Height can be measured in two ways. ... The traditional, orthometric height (H) is the height above an imaginary surface called the geoid, which is determined by the earth's gravity and approximated by MSL. The signed difference between the two heights—the difference between the ellipsoid and geoid—is the geoid height (N).

What is a geoid height?

A geoid height is the ellipsoidal height from an ellipsoidal datum to a geoid. • Hence, geoid height models are directly tied to the geoid and ellipsoid that define them (i.e., geoid height models are not interchangeable).

What is geoid value?

The geoid is a model of global mean sea level that is used to measure precise surface elevations. A depiction of the United States geoid. Areas in yellow and orange have a slightly stronger gravity field as a result of the Rocky Mountains.