## Directory of Map 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.

Aspects

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

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 cylindric 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.

Similar projections

Central cylindric projection also cannot show poles, but it is not conformal, and the spacing of parallels changes much more rapidly. Miller cylindric 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.

Origin

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

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.