Geoid Undulations

Geoid undulation (commonly denoted by N) is the height difference between the geoid and a reference ellipsoid at a given location. It represents the separation between the Earth’s physical equipotential surface (approximating mean sea level) and the mathematically defined reference ellipsoid.

In geodesy, geoid undulations are essential for transforming GNSS-derived ellipsoidal heights into orthometric heights using the relation:
H = h − N,
where H is orthometric height, h is ellipsoidal height, and N is geoid undulation. Accurate determination of geoid undulations from terrestrial, airborne, and satellite gravimetry is fundamental for height system realization, vertical datum unification, and precise geospatial applications.