What Is a Geodetic Datum?

A geodetic datum is the reference frame that anchors coordinates to the physical Earth: a reference ellipsoid, an origin (typically Earth's centre of mass), an axes orientation, and a gravity-field model. The same latitude and longitude can refer to different physical points under different datums.

A point on the ground has a fixed physical location. Its coordinates do not — they depend on which reference frame is used to describe it. Integrating data across systems requires agreeing on which reference frame each dataset is in, and that frame is the datum. This article covers the four components of a modern geodetic datum, the distinction between horizontal and vertical datums, the major operational datums in use today, how a datum is realized (the network of physical reference stations that actualise the abstract definition), and the transformation arithmetic between them. The companion pillar /learn/coordinate-systems-overview covers the CS-plus-datum split per ISO 19111; /learn/wgs84-explained and /learn/nad83-explained go deep on the two most widely-used datums.

The four components of a datum

Per ISO 19111:2019, a modern geodetic datum is defined by four interdependent parts:

The ellipsoid alone is a shape; the datum fixes that shape against physical Earth. Two datums can share the same ellipsoid and still be distinct — NAD 83, ETRS89, GDA2020, ITRF and JGD2011 all use the GRS80 ellipsoid but have different origins, orientations and realization epochs, and therefore name different physical points with the same nominal coordinates.

Horizontal vs vertical datums

Most coordinate workflows treat horizontal and vertical datums as separable:

A raw GPS receiver outputs ellipsoidal height (the third coordinate in the horizontal datum's 3D form, EPSG:4979 for WGS-84). Engineering, hydrology, aviation and any work where "water flows downhill" matters needs orthometric height instead, which lives in a separate vertical datum and requires a geoid model (EGM2008 global, GEOID18 US, OSGM15 UK, etc.) to convert.

Common horizontal datums

The dominant modern split is global (WGS-84, ITRF) versus continental (NAD 83, ETRS89, GDA2020, JGD2011). Continental datums are fixed to a specific tectonic plate so that coordinates stay numerically stable over time within that plate — an essential property for infrastructure records, deeds and surveys that must remain valid for decades. Global datums instead follow the geocentre and let coordinates drift with the plate they happen to sit on.

Common vertical datums

A height is "height above the reference surface." The reference is typically the geoid (a gravity-equipotential surface that approximately matches mean sea level). Orthometric heights are what civil engineers mean by "elevation"; ellipsoidal heights from raw GPS are geometrically simpler but require a geoid model to convert.

How a datum is realized

The definition of a datum is abstract — ellipsoid, origin, orientation. The realization is the network of physical reference points whose coordinates in that datum are observed, published, and periodically updated. Without realization, the definition has no operational meaning.

Modern realization is overwhelmingly GNSS-based: continuously-operating reference stations log satellite observations 24/7 and their positions are adjusted into the chosen datum at defined epochs. Older realizations (NAD 27, Tokyo Datum 1918, OSGB36) used triangulation networks of physical survey monuments connected by precisely measured angles and baselines; those networks remain in place as legacy references but are no longer the operational source of new coordinates.

Datum transformations

Converting coordinates from one datum to another is a transformation — distinct from a projection, which converts between coordinate systems within the same datum. Two main classes are in operational use:

A casual 7-parameter Helmert between NAD 27 and NAD 83 in CONUS gives metre-scale errors; an NTv2 grid-shift transformation (the NADCON5 / GEOCON workflow that NGS publishes) reduces those to sub-metre. The EPSG registry catalogues both Helmert parameters and grid-shift identifiers for hundreds of datum pairs. The PROJ library and NGS NCAT are the operational tools that consume them.

A worked example: Meades Ranch, Kansas

The NAD 27 origin monument at Meades Ranch (lat ~39.215° N, lon ~98.541° W) is the historical reference point of the old North American datum. Its coordinates in three datums:

The marker itself has not moved. The 28-metre apparent shift comes entirely from the datum change: NAD 27 used the Clarke 1866 ellipsoid with the topocentric Meades Ranch origin; NAD 83 uses GRS80 with a geocentric origin. Same monument, different reference frames, different numbers.

For the more common modern transformation — WGS-84 to NAD 83 in CONUS — the typical offset is 1-2 m. Both datums were intentionally aligned closely in their original definitions, and the 1-2 m gap is the cumulative drift of the North American plate against the global geocentric frame since 1986.

When datums matter

For everyday consumer-GPS navigation, datum choice rarely matters: the ~4.9 m positioning error of civilian GPS (SPS PS 2020) swamps the 1-2 m inter-datum offset. Three classes of work are different:

The NGS NCAT tool is the US authoritative service for any horizontal or vertical transformation that crosses datums. The PROJ library is the open-source operational tool for the rest of the world.

A short history of geodetic datums

Common misconceptions

Related pillars

The other seven pillar concepts on Coordinately:

• What is latitude and longitude? — the angular coordinates a datum anchors
• Coordinate formats explained — every format silently references a datum
• How GPS works — broadcasts in WGS-84, the canonical modern datum
• What is a map projection? — projections start from a datum
• Time zones explained — civil-time bands anchored to longitudes in a datum
• History of latitude and longitude — the evolution of datums from Ptolemy to ITRF
• Great-circle distance — Vincenty's formula on the WGS-84 ellipsoid