A geodetic datum is a reference system (or set of rules) that defines a coordinate system and an approximated shape of the Earth. A geodetic datum allows accurate positions to be defined and measurements to be taken on a curved surface, but it is more common to use projections for mapping and visualisation purposes as this provides coordinates on a flat surface.
Geodetic datum coordinates are normally represented by:
- latitude (Φ), which measures the distance north or south of the equator
- longitude (λ), which measures the distance east or west of Greenwich
- height (H), which measures the distance above or below the reference ellipsoid.
Geodetic datum coordinates are normally expressed as degrees (°), minutes (‘) and seconds (“), with metres (m) used for height.
For example:
-40° 06’ 28.6294” (or 40° 06’ 28.6294” S) is a latitude south of the equator.
175° 16’ 54.0124” (or 175° 16’ 54.0124” E) is a longitude east of Greenwich.
127.899 m is a height above the reference ellipsoid.
Coordinates such as latitude and longitude are defined in terms of a datum. The coordinates of a location expressed in terms different datums may have different values.
A geodetic datum includes a reference surface that approximates the shape of the Earth. It is usually an ellipsoid or spheroid. This allows efficient and consistent spatial referencing without the need for a detailed model of the Earth’s irregular shape. It does mean, however, that heights in relation to a reference ellipsoid are often an inaccurate reflection of the Earth’s topology, and a separate vertical datum is required to measure heights.
The official geodetic datums currently used in New Zealand are:
Locations in New Zealand may also be expressed in terms of global geodetic datums such as the International Terrestrial Reference Frame 2020 (ITRF2020) and the World Geodetic System 1984 (WGS84).
International Terrestrial Reference Frame 2020 (ITRF2020)
World Geodetic System 1984 (WGS84)