What Is a Gnomonic Projection?

A gnomonic projection is a non-conformal map projection obtained by projecting points on the surface of sphere from a sphere's center to points in a tangent plane. Developed by Thales in the 6th century B.C., it is considered the oldest map projection and is also called a great-circle chart.

A gnomonic map projection displays all great circles as straight lines; thus, any line segment on a gnomonic map shows the shortest route between the segment's two endpoints. No distortion occurs at the tangent point, but distortion increases rapidly away from it. The gnomonic projection is valuable for navigation when used in conjunction with the Mercator projection, and it is very useful in plotting great circle routes between arbitrary destinations.