5.3.3 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 space. As with Lambert's equal-areal 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 introducted 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 pictoral views at a small scale, only the spherical form is necessary.

To specify the orthographic projection you must supply

$\bullet$

Longitude and latitude of the projection center.

$\bullet$

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$^{o}$W/40$^{o}$N can therefore be generated by the following pscoast command:

#!/bin/sh
#    $Id: GMT_orthographic.sh,v 1.1 2001/03/21 04:10:21 pwessel Exp $
#

pscoast -R0/360/-90/90 -JG-75/40/4.5i -B15g15 -Dc -A5000 -G0 -P > GMT_orthographic.ps

Figure 5.15: Hemisphere map using the Orthographic projection.