Geoidal Undulation

Geoidal undulation (N), also called geoid height, is the separation between the geoid and the reference ellipsoid measured along the ellipsoidal normal. It links the two height systems through N = h minus H, where h is ellipsoidal height and H is orthometric height. N can be positive or negative and reaches tens of meters globally, from about minus 105 m near the Indian Ocean to over plus 80 m near New Guinea. Global models such as EGM2008 supply N on a grid. The undulation is what lets GNSS ellipsoidal heights be converted to the mean-sea-level-referenced orthometric heights used for charting and engineering.