Limitations in the NZGD2000 Deformation Model

Abstract

The New Zealand Geodetic Datum 2000 (NZGD2000) is a semi-dynamic datum, in that coordinates are fixed to their values at 1 January 2000 and velocities from a horizontal deformation model are used to transform the coordinates of data collected before or after that date.

The deformation model was calculated from GPS campaign data collected between 1991 and 1998, and was aligned with ITRF96. We examine the performance of this model in 2005 from two points of view: (1) how different are the ITRF2000 velocities from 1TRF96, and (2) for new stations, and older stations where additional data have been collected, how well do the newly estimated velocities match those in the deformation model (after the ITRF96-ITRF2000 transformation)?

We have calculated ITRF2000 velocities at points throughout New Zealand. The ITRF96 and ITRF2000 velocities differ by 4.8 mm/yr at azimuth −101° in the southwest of the country, and by 5.4 mm/yr at azimuth −111° in the northeast.

The differences between newly-calculated ITRF2000 site velocities and velocities from the deformation model transformed to ITRF2000 range between zero and about 4 mm/yr. Velocities of some continuous GPS stations installed in the past few years differ by more than this (in two cases by >7 mm/yr), in part because the new velocities cannot be estimated reliably from relatively short spans of data, and in part because the velocities at some sites are not linear. Significant vertical velocities up to a few mm/yr are estimated for continuous GPS stations that have been established for at least four years. These comparisons suggest that an upgrade to the deformation model should be considered. Alignment of the deformation model with ITRF2000 (or its successors) will have benefits in combining newly collected data with existing data, as the ITRF96 to ITRF2000 transformation step will no longer be needed.