Geographic coordinate system

A geographic coordinate system (GCS) uses latitude and longitude to describe positions on Earth. It is the oldest and most common type of spatial reference system, and many others are built upon it. Unlike a Cartesian system, a GCS is not planar; its coordinates are angles, not distances.

The system's invention is credited to Eratosthenes of Cyrene, who wrote his *Geography* in the 3rd century BC. Hipparchus of Nicaea later improved it by using stars for latitude and lunar eclipses for longitude. In the 1st or 2nd century, Marinus of Tyre created a world map with coordinates measured from a prime meridian at the Fortunate Isles (near the Canary or Cape Verde Islands) and a latitude baseline at Rhodes. Ptolemy’s 2nd-century *Geography* kept that prime meridian but measured latitude from the Equator. After translation into Arabic in the 9th century, Al-Khwārizmī corrected errors in the Mediterranean’s length, shifting the prime meridian about 10° east in medieval Arabic cartography. Ptolemy’s text was recovered by Maximus Planudes around 1300 and translated into Latin in Florence by Jacopo d’Angelo around 1407. In 1884, the International Meridian Conference in the United States saw 22 of 25 nations adopt the Royal Observatory in Greenwich as the zero meridian; the Dominican Republic voted against, while France and Brazil abstained. France switched to Greenwich Mean Time in 1911.

Latitude is defined as the angle between the equatorial plane and a line from the surface point to a second point on that plane. The second point varies: in an astronomical system, it is where the plumb bob vertical meets the plane; in a geodetic system, it is where the ellipsoid’s normal vector meets the plane; in a geocentric system, it is Earth’s center. Lines of equal latitude are called parallels, with the equator at 0° and the poles at 90° N and 90° S. Longitude is the angle east or west of a reference meridian. All meridians are halves of great ellipses converging at the poles. The international prime meridian runs through Greenwich, though some organizations use others. The 180° meridian is the antipode of Greenwich, but the International Date Line diverges from it in places for political reasons. Together, latitude and longitude specify any location on Earth’s surface (ignoring altitude). The grid they form is a graticule; its origin, at 0° latitude and 0° longitude in the Gulf of Guinea, is sometimes called Null Island.

To measure actual locations, a geodetic datum is required. A horizontal datum binds a reference ellipsoid to Earth for latitude and longitude; a vertical datum uses a geoid for elevation. Traditionally, these datums relied on networks of surveyed control points and were accurate only regionally. Newer datums are global.