This article includes information on NZGD2000 coordinates and how they are patched to accommodate changes to the land as earthquakes cause deformation every year.
NZGD2000 is a three-dimensional semi-dynamic datum. This means that it defines not just the coordinates at any physical point but also a deformation model that predicts the movement of that point through time.
The NZGD2000 coordinate of a point nominally represents its location on 1 January 2000 in terms of ITRF96 at epoch 2000.0 (but see comments below about reverse patches). The deformation model predicts the point's offset at any specified date - adding the offset to the coordinate gives the ITRF96 coordinate of the point at that date (up to the accuracy of the deformation model).
The deformation model includes both an secular velocity model, and "patches" which account for movements due to specific deformation events (earthquakes). The model is periodically updated to include earthquakes that have occurred and as better information becomes available.
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 as depicted below.
Earthquake patches are applied in two ways:
- forward patches, in which case the patch defines the displacements 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 displacements to be subtracted to calculate coordinates for dates before the event.
This means that although the NZGD2000 coordinates are nominally at epoch 1 January 2000, they include the displacements from specific earthquakes after that date. This somewhat counter-intuitive approach supports the majority of users who do not take account of the deformation model (for example due to limitations of their software) but who nonetheless need coordinates that represent the current relative positions of marks with a reasonable level of accuracy.
Each version of the deformation model is named by its approximate publication date. The versions that have been released are:
|20000101||The secular velocity model based on data from 1990 to 1998||No|
|20130801||Updated version of the secular velocity model and reverse patches for the following earthquakes:|
Secretary Island, 2003
Macquarie Island, 2004
George Sound, 2007
Dusky Sound, 2009
Darfield (Christchurch), September 2010
Christchurch, February 2011
Christchurch, June 2011
Christchurch, December 2011
|20140201||Adds forward patches for the 2013 Cook Strait and Lake Grassmere earthquakes||No|
The current version of the deformation model can be downloaded using the link 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 a reverse patch. This means that the NZGD2000 coordinates changed when this was implemented on 13 December 2013. See Related Content below: Applying adjustment files due to the South Island Coordinate Update, for guidance on applying this coordinate update to other data sets.
Deformation model download
NZGD2000 deformation model version 20140201 (attachment below)
Last updated 29 January 2014