The New Zealand Quasigeoid2009 (NZGeoid2009) can be used to convert GPS derived ellipsoidal heights to New Zealand Vertical Datum 2009 (NZVD2009) normal-orthometric heights that relate more closely to mean sea level and the local gravity field. NZGeoid2009 can also be used to transform heights to any of the 13 official local vertical datums used across New Zealand, more information on this transformation is available at http://www.linz.govt.nz/geodetic/conversion-coordinates/height-conversions.
NZGeoid2009 is formally defined in the LINZ standard LINZS25004 which can be obtained from http://www.linz.govt.nz/geodetic/standards-publications/standards. NZGeoid2009 models the difference between the GRS80 ellipsoid and the geoid over the New Zealand region. The geoid is a theoretical surface of equal gravity that roughly approximates the mean level of the sea across the Earth.
NZGeoid2009 is published by LINZ and was computed by enhancing the EGM2008 global gravity model with terrestrial gravity observations over New Zealand and DNSC08 satellite altimetry data over the oceans. The model is published on a one arc-minute grid covering New Zealand's extended exclusive economic zone.
The accuracy of NZGeoid2009 is nominally ±0.08 metres across New Zealand. More information on the accuracy in relation to the 13 local vertical datums is provided in LINZS25004.
NZGeoid2009 is published on a one arc-minute grid (approximately 1.8 kilometres) over the New Zealand continental shelf (160° E to 170° W, 25° S to 60° S). It has an expected accuracy of 6 centimetres based on comparisons with GPS-levelling throughout New Zealand. It was calculated by enhancing a global geopotential model (EGM2008) using a combination of land and satellite based gravity observations together with a digital elevation model. GPS-levelling observations were not used to compute NZGeoid2009.