Polyconic can refer either to a class of map projections or to a specific projection known less ambiguously as the American Polyconic. Polyconic as a class refers to those projections whose parallels are all non-concentric circular arcs, except for a straight equator, and the centers of these circles lie along a central axis. This description applies to projections in equatorial aspect.
As a specific projection, the American Polyconic is conceptualized as "rolling" a cone tangent to the Earth at all parallels of latitude, instead of a single cone as in a normal conic projection. Each parallel is a circular arc of true scale. The scale is also true on the central meridian of the projection. The projection was in common use by many map-making agencies of the United States from the time of its proposal by Ferdinand Rudolph Hassler in 1825 until the middle of the 20th century.
The projection is defined by:
where is the longitude of the point to be projected; is the latitude of the point to be projected; is the longitude of the central meridian, and is the latitude chosen to be the origin at . To avoid division by zero, the formulas above are extended so that if then and .
- An Album of Map Projections (US Geological Survey Professional Paper 1453), John P. Snyder & Philip M. Voxland, 1989, p. 4.
- Flattening the Earth: Two Thousand Years of Map Projections, John P. Snyder, 1993, pp. 117-122, ISBN 0-226-76747-7.
- Weisstein, Eric W., "Polyconic projection", MathWorld.
- Table of examples and properties of all common projections, from radicalcartography.net
- An interactive Java Applet to study the metric deformations of the Polyconic Projection.
|This cartography or mapping term article is a stub. You can help Wikipedia by expanding it.|