The GPS Altitude and Pressure Altitude instruments show different values because they measure altitude using different methods (Geometric height vs. pressure altitude):
-
GPS Altitude: Based on satellite signals, this value represents the geometric height above a defined reference surface, such as the WGS-84 ellipsoid. Due to signal distortions in the atmosphere, this value can vary by tens of meters.
-
Pressure Altitude: Derived from atmospheric pressure, assuming standard temperature and pressure conditions (International Standard Atmosphere, or ISA). It does not account for variations in temperature or pressure, leading to discrepancies from the geometric altitude, especially in non-standard weather conditions.
On warm days or in regions with varying atmospheric conditions, the difference between these two values becomes more pronounced due to the expansion or compression of the air column. Mike Borgelt's article "GPS Altitude vs. Pressure Altitude" is an interesting article on this topic.
Last Updated: