Short answer
The reference ellipsoid and datum used by GPS and the de-facto GNSS standard. Semi-major axis a = 6378137 m, flattening f = 1/298.257223563. Position differences from local datums (NAD83, ETRS89, GDA2020, CGCS2000) range from tens of cm to several metres depending on region.
Detailed explanation
WGS84 is the geodetic datum (reference ellipsoid + origin + orientation) used by GPS, and the de-facto GNSS standard adopted by every other constellation. It models Earth's geoid as an ellipsoid of revolution with semi-major axis a = 6,378,137 metres and flattening f = 1/298.257223563. The first eccentricity squared is e² ≈ 6.694×10⁻³.
WGS84 has been realised several times — WGS84(original) in 1984, then G730 (1994), G873 (1997), G1150 (2002), G1674 (2012), G1762 (2013), G2139 (2021). Each realisation is ~5–10 cm different from the previous; modern receivers use the latest. WGS84(G2139) is now within 1 cm of ITRF2014 at any survey point.
Local datums (NAD83 for North America, ETRS89 for Europe, GDA2020 for Australia, CGCS2000 for China, BJ54 for legacy Chinese surveying, etc.) differ from WGS84 by tens of centimetres to several metres depending on region. NAD83 vs WGS84 is ~1 m apart at the time of writing because NAD83 fixed the North American tectonic plate while WGS84 follows global motion.
GNSS receivers always compute in WGS84 and then transform to whatever datum the local survey requires using grid-shift files or 7-parameter Helmert transformations. The Coordinate Converter tool on this site does the WGS84-frame conversions (lat/lon ↔ UTM ↔ MGRS ↔ ECEF); local-datum transforms are out of scope.
Try it interactively
Coordinate Converter
Open the toolRelated terms
ECEF
A 3-D Cartesian coordinate system with origin at Earth's centre of mass, X axis through the prime meridian / equator, Z axis along Earth's rotation axis. The native frame in which GNSS receivers compute satellite and user positions before projecting to lat/lon.
UTM
A worldwide projection system that divides Earth into 60 6°-wide longitude zones, each projected as a transverse Mercator centred on its meridian. Distances and areas inside one zone are easy linear-metric computations; the projection is valid 80°S to 84°N latitude.
MGRS
A compact alphanumeric grid coordinate system built on top of UTM, used by NATO, US DoD, aviation, and search-and-rescue. A typical MGRS reference like 33TUM1234567890 encodes a position to 1 m precision in 15 characters — survives voice transmission and SMS reliably.
GPS
United States DoD-operated GNSS, fully operational since 1995. Broadcasts civil signals on L1 (1575.42 MHz), L2 (1227.6 MHz), and L5 (1176.45 MHz), plus restricted military M-code on L1 and L2.