Read about the New Zealand Quasigeoid 2016 (NZGeoid2016)
A geoid represents a hypothetical global sea level surface, shaped by Earth's gravitational field if oceans could flow unobstructed though land. A quasigeoid is a simplified geoid which is less accurate but still suitable for most applications.
Different geoid models have been developed depending on the data sources available, computation methods and regional requirements.
New Zealand Quasigeoid 2017 (NZGeoid2017)
New Zealand Quasigeoid 2017 (NZGeoid2017) is a re-computation of the New Zealand Quasigeoid 2016 (NZGeoid2016). NZGeoid2017 resolves academic inconstancies present in NZGeoid2016.
New Zealand Quasigeoid 2016 (NZGeoid2016)
NZGeoid2017 was computed using the same parameters as NZGeoid2016, so it was produced 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). Based on comparisons with GPS-levelling observations, it has an expected accuracy of 4.8 centimetres. It was calculated by enhancing the Eigen-6c4 global gravity model, using a combination of land, airborne and satellite-based gravity observations, together with a digital elevation model.
Eigen-6c4 global gravity model
Read more about gravity data collection and the computation of geoids and quasigeoids
NZGeoid2017 has 3 technical deviations from NZGeoid2016:
- It was computed using updated software.
- It used a modified Stokes integration to calculate the gravity anomaly.
- The global geopotential model computations were completed at the Earth surface.
The differences between NZGeoid2016 and NZGeoid2017 are not significant when positioning using standard survey methods. As NZGeoid2016 remains a close approximation of NZGeoid2017, it remains the official geoid of New Zealand Vertical Datum 2016 (NZVD2016).
New Zealand Vertical Datum 2016 (NZVD2016)
A paper outlining the computation and publication of NZGeoid2017 can be found in the Journal of Geodesy: McCubbine, J.C., Amos, M.J., Tontini, F.C. et al. The New Zealand gravimetric quasigeoid model 2017 that incorporates nationwide airborne gravimetry. J Geod 92, 923–937 (2018). https://doi.org/10.1007/s00190-017-1103-1
Data relating to NZGeoid2017 can be requested from crm_geodetic@linz.govt.nz
New Zealand Quasigeoid 2005 (NZGeoid05)
The New Zealand Quasigeoid 2005 (NZGeoid05) was the first regional geoid for New Zealand. It was superseded by New Zealand Quasigeoid 2009 (NZGeoid2009) in September 2009.
New Zealand Quasigeoid 2009 (NZGeoid2009)
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). Based on comparisons with GPS-levelling observations, it has an expected accuracy of 8 centimetres. It was calculated by enhancing the EGM96 global model using land, sea and satellite-based gravity observations.
The NZGeoid05 model can be downloaded from the page Geoid transformation grids
EIGEN-6c4
EIGEN-6c4 (European improved gravity model of the Earth by new techniques version 6c4) is a global gravity model developed in 2014. It offers a high-precision representation of the Earth’s gravitational field, combining satellite and surface datasets. The solution was extended by incorporating the DTU10 (Danish Technical University 2010, developed by the National Space Institute at the Technical University of Denmark) global gravity anomaly data grid.
EIGEN-6C4 can be downloaded from the International Centre for Global Earth Models
Earth Gravitational Model 2008 (EGM2008)
The Earth Gravitational Model 2008 (EGM2008) is the most recent global geoid model released by the National Geospatial-Intelligence Agency (NGA), replacing the Earth Gravitational Model 1996 (EGM96) model which had been widely used since its publication in 1998. A more recent version of the model, EGM2020, has been delayed several times, most recently due to the COVID-19 pandemic.
National Geospatial-Intelligence Agency (NGA)
EGM2008 was computed on a 5 arc-minute grid (approximately 6.5 kilometres) using gravity anomalies from land and satellite-based sources. The model provides complete information up to spherical harmonic degree and order 2159 (in respect to resolution and frequency of gravity waves). The EGM2008 model show global agreement to GPS-levelling to approximately 7 centimetres, and over New Zealand this agreement is approximately 6 centimetres.
Technical details and files to download are available on the NGA website
Earth Gravity Model 1996 (EGM96)
The Earth Gravity Model 1996 (EGM96) was jointly published in 1998 by the United States National Aeronautical and Space Agency (NASA) and the National Imagery and Mapping Agency (NIMA) in 1998. Prior to being replaced by EGM2008, EGM96 was widely used and become the most common global geoid model worldwide.
EGM96’s estimated accuracy is 1 to 2 metres at its maximum resolution of 56 kilometres. The model does not depict gravity field changes smaller than this resolution, and the 1 to 2 metre accuracy means that an error of this size may be introduced into heights that use the EGM96 in their calculation.
Technical details and files are available on the NASA GSFC and NIMA Joint Geopotential Model page