**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 projection 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:

x
=
cot
φ
sin
(
(
λ
−
λ
0
)
sin
φ
)
y
=
φ
−
φ
0
+
cot
φ
(
1
−
cos
(
(
λ
−
λ
0
)
sin
φ
)
)
where *λ* is the longitude of the point to be projected; *φ* is the latitude of the point to be projected; *λ*_{0} is the longitude of the central meridian, and *φ*_{0} is the latitude chosen to be the origin at *λ*_{0}. To avoid division by zero, the formulas above are extended so that if *φ*_{0} = 0 then *x* = *λ* − *λ*_{0} and *y* = 0.