Why do you need to know the Geoid Offset when Orthorectifying an image in ENVI?

The Geoid offset is a constant value that is added to every value in the DEM to account for the difference between a spheroid mean sea level (used in most available DEM data) and the constant geopotential surface known as the GEOID. The RPC coefficients are created based on geoid height, and that information must be used to provide accurate orthorectification.

The geoid offset is the difference between orthometric height (MSL) and ellipsoid elevation. Ellipsoid elevation (WGS-84 ellipsoid, specifically) is required for accurate orthorectification results.

Geoid heights can be used to convert between orthometric heights (approximately mean sea level) and ellipsoid heights according to the formula:

h = H + N

Where:

h = WGS 84 Ellipsoid height
H = Orthometric height
N = EGM96 Geoid height

Note that EGM96 applies only to the WGS 84 reference ellipsoid.

The difference between using a geoid offset and not can easily be 30 meters or more. That is enough of a difference to produce offsets that are noticeable with greater than 1 meter resolution data.

Review on 12/31/2013 MM