The World Geodetic System 1984 (WGS84) is a dynamic datum. Its coordinates change with time, and the datum has many different versions (or realisations). This means that an accurate transformation using WGS84 is only possible with a reference date. This is usually stored as the coordinate epoch.
World Geodetic System 1984 (WGS84)
Coordinate systems used in New Zealand
When you are calculating a coordinate transformation that includes a ground-fixed datum, additional parameters are needed to account for the time-dependent transformation and its deformation model/distortion grid. These provide the parameters for the internal deformation of the datum.
True WGS84 coordinates
Before converting WGS84 coordinates, it is important to confirm the data is WGS84 as opposed to it being in terms of a datum such as NZGD2000. The metadata and supporting documentation will show how the coordinates were calculated. Most coordinates in New Zealand are Zealand Geodetic Datum 2000 (NZGD2000) as they have been observed relative to a known geodetic mark.
Trig stations and geodetic marks
It has been common practice to assume that NZGD2000 and WGS84 are the same for most practical purposes (such as navigation). The Standard for New Zealand Geodetic Datum 2000 (effective 16 November 2007) describes WGS84 and NZGD2000 coordinates as 'identical, at the 1m level'. This is true, if not sufficient when accuracies better than 1m are required.
Find out more about WGS84 coordinates
Online coordinate conversions
Our online coordinate converter includes time-dependent transformations to and from WGS84.
Guidelines for coordinate transformation for when the time of the WGS84 data is unknown are shown in the following table.
| WGS84 coordinate accuracy | Suggested actions/required information |
|---|---|
| Less than 0.2m | To maintain the accuracy of the data, the correct realisation/version and coordinate epoch (time to the nearest day) is required. If this is not available, additional fieldwork can be used to check the coordinates to confirm the datum epoch. |
| Between 0.2m and 2m | To achieve this level of accuracy, the process is as above, except:
If the data is from a region affected by a significant earthquake, the coordinate epoch needs to indicate whether the data is pre- or post-earthquake. |
| More than 2m | No transformation required. At this level of accuracy, we can assume WGS84 coordinates are the same as NZGD2000 until 2040. |