# Geodetic datums

A geodetic datum defines a reference framework for specifying locations on the Earth's surface. Two geodetic datums are currently used in New Zealand, and others have been used in the past.

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.

Projections

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.

Vertical datums

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).