In ordinary conic projections, only one or two parallels, where the conic and spherical surfaces coincide, have correct scale. However, the map may be divided in strips of similar latitude, each fitted to a different cone. Cone constant varies from one at poles to infinity at the Equator, so the strips are not continuous, except along the central meridian. When infinitely many cones are used, each optimally tangent to a thin strip containing a single parallel, the gaps disappear; if the central meridian has constant correct scale, the result is the classic or common polyconic projection.

The Polyconic Projection is often used in BSB nautical maps for North America and for Brazilian grids. It is also called: American Polyconic.

To convert Polyconic coordinates, the following parameters have to be set:

- False Northing;
- False Easting;
- Latitude of Origin;
- Central Meridian;

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