Ellipsoidal height, also known as geodetic height or height above ellipsoid (HAE), is a vertical measurement representing the distance from a reference ellipsoid surface to a point of interest, measured along the perpendicular (normal) to the ellipsoid. This height measurement is the native vertical coordinate produced by GNSS receivers and differs fundamentally from the elevation values shown on topographic maps or used in everyday applications.

GNSS systems determine three-dimensional position by measuring distances to multiple satellites whose orbital positions are known in Earth-centered coordinates. The vertical component of this position naturally references the mathematical ellipsoid surface defined by the geodetic datum (such as WGS84). Because the ellipsoid is a smooth mathematical surface that approximates Earth’s overall shape, ellipsoidal heights do not directly correspond to physical concepts like sea level or the direction water flows.

The distinction between ellipsoidal height and orthometric height (elevation above mean sea level) is critical for many practical applications. Orthometric height references the geoid, an equipotential surface of Earth’s gravity field that closely approximates mean sea level. The difference between ellipsoid and geoid heights at any location is called geoid undulation or geoid height, which varies globally by roughly ±100 meters depending on local gravity anomalies and mass distributions within the Earth.

For applications requiring traditional elevation values, including construction, drainage engineering, flood mapping, and aviation, ellipsoidal heights from GNSS must be converted to orthometric heights using a geoid model. National and global geoid models such as GEOID18 (United States), EGM2008, or EGM2020 provide the undulation values needed for this conversion. Understanding and correctly handling the relationship between ellipsoidal and orthometric heights is essential for any professional using GNSS-derived coordinates in elevation-sensitive applications.