Geocart Projections

What is a projection?

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tilted perspective

Parameters: Azimuth intersecting top in degrees, Height above surface in kms, Tilt from vertical in degrees

Classifications

Geocart menu class: Azimuthal
Modified azimuthal
Perspective
Neither conformal nor equal area

Graticule

Meridians: Central meridian is straight; the point of perspective is in its plane, but this meridian may not appear on the map. Other meridians are normally elliptical arcs, but, for certain angles of tilt or centers, they may be straight lines, parabolas, or hyperbolas.
Parallels: One parallel, normally nearer to the pole than is the center of projection (depending on the position of the point of perspective), is a straight line. Other parallels are normally elliptical arcs but, for certain angles of tilt, may be parabolas or hyperbolas.
Poles: Points
Symmetry: Normally none, but there is symmetry about the central meridian if the direction of view is north or south

Range

Less than one hemisphere if the point of perspective is above the surface

Scale

Normally varies widely if a large area is covered

Distortion

Substantial distortion of shape, area, and scale if a large area is covered

Special features

Projection shows the Earth geometrically as it appears when photographed or otherwise imaged onto a plane from any point in space with any pointing of the “camera”.

Usage

Pictorial views of the Earth, resembling those seen from space

Origin

Developed primarily during the 20th century to relate to aerial and space photography and photogrammetry

Limiting forms

General Vertical Perspective projection
Orthographic projection, if the point of perspective is at infinite distance

Description adapted from J.P. Snyder and P.M. Voxland, An Album of Map Projections, U.S. Geological Survey Professional Paper 1453. United States Government Printing Office: 1989.