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.
- 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.
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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