The World Geodetic System 1984 (WGS84) is a globally defined datum developed and maintained by the United States National Geospatial-Intelligence Agency (NGA). It is consistent with the International Terrestrial Reference Frame (ITRF) at the 1cm level. WGS84 allows us to relate a 3-dimensional position to a consistent coordinate, in the format latitude (Φ), longitude (λ), ellipsoidal height (H), and time (t).
A geodetic datum is a reference system that defines how positions are represented. This includes:
- a coordinate system
- a reference ellipsoid
- additional models, such as a deformation model.
The WGS84 datum’s reference ellipsoid is also called WGS84 and has no deformation model. This means that when WGS84 is compared to fixed points in the ground the coordinates of those points will change over time. These changes are due to tectonic motion and variations in mass distribution in the Earth's interior. A changing coordinate system like this is referred to as dynamic or kinematic, making WGS84 a dynamic datum.
The tectonic plates under New Zealand move about 5cm per year, so the WGS84 coordinates are constantly changing. This means that coordinates in terms of WGS84 need to have a time associated with them (t), particularly when high accuracy is required.
| Name | World Geodetic System 1984 |
|---|---|
| Abbreviation | WGS84 |
| Coverage area | Worldwide, between 180°W –180°E and 90°N – 90°S |
| EPSG code | 4326 |
| Coordinate system | Geographic |
| Reference frame | Earth-centred, Earth-fixed |
| Reference ellipsoid | WGS84 |
| Prime meridian | Greenwich |
| Deformation model | None |
| Reference date | There are multiple realisations of WGS84, with each of these a separate datum. High accuracy coordinates must state the realisation the coordinates are referenced to. |
| Datum realisation | Implementation date | Reference epoch | Network (absolute) accuracy (m) (1-sigma) relative to ITRF2008 |
|---|---|---|---|
| WGS84(Doppler) | 1987, 1 January | NA | 1-2 |
| WGS84(G730) | 1994, 29 June | 1994.0 | 0.10 |
| WGS84(G873) | 1997, 29 January | 1997.0 | 0.05 |
| WGS84(G1150) | 2002, January | 2001.0 | 0.01 |
| WGS84(G1674) | 2012, 8 February | 2005.0 | 0.01 |
| WGS84(G1762) | 2013, 16 October | 2005.0 | 0.01 |
Understanding issues with WGS84 coordinates in New Zealand
Inaccurate, missing or assumed metadata can make it difficult to determine the datum of geospatial data. For example, WGS84 coordinates may not specify the associated coordinate epoch or the version of WGS84 that was used. It is also common for coordinates described as being WGS84 to actually be in terms of New Zealand Geodetic Datum (NZGD2000).
New Zealand Geodetic Datum (NZGD2000)
True WGS84 coordinates change with time and are not generally available in New Zealand. It is often assumed that a coordinate is in the WGS84 datum because GPS satellite orbits are broadcast in terms of WGS84. However, when using accurate positioning methods it is the coordinates of the ground control, not the satellites, which determines the datum of the coordinates.
In New Zealand, precise positioning uses connections to NZGD2000 geodetic control or is aligned to some other NZGD2000 data. As a result, most coordinates in New Zealand are NZGD2000. For example, centimetre accurate GNSS coordinates are referenced to the NZGD2000 coordinates of a base station. In other cases it is the site transformation, made in the field, that sets the survey in terms of NZGD2000 coordinates.
Publications such as the Standard for New Zealand Geodetic Datum 2000 (effective16 November 2007) describe WGS84 and NZGD2000 coordinates as ‘identical at the 1m level’, and other sources which state that WGS84 and NZGD2000 are ‘the same for most practical purposes’. This is true, but NZGD2000 and WGS84 are not equivalent if accuracies better than 1m are required.