A geographic coordinate system, commonly abbreviated GCS, is a reference system used to specify the location of places and features on the Earth using angular coordinates rather than ordinary linear distances. Its fundamental coordinates are latitude and longitude, and a three dimensional geographic system may also include ellipsoidal height. Latitude describes angular position north or south of the Equator, while longitude describes angular position east or west of a defined prime meridian.
The Equator is assigned 0 degrees latitude, and latitude extends to 90 degrees north and 90 degrees south. Longitude is normally measured from 0 degrees at the Greenwich Meridian to 180 degrees east or west. A coordinate such as 51.5074 degrees N, 0.1278 degrees W therefore identifies a position by its angular relationship to the reference surfaces and axes of the coordinate system. A GCS is not itself a map projection. It describes positions on a curved reference surface, whereas a projected coordinate system converts those positions to a planar map using a mathematical projection.
A geographic coordinate system depends on a geodetic datum or reference frame because latitude and longitude have meaning only in relation to a specified model and orientation of the Earth. Modern systems normally use a reference ellipsoid, which approximates the Earth's overall shape mathematically. For example, WGS 84 is a globally used geodetic reference system associated with the World Geodetic System and is widely used by GPS and civilian positioning software.
NOAA describes WGS 84 as commonly used within civilian GPS software, while the International Terrestrial Reference Frame provides a global reference framework for highly precise geodetic work. Different datums can assign slightly different latitude, longitude and height values to the same physical point because their origins, orientations, ellipsoids, reference epochs and realizations differ. Consequently, a coordinate without its datum or reference system can be incomplete for high accuracy work.
Latitude and longitude are angular quantities, so they are fundamentally different from metres, feet or other linear coordinate units. One degree of longitude does not represent the same ground distance everywhere because meridians converge toward the poles. At the Equator, one degree of longitude corresponds to roughly 111 kilometres, while at the poles the longitudinal distance represented by one degree approaches zero.
A degree of latitude has a much more consistent but not perfectly constant physical length because Earth is an oblate ellipsoid rather than a perfect sphere. Degrees can be subdivided into decimal degrees or degrees, minutes and seconds. Decimal notation is especially common in digital geographic information systems. A positive latitude conventionally represents north and a negative latitude south, while positive longitude can represent east and negative longitude west, although other longitude conventions are possible. These numerical conventions allow databases, navigation systems, surveying instruments and geographic information systems to represent locations consistently.
A GCS should be distinguished from a coordinate reference system in the broader sense. A coordinate reference system can be geographic, projected or geocentric, whereas a geographic coordinate system specifically uses angular coordinates associated with a curved reference surface. It should also be distinguished from a datum, although the terms are sometimes used loosely. A datum establishes the geodetic reference framework to which coordinates relate, while a geographic coordinate system specifies how geographic position is represented within that framework.
In practical GIS terminology, software often stores a complete coordinate reference system containing a datum, ellipsoid, coordinate axes, units and related definitions. EPSG codes are commonly used to identify standardized coordinate reference systems. WGS 84 geographic coordinates are commonly identified through EPSG terminology as EPSG:4326, although the broader WGS 84 family contains time dependent realizations that matter in high precision applications.
Geographic coordinate systems are fundamental to cartography, geographic information systems, satellite navigation, remote sensing, surveying, geocoding, environmental science, meteorology and spatial databases. A road can be represented as a sequence of latitude and longitude coordinates, a weather observation can be assigned to a geographic position, a satellite image can be georeferenced to geographic coordinates, and a property boundary can be associated with surveyed positions.
GPS receivers determine positions in a global geodetic reference framework and commonly expose those positions as latitude and longitude. Web mapping systems frequently transform geographic coordinates into a projected coordinate system for display because flat computer screens require planar coordinates. A GIS can therefore store data in a geographic coordinate system and dynamically project it for visualization. The distinction matters because calculations involving distance, area and direction are often more appropriate in a suitable projected or geodesic framework than by treating latitude and longitude as ordinary Cartesian x and y coordinates.
The historical development of geographic coordinate systems is closely connected with astronomy, navigation, surveying and geodesy. Ancient Greek scholars developed systematic geographical concepts involving latitude and longitude, and Claudius Ptolemy's Geographia provided an influential coordinate based framework for describing locations. Longitude was historically difficult to determine accurately because it requires accurate time or equivalent astronomical information, whereas latitude can be determined more directly from observations of celestial objects.
The development of accurate clocks, astronomical techniques, triangulation, marine chronometers, radio navigation, electronic surveying and satellite positioning progressively improved coordinate determination. Modern systems no longer depend primarily on local astronomical observations and terrestrial triangulation. Global satellite observations provide highly precise measurements tied to Earth centred reference frames, allowing geographic coordinates to be maintained consistently across continents and oceans.
Geographic coordinates also require attention to longitude conventions, coordinate order and precision. Some software writes longitude first and latitude second, while other systems use latitude first. A coordinate written as 51.5, minus 0.1 can therefore be misinterpreted if the software expects a different order. The antimeridian near 180 degrees longitude creates additional complications because a geographic feature can cross the numerical boundary between positive and negative longitude.
The poles create another special case because longitude becomes mathematically undefined at the exact poles. Precision also has practical limits. Recording many decimal places does not automatically make a coordinate more accurate because the true uncertainty depends on the measurement technique, reference frame, atmospheric effects, receiver quality, survey method and data processing. Coordinate metadata is therefore essential when geographic information is exchanged between systems.
A geographic coordinate system ultimately provides a standardized mathematical language for describing where something is on Earth. Its apparent simplicity hides several layers of geodetic definition. Latitude and longitude require an origin, axes, angular units, reference ellipsoid and datum or reference frame, and modern high precision systems may additionally require an epoch because tectonic plates and other parts of the Earth's surface move. A coordinate is therefore not merely a pair of numbers independent of context.
The same physical feature can receive different numerical coordinates under different reference systems, and coordinates may change over time when expressed in a dynamic reference frame. For ordinary mapping these differences may be negligible, but for surveying, engineering, crustal deformation monitoring, satellite positioning and scientific research they can become significant. Geographic coordinate systems are consequently best understood as structured reference frameworks that connect numerical coordinates to a physical model and realization of the Earth.
