The New Zealand Quasigeoid 2009 (NZGeoid2009) is the reference surface for the New Zealand Vertical Datum 2009 (NZVD2009).
The geoid is an undulating surface that is related to the Earth's gravitational field that approximates level of the seas over the entire Earth within a few metres. Because it is related to gravity (and therefore the level of the sea), it is a more intuitive reference surface than the mathematical ellipsoid.
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.
NZGeoid2009 can be used to transform between NZVD2009 normal-orthometric heights and NZGD2000 ellipsoidal heights, and with the published offsets, to the 13 local mean sea level datums. Note that NZGeoid2009 is not a datum in its own right, heights should not be referenced to the geoid surface directly.
The NZGeoid2009 model is available for download as Esri ASCII grid or GeoTIFF on the LINZ Data Service: