## Geocart Projections

What is a projection?

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central cylindric

perspective cylindric

simple perspective cylindric

Classifications

Cylindrical
Perspective
Neither conformal nor equal area

Graticule

Meridians: Equally spaced straight parallel lines
Parallels: Unequally spaced straight parallel lines, closest near the Equator, but spacing increases poleward at a much greater rate than it does on the Mercator. Perpendicular to meridians.
Poles: Cannot be shown
Symmetry: About any meridian or the Equator

Scale

True along the Equator
Increases with distance from the Equator to infinity at the poles
Changes with direction at any given point, except at the Equator, but scale in a given direction is constant at any given latitude or at the latitude of opposite sign

Distortion

Shape, area, and scale distortion increase rapidly away from the Equator, where there is no distortion.

Other features

Projection is produced geometrically by projecting the Earth's surface perspectively from its center onto a cylinder tangent at the Equator.
Should not be confused with the Mercator, which is not perspective.

Usage

Distortion is too great for any usage except showing the appearance of the Earth when so projected and contrasting with the Mercator.

Origin

Uncertain

Aspects

Normal is described above. Transverse aspect is called the Wetch projection, since it was discussed by J. Wetch in the early 19th century.

Other names

Simple Perspective Cylindrical

Similar projections

Mercator projection, which is not perspective, also cannot show the poles, but the poleward increase in the spacing of the parallels does not occur as rapidly as it does on the Central Cylindrical projection.
Call and other perspective cylindrical projections can be produced by moving the point of perspective away from the center of the Earth. The poles can then be shown.
Gnomonic projection is projected perspectively from the center of the Earth onto a tangent plane rather than a cylinder and is very different in appearance.

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.