GPSAltitude

GPSAltitude is a term used to describe the altitude value provided by a GNSS receiver based on GPS measurements. In most GPS systems the computed height is relative to a reference ellipsoid, typically WGS84, and is known as the ellipsoidal height (h). To report altitude above mean sea level, users usually see the orthometric height (H), which is derived by subtracting the geoid height N from h (H = h − N). The geoid height is the difference between the ellipsoid and the geoid and is supplied by global geoid models such as EGM96 or EGM2008, either preloaded or accessed online.

GPSAltitude data are obtained from solving the navigation equations using pseudorange and carrier-phase measurements from satellites. The vertical solution is often less precise than the horizontal one because of weaker geometry, atmospheric corrections, multipath, and receiver clock errors. Consequently, GPS-only vertical accuracy can range from meters to tens of meters on typical consumer devices; vertical precision improves with augmentation methods such as differential GPS (DGPS), real-time kinematic (RTK), or precise point positioning.

Some devices display altitude above mean sea level, while others expose ellipsoidal height. Barometric sensors may be used to complement GPS altitude, especially for short-term changes, but barometer readings can drift with weather and calibration.

In practice, it is important to distinguish GPS ellipsoidal height (h) from orthometric height (H) and to understand whether a given device reports AMSL or ellipsoidal altitude.