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The Gall–Peters projection, named after James Gall and Arno Peters, is one specialization of a configurable equalarea map projection known as the equalarea cylindric or cylindrical equalarea projection. The Gall–Peters achieved considerable notoriety in the late 20th century as the centerpiece of a controversy surrounding the political implications of map design. Maps based on the projection continue to see use in some circles and are readily available, though few major map publishers produce them.
Contents
Description
Formulae
The projection is conventionally defined as:
where λ is the longitude from the central meridian in degrees, φ is the latitude, and R is the radius of the globe used as the model of the earth for projection. For longitude given in radians, remove the π/180° factors.
Simplified formula
Stripping out unit conversion and uniform scaling, the formulae may be written:
where λ is the longitude from the central meridian (in radians), φ is the latitude, and R is the radius of the globe used as the model of the earth for projection. Hence the sphere is mapped onto the vertical cylinder, and the cylinder is stretched to double its length. The stretch factor, 2 in this case, is what distinguishes the variations of cylindric equalarea projection.
Discussion
The various specializations of the cylindric equalarea projection differ only in the ratio of the vertical to horizontal axis. This ratio determines the standard parallel of the projection, which is the parallel at which there is no distortion and along which distances match the stated scale. There are always two standard parallels on the cylindric equalarea projection, each at the same distance north and south of the equator. The standard parallels of the Gall–Peters are 45° N and 45° S. Several other specializations of the equalarea cylindric have been described, promoted, or otherwise named.^{[1]}^{[2]}^{[3]}
Origins and naming
The Gall–Peters projection was first described in 1855 by clergyman James Gall, who presented it along with two other projections at the Glasgow meeting of the British Association for the Advancement of Science (the BA). He gave it the name "orthographic" (no relation to the Orthographic projection), and formally published his work in 1885 in the Scottish Geographical Magazine.^{[4]}
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