Gnomonic Projection



An azimuthal projection that is a perspective projection and is neither conformal nor equal-area. Also called the Gnomic or Central projection and was known as the horologium (meaning "sundial" or "clock") in earlier times.




Scale true at the center only. Directions from the center are true.




No distortion at the center only. Distortion and scale rapidly increase away from the center.




The only useful feature of the Gnomonic projection is that all great-circle arcs project as straight lines on this projection. The scale is badly distorted along such a plotted great circle, but the route is precise.


Except at the center, distortion of shape, area and scale on the Gnomonic projection is so great that it has seldom been used for atlas maps. Generally, the projection is used for plotting great-circle paths.


Several sets of star maps from the late 18th century and some terrestrial maps of 1803 used this projection with the sphere projected onto the six faces of a tangent cube. Also used from the mid-16th to the mid-20th centuries to project the terrestrial globe onto the faces of other polyhedra.




Use only for a single hemisphere.




Used by Thales of Miletus (636? - 546? BC) for star maps.


Limiting Forms


Used in Spherical form only.