The NZGD2000 deformation model defines the relationship between the NZGD2000 datum and global datums such as WGS84 and ITRF2008.
The deformation model is part of the definition of the NZGD2000 datum. It is a time dependent transformation function that can convert coordinates between NZGD2000 - the official coordinate system of New Zealand - and global coordinate systems such as WGS84 and ITRF2008, which are used by Global Navigation Satellite Systems (GNSS) such as GPS. When NZGD2000 was originally defined it was aligned with the ITRF96 global datum at 1 January 2000. The deformation model represents the movement and distortion of New Zealand since then due to the continuous movement of tectonic plates, as well as deformation from significant earthquakes since then.
It is common practice to ignore the deformation model as most geospatial and positioning software does not account for it and cannot transform coordinates correctly between NZGD2000 and the global datums. For many applications this is acceptable, but it is important to be aware of the potential errors in doing so.
For low accuracy applications where errors of up to a metre are not important, the deformation model can be safely ignored. In this case WGS84 or ITRF2008 coordinates can be treated as equivalent to NZGD2000 coordinates. The error in doing this is about 5cm per year since 2000. In 2014 the error is up to 70cm.
The deformation model may also be ignored if only local relative accuracy of concern – for example making observations or calculations between nearby points. We need coordinates to be consistent in a local area, but we are not concerned if they are inconsistent with distant points. The NZGD2000 datum is being progressively distorted by deformation so the local relative accuracy is compromised. At worst the error incurred by ignoring deformation is about 0.7mm per km for each year since 2000. For example the error in a 10km line in 2014 may be up to 10cm.
The magnitude of this distortion error is plotted below. The units of ppm per year are equivalent to millimetres per kilometre per year after 2000. Around Wellington and the Southern Alps the distortion is up to 0.7mm per km per year. However in the north west and south east of New Zealand it is much less.
The deformation model is a time dependent transformation, which means that in order to convert between NZGD2000 and global systems you need to know not only the coordinates of the point being converted, but also the time at which the conversion applies. Typically this will be the time at which a coordinate is measured or relocated.
The model includes both a secular velocity model, and "patches" which account for movements due to specific deformation events (earthquakes).
The secular model represents the continuous deformation of New Zealand caused by the convergence of the Australasian and Pacific tectonic plates whose boundary New Zealand straddles. This causes approximately 5cm per year deformation across the country.
Earthquake patches are applied in two ways:
- forward patches, in which case the patch defines the corrections to apply after the event, or
- reverse patches, in which case the NZGD2000 coordinates are updated to include the deformation due to the earthquake, and the patch defines the corrections to be subtracted to calculate coordinates for dates before the event.
Forward patches are simple to implement as they only affect the deformation model, which most applications do not use at all. Reverse patches have much more impact on users of NZGD2000 as they require all NZGD2000 coordinates in the affected area to be changed. This is equivalent to transforming to a new coordinate system. Nonetheless where there is significant deformation from earthquakes reverse patches are preferred as this ensures that the relative local accuracy of coordinates is maintained.
The model is periodically updated to include earthquakes that have occurred and as better information becomes available. Each new version of the deformation model is equivalent to a new version of the datum.
Versions of the deformation model are named by their approximate publication date. The versions that have been released are:
The secular velocity model based on data from 1990 to 1998
Updated version of the secular velocity model and reverse patches for the following earthquakes:
Adds forward patches for the 2013 Cook Strait and Lake Grassmere earthquakes
The current version of the deformation model can be downloaded (see attachment below). The model is provided as a zip file containing the data of the deformation file in a set of CSV (comma separated value) files, documentation about how to use the model, and an example python program that can be used to calculate the deformation at any particular time. The deformation model is also incorporated into the SNAP and concord software available on this site.
Version 20130801 incorporates reverse patches. This means that the NZGD2000 coordinates changed when this was finally implemented on 13 December 2013. See Applying adjustment files due to the South Island Coordinate Update, for guidance on applying this coordinate update to your data sets.