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| [[File:Van der Grinten projection SW.jpg|250px|thumb|Van der Grinten projection of the world.]]
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| The '''van der Grinten projection''' is a compromise [[map projection]] that is neither [[Map projection#Equal-area|equal-area]] nor [[conformal map projection|conformal]]. It projects the entire Earth into a circle, though the polar regions are subject to extreme distortion. The projection was the first of four proposed by Alphons J. van der Grinten in 1904, and, unlike most projections, is an arbitrary geometric construction on the plane. It was made famous when the [[National Geographic Society]] adopted it as their reference map of the world from 1922 until 1988.<ref>''Flattening the Earth: Two Thousand Years of Map Projections'', John P. Snyder, 1993, pp.258-262, ISBN 0-226-76747-7.</ref>
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| The geometric construction given by van der Grinten can be written algebraically:<ref>[http://pubs.er.usgs.gov/usgspubs/pp/pp1395 ''Map Projections - A Working Manual''], [[United States Geological Survey|USGS]] Professional Paper 1395, John P. Snyder, 1987, pp.239-242</ref>
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| :<math>x = \frac {\pm \pi \left(A\left(G - P^2\right) + \sqrt {A^2 \left(G - P^2\right)^2 - \left(P^2 + A^2\right)\left(G^2 - P^2\right)}\right)} {P^2 + A^2}\,</math>
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| :<math>y = \frac {\pm \pi \left(P Q - A \sqrt{\left(A^2 + 1\right)\left(P^2 + A^2\right) - Q^2} \right)} {P^2 + A^2}</math>
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| where <math>x\,</math> takes the sign of <math>\lambda - \lambda_0\,</math>, <math>y\,</math> takes the sign of <math>\phi\,</math> and
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| :<math>A = \frac {1} {2}|\frac {\pi} {\lambda - \lambda_0} - \frac {\lambda - \lambda_0} {\pi}|</math>
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| :<math>G = \frac {\cos \theta} {\sin \theta + \cos \theta - 1}</math>
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| :<math>P = G\left(\frac {2} {\sin \theta} - 1\right)</math>
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| :<math>\theta = \arcsin |\frac {2 \phi} {\pi}|</math>
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| :<math>Q = A^2 + G\,</math>
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| Should it occur that <math>\phi = 0\,</math>, then
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| :<math>x = \left(\lambda - \lambda_0\right)\,</math>
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| :<math>y = 0\,</math>
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| Similarly, if <math>\lambda = \lambda_0\,</math> or <math>\phi = \pm \pi / 2\,</math>, then
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| :<math>x = 0\,</math>
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| :<math>y = \pm \pi \tan {\theta / 2 }</math>
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| In all cases, <math>\phi\,</math> is the latitude, <math>\lambda\,</math> is the longitude, and <math>\lambda_0\,</math> is the central meridian of the projection.
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| ==See also==
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| {{Portal|Atlas}}
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| * [[List of map projections]]
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| ==References==
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| {{reflist}}
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| === Sources ===
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| * {{cite web|url=http://www.progonos.com/furuti/MapProj/Normal/ProjOth/projOth.html|title=Projections by Van der Grinten, and variations}}
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| {{Map Projections}}
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| [[Category:Cartographic projections]]
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| {{cartography-stub}}
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