Geoid Height

Use the Coordinates > Geoid Height menu option (GEOID GX) to create a channel of the geoid height relative to the GRS80 The Geodetic Reference System 1980 (GRS80) consists of a global reference ellipsoid and a normal gravity model. spheroid for a given pair of coordinate channels. This GX requires a geoid model in the Oasis montaj GRD format that represents the elevation of the geoid in metres relative to the GRS80 spheroid. The GX then samples the selected geoid grid to the current database.

Geoid Height dialog options

X location channel Y location channel	Select the X and Y channel pair to look up the geoid elevation. The coordinate pair can be either Latitude and Longitude channels or projected ground coordinates with a defined coordinate system. Script Parameters: GEOID.X, GEOID.Y
Output geoid height channel	Specify the channel where the geoid height will be placed. By default, the name is set to "GeoidHeight". If the specified channel does not exist, it will be created. Creating a geoid height channel is useful for converting GPS elevations to elevations relative to a geoid. Script Parameter: GEOID.GEOID
Geoid model	Select the geoid model from the list of geoid elevations included with Oasis montaj. See the Application Notes for further details. Select the Other grid option if you want to use your own model grid file. The Select Geoid Model Grid dialog will open: Grid file Expand the list or click the browse button [...] to find and select a model grid file. To create your own model grid, you need to generate a grid file that contains the elevation of the geoid in metres relative to the GRS80 spheroid. Ensure the grid has a defined coordinate system. If the current database does not overlap or only partially overlaps with the selected geoid model, an error or warning prompt will be triggered. Script Parameter: GEOID.GRID

Application Notes

The geoid is the shape of the Earth that would be represented by mean sea level surface extended continuously through the continents. It is a theoretically continuous surface that is perpendicular at every point to the direction of gravity (the plumb line). Older geodetic datums measure elevations with respect to the geoid. Currently the recorded GPS elevations (geocentric elevations) are relative to the GRS80 reference ellipsoid, which is a theoretical spheroid shape that closely models the Earth’s shape without local variations due to gravity. All modern datums are based on the GRS80 spheroid, including CHTRF95, EST92, ETRF89, GDA94, GGRS87, Israel, KUDAMS, LKS94, NAD83, and WGS84.

The GRS80 ellipsoid is designed to best fit Earth's geoid, while the WGS84 ellipsoid is essentially the GRS80 ellipsoid shifted to Earth's center of mass. Initially, WGS84 was intended to be just that, but "refinements" were made to the axis size after several years of satellite observations to minimize anomalies with Earth's surface/geoid. WGS84 coordinates agree with GRS80 coordinates within 10 cm.

Geoid Models

The goal of a geoid model is to be consistent with actual physical characteristics as determined from current observations.

Older geoid models, such as GEOID99, were built using observation data from the time of their formation but are considered no longer consistent with the physical Earth. Location coordinates can change for several reasons. They may change due to natural causes, such as tectonic plate shifts in areas of the western United States or Alaska, sediment deposition and consolidation in southern Louisiana, or glacial rebound in the Great Lakes region. They also may change due to human activities, such as mineral extraction or large-scale water extraction.

Hence, new geoid models are periodically produced to reflect changes in both types of heights (orthometric heights are also periodically updated) and to include new observations. Below is an illustration of the ellipsoid, geoid, and topographic surface (including both landmass topography and ocean bathymetry).

Khal et al, 2020 ^[1]:  H—elevation above the geoid, h—height of the ellipsoid, N—height of the geoid (undulation) above the ellipsoid.

The Geoid model list includes recently published geoids (relative to the GRS80 reference ellipsoid) sourced from the following: 

GEOID Model Homepage - National Geodetic Survey (NGS) provides links to the latest NOAA geoid models and elevation grids for North America. The files are available in ASCII and binary BYN format, each covering defined latitude and longitude windows.

AUSGeoid2020 | Geoscience Australia provides the GDA2020 geoid model and the ASCII code at every minute of latitude/longitude for the GDA2020 geoid, covering all of Australia.

The EGM2008 The Earth Gravitational Model 2008. It is the successor to EGM96 and EGM84, and it is provided by the U.S. National Geospatial-Intelligence Agency (NGA) EGM Development Team. The EGM2008 model has a cell size of 1.0 × 1.0 minutes, resulting in a grid of 10,801 rows × 21,600 columns, containing 4-byte IEEE float values that define the difference between the WGS84 ellipsoid height and Mean Sea Level (MSL). global model is available as a geoid model grid from the public DAP Server. To download the grid into your project, launch Seeker, navigate to the Search page, and enter "EGM2008" in the Search bar. Then, go to the Results page and expand the Geosoft Public DAP Server node to locate the geoid model grid. You can download the full grid dataset, which covers the entire world, or select a specific portion. Refer to the Seeker - Search topic on defining an area of interest (AoI) or selecting a specific region. If your project contains an active map, you will automatically download the portion of the grid that matches the AoI defined by the active map.
After downloading the grid, rerun the GEOID GX and select the Other grid option from the Geoid model list to select the grid and generate the geoid height channel in the current database with interpolated elevations from the selected file.

You can also access the Public DAP Server online at: https://public.dap.seequent.com

For each geoid model in the list, the corresponding ASCII or binary elevation files have been converted to Geosoft grids (gridded at increments such as 1' or 3' in longitude and latitude). The geoid grid files (in longitude/latitude X/Y units) are located in the C:\Program Files\Geosoft\Desktop Applications\etc folder. When the GEOID GX runs, it generates the geoid height channel in the current database with interpolated elevations from the selected geoid grid file.

Longitudes in a Geosoft database are commonly provided in the range of -180 to +180 degrees. However, some geoid grids are provided in the 0-360 degree range. This discrepancy can cause issues in areas like Alaska, where geoid longitudes assume a 0-360 range, but survey data may be in the 0 to -180 range. The geoid’s interpolation process will assume a circular coordinate system (i.e., ensuring that the X range of the geoid grid and the survey database are the same).

Reference