A geographic coordinate system enables every location on the Earth to be specified in three coordinates, using mainly a spherical coordinate system. The Earth is not a sphere, but an irregular shape approximating an ellipsoid; the challenge is to define a coordinate system that can accurately state each topographical feature as an unambiguous set of numbers.

Latitude is the angle from a point on the Earth's surface and the equatorial plane, measured from the centre of the sphere. The North Pole is 90° N; the South Pole is 90° S. The 0° parallel of latitude is designated the equator. The equator is the fundamental plane of all geographic coordinate systems. The equator divides the globe into Northern and Southern Hemispheres. Longitude is the angle east or west of a reference meridian between the two geographical poles to another meridian that passes through an arbitrary point. All meridians are halves of great circles, and are not parallel. They converge at the North and South poles.