New Zealand Geodetic Datum 2000 (NZGD2000)

The New Zealand Geodetic Datum 2000 (NZGD2000) is the official geodetic datum for New Zealand and its offshore islands. The datum is formally defined in LINZS25000 and was implemented in 1998 when it replaced the NZGD1949.

NZGD2000 is a three-dimensional semi-dynamic datum. A three-dimensional datum provides positions that are described by coordinates in terms of a latitude, longitude and ellipsoidal height. A semi-dynamic datum uses a deformation model to correct coordinates and survey observations for the effect of regional-scale tectonic movements of the Earth's crust. The model causes all coordinates and observations to be stored as if they were observed at the same time or reference epoch, which in the case of NZGD2000 is 1 January 2000.

Prior to NZGD2000, New Zealand, like most countries, used a static datum. Static datums fix the positions of a small number of reference points and then adjust all of the other observations to fit them. This approach is acceptable where an area is internally stable (eg Australia). However when internal deformation occurs regularly, such as that caused by tectonic movements, a static datum will quickly become inaccurate. This was the case with the predecessor to NZGD2000, New Zealand Geodetic Datum 1949 (NZGD1949), and was a primary reason for its replacement.

In comparison a fully-dynamic datum uses a deformation model to continuously update coordinates to reflect movements of the Earth's surface. This means that positions will change from one day to the next.

For many users, NZGD2000 has the appearance of a static datum, which facilitates use of the datum by the GIS and mapping community who do not have the tools to manage continuously changing coordinates.

NZGD2000 provides ellipsoidal heights that are given in relation to the GRS80 ellipsoid rather than sea level. Ellipsoidal heights do not relate to the local gravity field and so may not necessarily predict the flow of fluids (eg water may appear to flow up-hill). Ellipsoidal heights can be related to the 13 mean sea level datums by using a geoid model (eg NZGeoid05).

NZGD2000 is essentially coincident with the World Geodetic System 1984 (WGS84). This is the reference system that is used by GPS receivers. It means that for most practical purposes WGS84 coordinates can be assumed to be the same as NZGD2000 coordinates.

NZGD2000 is formally defined in the LINZS25000 (Standard for New Zealand Geodetic Datum 2000). The key parameters from this standard are summarised below:

Name:	New Zealand Geodetic Datum 2000
Abbreviation:	NZGD2000
Reference ellipsoid:	GRS80
Reference frame:	ITRF96
Reference epoch:	1 January 2000
Deformation model:	LINZ deformation model

The New Zealand Geodetic Datum 1949 (NZGD1949), was based on a different reference ellipsoid to NZGD2000. This means that coordinates are approximately 200 metres different between datums. There are three official transformations between NZGD2000 and NZGD1949.

The accuracy of NZGD2000 coordinate is classified by a series of Orders. The Order of a coordinate is related to the accuracy of the observations used to derive it and the type of ground mark that is used to physically represent it. The Order classifications between datums are not consistent.