## Geocart Projections

What is a projection?

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Mercator

Wright

Classifications

Cylindric
Conformal

Graticule

Meridians: Equally spaced straight parallel lines
Parallels: Unequally spaced straight parallel lines, closest near the Equator, perpendicular to meridians
Poles: Cannot be shown
Symmetry: About any meridian or the Equator

Scale

True along the Equator or along two parallels equidistant from the Equator
Increases with distance from the Equator to infinity at the poles
Constant along any given parallel; same scale at parallel of opposite sign (north +, south -)
Same in all directions near any given point

Distortion

Infinitesimally small circles (indicatrices) of equal size on the globe appear as circles on the map (indicating conformality) but increase in size away from the Equator (indicating area distortion). Great distortion of area in polar regions. Conformality (and therefore local angle preservation) fails at the poles.

Other features

All loxodromes or rhumb lines (lines that make equal angles with all meridians and are therefore lines of constant true bearing) are straight lines.
Meridians can be geometrically projected onto a cylinder, the axis of which is the same as that of the globe. Parallels cannot be geometrically (or perspectively) projected. Meridians cannot be compressed relative to parallels, as they can on Cylindrical Equal Area and Equirectangular projections, since conformality would be lost.

Usage

Designed and recommended for navigational usage because of straight rhumb lines; standard for marine charts
Recommended and used for conformal mapping of regions predominantly bordering the Equator
Often and inappropriately used as a world map in atlases and for wall charts. It presents a misleading view of the world because of the excessive distortion of area.

Origin

Presented by Gerardus Mercator (1512-94) of Flanders in 1569 on a large world map "for use in navigation"

Aspects

Normal is described here. Transverse and Oblique aspects are listed separately because of importance and common treatment as separate projections.

Other names

Wright (rare) (after Edward Wright of England, who developed the mathematics in 1599)

Similar projections

Central Cylindrical projection also cannot show poles, but it is not conformal, and the spacing of parallels changes much more rapidly. Miller Cylindrical projection shows the poles, is not conformal, and has more gradual spacing of parallels. Gall projection shows the poles, is not conformal, and has more gradual spacing of parallels.

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.