Geodetic datum

A geodetic datum or geodetic system is a global datum reference or reference frame for unambiguously representing the position of locations on Earth by means of either geodetic coordinates or geocentric coordinates. Datums are crucial to any technology or technique based on spatial location, including geodesy, navigation, surveying, geographic information systems, remote sensing, and cartography. A horizontal datum is used to measure a horizontal position, across the Earth's surface, in latitude and longitude or another related coordinate system.

A vertical datum is used to measure the elevation or depth relative to a standard origin, such as mean sea level (MSL).
A three-dimensional datum enables the expression of both horizontal and vertical position components in a unified form.
The concept can be generalized for other celestial bodies as in planetary datums.

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