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## 6.2.3 Perspective projection (-Jg -JG)

The perspective projection imitates in 2 dimensions the 3-dimensional view of the earth from space. The implementation in GMT is very flexible, and thus requires many input variables. Those are listed and explained below, with the values used in Figure 6.11 between brackets.

- Longitude and latitude of the projection center (4°E/52°N).
- Altitude of the viewer above sea level in kilometers (230 km). If this value is less than 10, it is assumed to be the distance of the viewer from the center of the earth in earth radii. If an ``r'' is appended, it is the distance from the center of the earth in kilometers.
- Azimuth in degrees (90°, due east). This is the direction in which you are looking, measured clockwise from north.
- Tilt in degrees (60°). This is the viewing angle relative to zenith. So a tilt of 0° is looking straight down, 60° is looking from 30° above the horizon.
- Twist in degrees (180°). This is the boresight rotation (clockwise) of the image. The twist of 180° in the example mimics the fact that the Space Shuttle flies upside down.
- Width and height of the viewpoint in degrees (60°). This number depends on whether you are looking with the naked eye (in which case you view is about 60° wide), or with binoculars, for example.
- 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) (5 inches).

The imagined view of northwest Europe from a Space Shuttle at 230 km looking due east is thus accomplished by the following pscoast command:

    pscoast -Rg -JG4/52/230/90/60/180/60/60/5i -B2g2/1g1 -Ia -Di -Glightgray -Wthinnest -P \
            --ANNOT_MIN_SPACING=0.25i > GMT_perspective.ps

Figure 6.11: View from the Space Shuttle in Perspective projection.

Next: 6.2.4 Orthographic projection (-Jg Up: 6.2 Azimuthal projections Previous: 6.2.2.3 General stereographic map Contents Index