The American Polyconic projection§ was originated by Ferdinand Rudolph Hassler about 1820. It became the nearly exclusive standard for large-scale mapping of the United States through the late 1950's.  As its name implies, the Polyconic projection is derived from "many cones," indeed infinitely many in principle, as a tangent cone is calculated for each point represented on the map.  Parallels of latitude are arcs of nonconcentric circles, except for the straight Equator. In the spherical form, as shown here, parallels are equidistant along any given meridian.  Meridians are complex curves, except for the straight central meridian, chosen here at 30° West longitude near the center of the North Atlantic Ocean.  Although the Polyconic projection is neither conformal nor equal area, it had a great advantage in a time when maps were compiled by hand using lookup tables: a table of rectangular coordinates calculated for a single quadrangle is universal in that it may be used for any quadrangle of the same ellipsoid merely by applying the proper scale and central meridian.

The manner of geometrical construction, with parallels that arc gently towards the poles, also makes it perfectly suited to use for globe gores.

Projected by Ptolemyä from the Central Intelligence Agency Digital Chart of the World.

§Source:  John P. Snyder, Map Projections--A Working Manual; U.S.G.S Professional Paper 1395, U.S. Government Printing Office: Washington, D.C. 1987, pp. 124-137.

Polyconic

Polyconic 30° West Copyright © 1993 W. Murray Sexton.  All rights reserved.

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