Geographic coordinate system

A geographic coordinate system identifies a position by two angular coordinates: latitude and longitude. Latitude measures angular distance north or south of the equator, from 0° at the equator to 90° at either pole; longitude measures angular distance east or west of a selected prime meridian, commonly extending to 180° in either direction.¹

The coordinates refer to a mathematical model of Earth rather than to a perfectly spherical surface. Modern systems generally use an oblate ellipsoid, whose dimensions and orientation are defined by a geodetic datum. A complete coordinate reference therefore includes more than a pair of numbers: it specifies the datum, ellipsoid, prime meridian, coordinate order, and units. The same numerical latitude and longitude can identify slightly different physical locations when expressed in different datums.

A geodetic datum determines how geographic coordinates are anchored to Earth. Global datums such as WGS 84 approximate the planet with a reference ellipsoid and geocentric orientation, while regional datums may be fitted more closely to a particular continent or survey network.

Latitude can be geodetic, measured relative to the ellipsoid normal, or geocentric, measured relative to a line from Earth’s center. These values differ except at the equator and poles. A third quantity, astronomical latitude, is derived from the direction of gravity and can differ because Earth’s mass distribution produces a nonuniform gravity field. Height is a separate issue: ellipsoidal height is measured from the reference ellipsoid, whereas orthometric height approximates elevation above mean sea level.

Geographic coordinates may be written in decimal degrees or in degrees, minutes, and seconds. For example, 40.5° north is equivalent to 40° 30′ 00″ N. Conventionally, north and east are positive and south and west are negative, although software and datasets can use explicit directional letters. Coordinate order is not universal: some standards list latitude before longitude, while many programming interfaces use longitude before latitude.

Coordinates can be transformed between datums or converted to a projected coordinate system for mapping and measurement. Projection changes are necessary when a curved geographic surface is displayed on a plane, and every map projection introduces characteristic distortion in distance, area, shape, or direction. Precision should match the measurement: extra decimal places do not create greater positional accuracy, particularly when the original observation, datum, or transformation is uncertain.

Several edge cases make geographic coordinates less intuitive than a simple grid. Longitude is discontinuous at the antimeridian, so nearby points can have values near +180° and −180°; software that sorts longitudes numerically may incorrectly place them far apart. Latitude and longitude also describe a curved, nonuniform grid: lines of longitude converge toward the poles, so one degree of longitude spans progressively less ground distance with increasing latitude.

Coordinate reference systems can be three-dimensional or time-dependent. Plate motion changes the position of surveyed points, so high-precision geodesy may require an epoch as well as a datum. Satellite navigation commonly supplies ellipsoidal height, while civil engineering and topographic work often needs a gravity-related height derived through a vertical datum. Standards registries such as the EPSG dataset provide identifiers and parameters that help software exchange these definitions without relying on ambiguous names.

Latitude

Angular coordinate measuring position north or south of the equator.

Longitude

Angular coordinate measuring position east or west of a prime meridian.

Geodetic datum

A defined reference framework, including an ellipsoid and its position and orientation, used to assign coordinates.

Ellipsoid

A mathematical surface approximating the shape of Earth for geodetic computation.

Prime meridian

The meridian assigned zero longitude in a coordinate reference system.

Antimeridian

The meridian approximately opposite the prime meridian, near 180° longitude.

Epoch

A specified date or time used to define coordinates when positions change through tectonic motion.