EGM96 and EGM2008 Geoids

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.

EGM2008

• Heights above the WGS 84 ellipsoid--add to the ellipsoid height to get the geoid height
• Grid has 5 arc minute resolution (NGA has 2.5 and 1 minute versions available)
• http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm2008/egm08_gis.html
• http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm2008/
• EGM2008: The development and evaluation of the Earth Gravitational Model 2008 (EGM2008) - Nikolaos K. Pavlis, Simon A. Holmes, Steve C. Kenyon, John K. Factor; Journal of Geophysical Research: Solid Earth (1978-2012) Volume 117, Issue B4, April 2012.  
  https://doi.org/10.1029/2011JB008916

EGM96

• Grid has 15 arc minute resolution
• http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm96/binary/readme.txt
• http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm96/binary/binarygeoid.html

Using in MICRODEM

The grids are downloaded with the geology download, and then files will be in  available in  C:\mapdata\geology\geology_grids

• Grid:  EGM2008 can be used to do a vertical datum shift on the edit menu.
• Point data base (such as ICEsat2): EGM2008 can be used to add a field VDATM_SHFT, Edit, Map coordinates.  You can then
  • add to an elevation field in the database to get the geoid height if you ellipsoid heights
  • subtract from an elevation field in the database to find the ellipsoid heights if you have geoid heights.

GEOID12B_CONUS

GEOID 18 US

• Fetch the 2018 geoid.  >wget https://geodesy.noaa.gov/PC_PROD/GEOID18/Format_pc/g2018u0.bin  Convert to Geotiff with GDAL

GDAL reads a number of the grid shift files needed for the vertical datum shifts:

• NRCAN (Canada, but might have trouble with all the subsets)
• NTv2

References

• https://www.fig.net/resources/proceedings/fig_proceedings/fig2010/papers/ts01c/ts01c_roman_wang_et_al_3768.pdf
• https://proj.org/resource_files.html has links to download GTX files, which GDAL should convert to Geotiff
• Other vertical datum shift files:  https://www.agisoft.com/downloads/geoids/
• https://vdatum.noaa.gov/docs/gtx_info.html
• https://groups.google.com/forum/#!searchin/lastools/las2las$20coodinate$20transformation%7Csort:date/lastools/1A36bxhcZUM/jE-6D3lJAAAJ