How it works
A cone touches the sphere along one parallel, the standard parallel. Cut along the meridian
opposite the central one and laid flat, it becomes a sector of a circle: the cone constant is
the fraction of the circle it fills, and it equals the sine of the standard parallel.
Move the standard parallel to the equator and the cone becomes a cylinder; move it to a pole and
it becomes a plane. Each property is one family across all three surfaces:
| Property | Cylinder | Cone | Plane |
|---|
| Equidistant | eqc | eqdc | aeqd |
| Conformal | merc | lcc | stere |
| Equal-area | cea | aea | laea |
Unlike the perspective projections there is no point of light: the property decides how far
along the surface each parallel lands. The links join points on the globe to where they land
on the surface. Unroll opens the cone with its apex fixed, the way a paper cone opens.
The flat map is drawn by proj4js from the PROJ
string, so it checks the construction.