Accurate transformations between WGS84 and NZGD2000 require a date, since WGS84 coordinates change with time
Before carrying out a transformation involving WGS84, it is important to be aware of the characteristics of WGS84, such as the fact that WGS84 coordinates change with time. It is also important to consider whether the data being transformed is truly WGS84, or whether it is actually already in terms of a datum such as NZGD2000.
Because WGS84 coordinates for a ground-fixed object change with time, the transformation software needs a date to calculate from or to a local datum such as NZGD2000. This is sometimes referred to as the coordinate epoch. The overall transformation process involves using time-dependent transformation parameters and a deformation model.
It has been common practice to assume 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.
The LINZ online coordinate converter was updated 27 June 2016 to carry out rigorous time-dependent transformations involving WGS84. Prior to this it used null transformation parameters, which treat NZGD2000 and WGS84 coordinates as being the same (except in some earthquake-affected areas).
The table below provides some suggestions on how to proceed where there is uncertainty about a set of WGS84 coordinates.
WGS84 Coordinate Accuracy
Suggested Actions/Required Information
Better than 0.2m
Confirm whether the data is actually WGS84 (consider that it may be NZGD2000 already), by assessing metadata and any supporting documentation to determine how the WGS84 coordinates were calculated. If the data is confirmed as WGS84, both the correct realisation and coordinate epoch will be required (preferably to the nearest day) if the transformation is to maintain the accuracy of the data. If insufficient information is available, fieldwork may be required to check some of the coordinates and therefore confirm the datum and/or coordinate epoch.
Between 0.2m and 2m
As above, except the specific realisation of WGS84 is not essential. The coordinate epoch can also be approximate (eg to the nearest year) at this level of accuracy. Note that if the data covers a region affected by a significant earthquake, the coordinate epoch needs to be accurate enough to reflect whether the data is pre or post-earthquake.
Worse than 2m
No transformation required. At this low level of accuracy, it is reasonable to assume coordinates in any of the WGS84 realisations are the same as NZGD2000, both now and for the next 20 years.