Geoid Height

Use the Coordinates > Geoid Height menu option (GEOID GX) to create a channel of the geoid height relative to the GRS 80 spheroid for a given pair of coordinate channels. This GX requires a grid of the geoid model, which is the elevation of the geoid in metres relative to the GRS 80 spheroid.

Geoid Height dialog options

X location channel Y location channel	Select the X,Y channel pair at which to look up the geoid elevation. The coordinate pair must have a defined coordinate system. Script Parameter: GEOID.X, GEOID.Y
Output geoid height channel	Enter the name of a channel in which to place the geoid height. If a channel does not exist it will be created. Script Parameter: GEOID.GEOID
Geoid model	Select a geoid model from the list of models installed with Oasis montaj, or choose "Other grid" to select your own model grid file. 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 plumbline). Older geodetic datums measure elevations with respect to the geoid. GPS measured elevations (geocentric elevations) are with respect to the GRS 1980 spheroid, 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 GRS 1980 spheroid, including CHTRF95, EST92, ETRF89, GDA94, GGRS87, Israel, KUDAMS, LKS94, NAD83 and WGS84. It is useful to create a geoid height channel if you would like to convert GPS elevations to elevations relative to the geoid.

To create your own model grid, create a grid file that contains the elevation of the geoid in metres relative to the GRS 1980 ellipsoid. The grid must have a defined coordinate system.