Geoid models created by NGA as a grid. These have the height above the ellipsoid in meters, and should range up to approximately 100 m in absolute value. NGA also produces the coefficients, and code to create your own grids or values at any location. The RMS difference between EGM96 and EGM2008 is about 0.5 m, with the maximum differences of about 10 m in places like ocean trenches. Add the grid values to to the ellipsoid height to get the geoid height.
| Gravity Model | Grid downloads | References | Notes |
| EGM2020 | EGM2020 is to be a new release with the same structure as EGM2008, but with improved accuracy by incorporating newer data. It is at least a year late (July 2021). | ||
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EGM2008 is provided as a set of normalized, geopotential coefficients complete to degree and order 2159, and contains additional spherical harmonic coefficients extending to degree 2190 and order 2159. Also provided is a 2.5-minute worldwide geoid height file, precomputed from the EGM2008. | |
| EGM96 | Grid has 15 arc minute resolution
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EGM96 is provided as a set of normalized, geopotential coefficients to degree and order 360. Also provided is a 15-minute worldwide geoid height file, precomputed from the EGM96. |
The grids are downloaded with the geology download, and then files will be in available in C:\mapdata\geology\geology_grids
NAVD88 to other datums: https://vdatum.noaa.gov/download.php#download_geoid has GTX files
GDAL reads a number of the grid shift files needed for the vertical datum shifts and can convert to Geotiff:
References
Last revision 7/7/2021