 6.11: Hemisphere map using the Orthographic projection.

Next: 6.2.5 Azimuthal Equidistant projection Up: 6.2 Azimuthal projections Previous: 6.2.3 Perspective projection (-Jg Contents Index