The World Geodetic System 1984 (WGS84) is a datum featuring coordinates that change with time
WGS84 is defined and maintained by the United States National Geospatial-Intelligence Agency (NGA). It is consistent, to about 1cm, with the International Terrestrial Reference Frame (ITRF). It is a global datum, which means that coordinates change over time for objects which are fixed in the ground. This is because the tectonic plates on which New Zealand sits are constantly moving, albeit reasonably slowly. In New Zealand this movement is about 5cm per year. This continuous ground movement means that even in the absence of earthquakes and other localised land movements, WGS84 coordinates are constantly changing. These are often referred to as dynamic or kinematic coordinates. Therefore it is important that coordinates in terms of WGS84 have a time associated with them, especially where the best levels of accuracy are required.
Note: There are multiple realisations of WGS84. Each of these realisations is a separate datum. For highly-accurate coordinates, it is important to know which realisation the coordinates are referenced to.
Network (Absolute) Accuracy (m) (1-sigma)
1987, 1 January
1994, 29 June
1997, 29 January
2012, 8 February
2013, 16 October
Issues with WGS84 Coordinates
It is common for geospatial data to be referenced to the WGS84 datum, with no associated coordinate epoch and/or no information about which realisation of WGS84 was used.
It is also common for coordinates described as being WGS84 to actually be in terms of NZGD2000.
This is because most accurate coordinates in New Zealand were calculated using connections to NZGD2000 geodetic control or aligned to some other NZGD2000 data. For example, it is common for centimetre-accurate GNSS coordinates to be referenced to the NZGD2000 coordinates of a base station. In other cases, a site transformation may have been calculated in the field using NZGD2000 coordinates. The WGS84 confusion usually occurs because the GPS satellite orbits are broadcast in terms of WGS84. But for accurate positioning, it is the coordinates of the ground control, not the satellites, which determines the datum of the coordinates.
Some of the confusion about WGS84 stems from the assumption that NZGD2000 and WGS84 are the same, for practical purposes. For example, the Standard for New Zealand Geodetic Datum 2000 (effective 16 November 2007) describes WGS84 and NZGD2000 coordinates as identical, at the 1m level. This assumption is not valid where accuracies better than 1m are required.
It is worth noting that true WGS84 coordinates (ie that change with time) are not generally available in New Zealand. It is therefore unlikely that a centimetre or decimetre-accurate WGS84 dataset in New Zealand is truly in terms of WGS84. It is more likely that they are actually NZGD2000, or some other local coordinate system.