Geodetic datum

Builds on Reference ellipsoid Geodetic coordinates

Definition

A geodetic datum consists of the following data:

a reference ellipsoid, given by its semi-major axis $a$ and flattening $f$ ;
a Cartesian frame fixed to the Earth: an origin, an axis direction for $z$ , and a direction for $x$ in the plane $z = 0$ ;
the convention that the ellipsoid is centred at the origin with its axis along $z$ .

The direction of $x$ fixes the prime meridian $\lambda = 0$ . Given a datum, each point of space receives geodetic coordinates $(\varphi, \lambda, h)$ with respect to the placed ellipsoid.

Remarks

Example: WGS 84 uses the ellipsoid with $a = 6378137$ m and $1/f = 298.257223563$ , an origin at the Earth's centre of mass, and the $z$ -axis along the conventional mean rotation axis. Non-example: the ellipsoid GRS 80 alone is not a datum, since it says nothing about where the ellipsoid is placed relative to the Earth (clause 2 fails). A datum whose origin is the centre of mass is called geocentric. In practice a datum is realised by the coordinates of a network of physical markers or stations.

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Tag	`X4ZCF21SGT`
URI	`m2://X4ZCF21SGT`
Kind	definition
Writing	W0 stub
MSC	86A30
Defines	Geodetic datum
Uses	Reference ellipsoid, Geodetic coordinates
Changed	3 November 2026

Geodetic datum

Definition

Remarks

Reading path

Further reading

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