Geodetic datum
A reference frame for unambiguously representing Earth positions.
A geodetic datum (or geodetic system) is a global reference frame for unambiguously representing positions on Earth using geodetic coordinates or geocentric coordinates. Datums are crucial to technologies based on spatial location, including geodesy, navigation, surveying, geographic information systems, remote sensing, and cartography. A horizontal datum measures horizontal position in latitude and longitude; a vertical datum measures elevation or depth relative to a standard origin such as mean sea level; a three-dimensional datum unifies both.
A standard datum specification includes a model for Earth’s shape and dimensions, such as a reference ellipsoid or geoid; an origin tied to a known, often monumented, location on or inside Earth; and multiple precisely measured control points physically monumented. Coordinates of other places are then determined by surveying from the nearest control point. Because the ellipsoid or geoid, origins, and spatial orientation differ between datums, the relationship between coordinates in different datums is undefined and can only be approximated. This disparity, called datum shift or datum transformation, can cause points with identical coordinates in two datums to be kilometers apart on the ground if far from either origin. Since the rise of GPS, the ellipsoid and datum WGS 84 have supplanted most others in many applications, as it is intended for global use. However, because Earth is an imperfect ellipsoid, local datums can represent a specific area more accurately than WGS 84; for example, OSGB36 better approximates the geoid over the British Isles. Nevertheless, the advantages of a global system have led to widespread adoption of WGS 84.
Historically, the spherical nature of Earth was known to ancient Greeks, who developed latitude, longitude, and early astronomical measurement methods. These methods, preserved by Muslim and Indian astronomers, sufficed for global exploration. The Age of Enlightenment revealed measurement errors and demanded greater precision, leading to innovations like the marine chronometer and a reconsideration of Earth’s shape. Isaac Newton proposed an oblate shape, while early surveys suggested a prolate one; later French geodesic missions to Lapland and Peru confirmed Newton’s oblate model and revealed gravity variations that led to the geoid concept. Trigonometric surveys, beginning in Franc
- field
- Geodesy, Surveying, Cartography
- known_for
- Providing a reference frame for spatial location on Earth and other celestial bodies
- types
- Horizontal, vertical, and three-dimensional datums
- global_example
- WGS 84, used by GPS and intended for global use
- local_example
- OSGB36, a better approximation for the British Isles than WGS 84
Lore & Background
The spherical nature of Earth was known by ancient Greeks, who developed latitude, longitude, and astronomical measurement methods. These methods, preserved by Muslim and Indian astronomers, sufficed for global exploration in the 15th and 16th centuries. However, the scientific advances of the Age of Enlightenment revealed errors in these measurements and demanded greater precision. This spurred innovations like the marine chronometer and a reconsideration of Earth’s shape. Isaac Newton theorized an oblate shape, while early surveys by Jacques Cassini suggested a prolate shape. French geodesic missions to Lapland and Peru confirmed Newton’s oblate model and also discovered gravity variations that later led to the geoid concept. Trigonometric surveys, beginning with Cassini’s work and the Anglo-French Survey, established control networks across France and the United Kingdom by the late 18th century. More ambitious projects, such as the Struve Geodetic Arc across Eastern Europe and the Great Trigonometrical Survey of India, took decades but improved estimates of Earth’s ellipsoid. The first triangulation across the United States was completed in 1899. This work produced the North American Datum of 1927 and the Vertical Datum of 1929, the first standard public datums. Later, improved measurements and early satellites enabled more accurate regional datums like NAD83, ETRS89, and GDA94, as well as global datums for satellite navigation, notably WGS84.
Reader's Guide
Geodetic datums are foundational to any technology or technique based on spatial location. Before GPS, there was no precise way to measure positions far from reference points such as the Prime Meridian at Greenwich Observatory, the Equator, or the nearest coast. Astronomical and chronological methods had limited precision over long distances. The rise of GPS and its WGS 84 datum supplanted most earlier datums in many applications, as WGS 84 is intended for global use. However, because Earth is an imperfect ellipsoid, local datums can give more accurate representation of specific areas—for example, OSGB36 approximates the geoid over the British Isles better than WGS 84. Despite this, the benefits of a global system have led to widespread adoption of WGS 84. A standard datum specification includes a model for Earth's shape (reference ellipsoid or geoid), an origin tied to a known location, and multiple control points. Coordinates referred to different datums have an undefined relationship and can only be approximated; datum shift can reach kilometers if a point is far from the origin of one or both datums.
Did You Know?
- A horizontal datum binds a reference ellipsoid to the physical Earth using monumented geodetic control points.
- The WGS 84 datum is almost identical to NAD 83 in North America and ETRS89 in Europe.
- Mars has no oceans and no sea level, but at least two martian datums have been used to locate places there.
Frequently Asked Questions
What exactly is a geodetic datum?
It is a standardized reference framework that lets us pin down any point on Earth (or another celestial body) with unambiguous coordinates. Think of it as the agreed-upon origin and coordinate rules that make every latitude, longitude, or elevation reading consistent and comparable across users.
What are the main types of geodetic datums?
There are three: horizontal datums that handle latitude and longitude, vertical datums that handle elevation or depth relative to a reference like mean sea level, and three-dimensional datums that unify both into a single system.
What is WGS 84 and why is it so widely recognized?
WGS 84 is the global geodetic datum embedded in the GPS system, so virtually every smartphone, car navigator, and satellite phone reports positions in its coordinate scheme. It was designed to work acceptably everywhere on the planet, even if it is not the most locally precise option available.
Why would a country prefer a local datum over WGS 84?
A local datum such as OSGB36 is fitted more tightly to a specific region, giving it finer positional accuracy for the British Isles than the one-size-fits-all WGS 84. The trade-off is local precision versus the global interoperability that WGS 84 provides.
Why does a geodetic datum matter to everyday technology?
Without a shared datum, surveying, navigation, GIS mapping, remote sensing, and cartography would all be speaking different coordinate languages and could not reliably overlay or compare data. It is the invisible backbone that makes a 'you are here' reading actually mean something universal.
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