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, it is a more intuitive reference surface than the ellipsoid.
The official geoid model for New Zealand region is the New Zealand Quasigeoid 2009 (NZGeoid2009). Other geoid models are also available for the New Zealand region however they can not be used to generate official heights in terms of NZVD2009, NZGD2000 or the local mean sea level datums. Official heights must be generated using NZGeoid2009.
New Zealand Quasigeoid 2005 (NZGeoid05)
The New Zealand Quasigeoid 2005 (NZGeoid05) was the official geoid for New Zealand until it was replaced by the NZGeoid2009 in September 2009.
NZGeoid05 is produced on a two arc-minute grid (approximately 3.7 kilometres) over the New Zealand continental shelf (160° E to 170° W, 25° S to 60° S). It has an expected accuracy of 8 centimetres based on comparisons with GPS-levelling observations. It was calculated by enhancing the EGM96 global model using a combination of land, sea and satellite based gravity observations. GPS-levelling observations were not used to compute NZGeoid05.
The NZGeoid05 model can be downloaded as either an ASCII grid (file below) or coordinate triplets (file below) for the entire computation area (160° E - 170° W; 25°S - 60°S).
The Earth Gravitational Model 2008 (EGM2008) is the latest global geoid model published by the National Geospatial-Intelligence Agency (NGA). It replaced the EGM96 model which had been the default global geoid since its publication in 1996.
The model was computed from a global 5 arc-minute grid of gravity anomalies from land and satellite based sources. The model is provided complete to spherical harmonic degree and order 2159, which equates to a grid size of approximately 6.5 km. The global agreement to GPS-levelling is approximately 7cm. Over New Zealand the agreement is approximately 6cm.
EGM2008 is available from the NGA website. It is provided in terms of spherical harmonic coefficients which generally need to be converted into a grid of geoid undulations before they can be used (software to do this is on the NGA website).
Earth Gravity Model 1996 (EGM96)
The Earth Gravity Model 1996 (EGM96) was published jointly by the United States National Aeronautical and Space Agency (NASA) and the National Imagery and Mapping Agency (NIMA) in 1998. Until it was replaced by the EGM2008 it was the most common global geoid model used worldwide. Technical details of the EGM96 model are available on the NASA website - NASA GSFC and NIMA Joint Geopotential Model.
The estimated accuracy of the EGM96 model is 1-2 metres at its maximum resolution of 56 kilometres. This means that gravity field changes that are smaller than this resolution will not be depicted by it. Similarly the 1-2 metre accuracy level means that an error of this size may be introduced into heights that use the EGM96 in their calculation.
EGM96 is available from the NASA website. It is provided in terms of spherical harmonic coefficients which generally need to be converted into a grid of geoid undulations before they can be used.