Due to crustal motion, all points on the Earth’s surface are in motion so in global datums such as the International Terrestrial Reference Frame (ITRF), coordinates change continuously with time. However, users of national datums prefer that coordinates are static. In addition, there must be well defined transformations between national datums and the ITRF to facilitate the use of GNSS for surveying measurements. There are two basic techniques that national datums use to provide static coordinates while still providing a link to the global systems. Countries that lie entirely within a tectonic plate are able to incorporate the motion of the tectonic plate into the definition of the datum. However, for a country like New Zealand that lies across a plate boundary a different strategy must be adopted. In common with other developed countries located on plate boundary zones, New Zealand has adopted a semi-dynamic datum, New Zealand Geodetic Datum 2000 (NZGD2000), where coordinates are propagated to a standard epoch (2000.0) using a numerical model of deformation across the plate boundary. These models contain separate models of the secular (continuous) velocity field associated with on-going deep seated tectonic processes and displacements associated with significant earthquakes. In New Zealand’s case this is called the National Deformation Model (NDM). Currently it contains models representing velocity differences across the country representing the continuous tectonic deformation and co-seismic deformation associated with major earthquakes (including the Dusky Sound earthquake of 15 July 2009 and the four earthquakes between September 2010 and December 2011 that make up the Canterbury earthquake sequence). The NDM deals with earthquake displacements in two different ways. In the first, known as a forward patch, the reference coordinates are unchanged, and the deformation model accounts for the deformation associated with the co-seismic and secular parts of the model. In the second, known as a reverse patch, the reference coordinates are changed to reflect the co-seismic portion of the deformation and the deformation model only needs to account for the secular part of the deformation. A consequence of the use of reverse patches is that the epoch 2000 coordinates change to reflect the changes caused by the earthquake, even though these occurred well after the earthquake.

The NDM is important to GIS professionals because it affects all coordinates in NZGD2000 and in some cases its use is required for coordinate transformations. However, it is not supported by the coordinate transformations provided by GIS packages. Unfortunately, if the epoch date of the coordinates is not taken into account, transformations to and from the ITRF may be inaccurate. In addition, the introduction of reverse patches into the NDM in 2013 has caused coordinates to change by up to several metres. The purpose of this talk is to make GIS professionals aware of the affect that ignoring crustal deformation can have on GIS databases, and demonstrate some practical ways that these effects can be mitigated.