Height Above Ellipsoid (HAE), also known as ellipsoidal height or geodetic height, is the vertical distance measured from a mathematical reference ellipsoid surface to a point of interest, calculated along the line perpendicular (normal) to the ellipsoid at that location. HAE is the native height output from GNSS receivers because satellite positioning naturally determines coordinates relative to the geodetic datum’s ellipsoid rather than to physical surfaces like sea level.
Understanding HAE requires recognizing that GNSS positioning operates within a geometric framework defined by the ellipsoid, not within a gravity-based framework tied to sea level. When a GNSS receiver calculates its three-dimensional position from satellite ranges, the vertical component is inherently measured relative to the smooth ellipsoid surface. This differs fundamentally from orthometric height (elevation), which is measured relative to the geoid, an irregular surface representing mean sea level and the direction of gravity.
The distinction between HAE and orthometric height is critical for many practical applications. Water flows downhill in the gravitational sense (toward lower geoid heights), not necessarily toward lower ellipsoidal heights. Construction projects, drainage engineering, flood mapping, and any application involving fluid flow require orthometric heights. Converting HAE to orthometric height requires knowing the geoid undulation (N) at the location, the separation between ellipsoid and geoid surfaces, which varies globally by approximately ±100 meters.
For GNSS users, properly handling HAE and understanding when conversion to orthometric height is necessary prevents potentially serious errors. Survey-grade applications routinely apply geoid models (such as GEOID18 in the United States or EGM2020 globally) to convert GNSS-derived HAE values to practical elevations. Real-time applications may perform this conversion automatically, while post-processing workflows typically apply geoid corrections during data analysis. Any time GNSS heights must match existing elevation data, topographic maps, or gravity-dependent engineering requirements, proper geoid model application is essential.
