Geocart Projections

What is a projection?

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perspective conic

Parameters: Latitude of origin, Standard parallel 1, Standard parallel 2

Projection Classifications

Geocart menu class: Conic
Conic
Perspective
Neither conformal nor equal area

Graticule

Meridians: Equally spaced straight lines converging at a common point, which is one of the poles. The angles between them are less than the true angles.
Parallels: Unequally spaced concentric circular arcs centered on the pole of convergence of the meridians. The meridians are therefore radii of the circular arcs. Spacing of parallels increases away from the central latitudes.
Poles: One pole is a point; the other pole cannot generally be shown, but, under some conditions, it is a circular arc
Symmetry: About any meridian

Scale

True along one or two chosen standard parallels, which may be on the same side of or both sides of the Equator
Scale is constant along any given parallel.

Distortion

Free of distortion only along the one or two standard parallels. Distortion is constant along any given parallel but changes more rapidly in a north-south direction than it does on conformal or equal-area conics, and the Perspective Conic has no compensating advantage. Other projections should be used instead.

Other features

Projection is produced geometrically by projecting the Earth perspectively from the center (or from some other point) onto a cone tangent or secant along the standard parallels.

Limiting forms

Polar Gnomonic projection, if the pole is the single standard parallel. The cone of projection thereby becomes a plane. Central Cylindrical projection, if the Equator is the single standard parallel. The cone of projection thereby becomes a cylinder. Standard conic formulas must be rewritten.

Similar projection

A stereographic perspective conic was presented by Carl Braun in 1867. The map is projected from the South Pole onto a cone tangent at latitude 30° N.

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.