Geographic Coordinate System

A geographic coordinate system is a mathematical method for specifying any location on Earth's surface using two or three numbers. The two most common are latitude and longitude, while a third number can be altitude when precise three-dimensional positioning is required.

This system forms the basis of cartography, navigation, aviation, and GPS devices. Without a clear definition of Earth's shape and a starting point for measurement, coordinate numbers cannot be reliably compared.

Latitude and longitude

Latitude measures whether a location lies north or south of the equator. It ranges from 0° at the equator to 90° north (the North Pole) and 90° south. Longitude measures whether a location lies east or west of the prime meridian, conventionally placed at Greenwich in London, and ranges from 0° to 180° east and 180° west. Both can be expressed in degrees, minutes, and seconds (for example 2° 02′ N, 45° 20′ E) or as decimal degrees (2.03, 45.34).

Datum and Earth's shape

Earth is not a perfect sphere; it resembles an ellipsoid, flattened at the poles. For this reason coordinate systems depend on a datum—a model that defines the size and position of the ellipsoid being used. The global datum used by GPS is WGS 84. The same location can have different coordinate values if different datums are used, which introduces measurement error if the data is not transformed.

Converting between flat maps

When spherical coordinates are displayed on a flat map, a projection (such as Mercator or UTM) must be applied. Each projection preserves one property—shape, area, distance, or direction—because it is impossible to preserve all of them simultaneously. Modern maps and GIS software typically specify which datum and projection have been used.