WGS 84

WGS 84 (World Geodetic System 1984) is the global reference frame that underpins the Global Positioning System (GPS) and serves as the default coordinate reference system for the vast majority of geospatial data in use today. When you see coordinates expressed as latitude and longitude — whether on a smartphone, in a GeoJSON file, or in satellite imagery metadata — they are almost certainly in WGS 84. Its EPSG code is 4326, one of the most frequently referenced identifiers in geospatial computing.

WGS 84 defines two things: a reference ellipsoid (a mathematical model of the Earth's shape) and a coordinate system anchored to it. The ellipsoid has a semi-major axis of 6,378,137 meters and a flattening of 1/298.257223563. Positions are expressed as geodetic latitude (angle north or south of the equator), geodetic longitude (angle east or west of the Prime Meridian at Greenwich), and ellipsoidal height (distance above the ellipsoid surface). This system provides a consistent global framework — the same coordinate always refers to the same point on Earth, regardless of which country you are in or which software you are using.

WGS 84 is maintained and periodically refined by the U.S. National Geospatial-Intelligence Agency (NGA). The reference frame is realized through a global network of GPS monitoring stations whose precise positions are known. The current realization (G2139, adopted in 2021) aligns WGS 84 to the International Terrestrial Reference Frame (ITRF) at the centimeter level.

Because the Earth's tectonic plates move several centimeters per year, the positions of points on the surface are constantly changing relative to any fixed reference frame. WGS 84 is an Earth-Centered, Earth-Fixed (ECEF) frame — its origin is at the Earth's center of mass, and its axes are fixed to the rotating Earth. For most practical purposes, WGS 84 coordinates are stable enough that users do not need to account for tectonic motion. However, for centimeter-precision applications (geodetic surveying, InSAR), the epoch (time of measurement) matters because WGS 84 coordinates shift by millimeters to centimeters per year due to plate tectonics.

The WGS 84 ellipsoid is not the same as the geoid (the equipotential gravity surface that approximates mean sea level). GPS heights are measured relative to the ellipsoid, not sea level. Converting between ellipsoidal height and orthometric height (above the geoid) requires a geoid model such as EGM2008 or EGM96. This distinction matters for elevation data: a GPS receiver reporting 100 m height is measuring from the ellipsoid, which may differ from sea level by ±100 m depending on location.

WGS 84 coordinates are in degrees, which cannot be used directly for metric distance or area calculations because the length of a degree varies with latitude (a degree of longitude is ~111 km at the equator but 0 km at the poles). For measurement, data must be projected to a local CRS like UTM. The ellipsoidal height from GPS does not correspond to sea level — converting to orthometric height requires a geoid model. For centimeter-precision work, the specific WGS 84 realization and epoch matter because of tectonic plate motion. Some legacy datasets use older WGS 84 realizations that differ from the current one by up to 2 meters.

The U.S. Department of Defense developed WGS 84 in 1984 to replace WGS 72 as the reference system for military mapping and navigation. It was designed as a global geocentric datum compatible with satellite positioning. When GPS was opened to civilian use in the 1980s and selective availability was removed in 2000, WGS 84 became the universal coordinate system for consumer navigation. The rise of web mapping (Google Maps, 2005) and the GeoJSON specification (RFC 7946, 2016) cemented WGS 84 as the default CRS for digital geospatial data. The datum has been refined multiple times — G730 (1994), G873 (1997), G1150 (2002), G1762 (2013), G2139 (2021) — each improving alignment with ITRF.