Altitude

PX4 reports two altitude types - AMSL (Above Mean Sea Level) and AHL (Above Home Location). Altitude AHL is displayed as Raw altitude in the client.

Vehicle altitude AMSL is calculated from Raw altitude as:
Vehicle altitude AMSL = Takeoff altitude + Reported altitude AHL

If the Takeoff altitude is not specified in UgCS then:
Vehicle altitude AMSL = Reported altitude AMSL

Current altitude AGL (Above Ground Level) is calculated as:
Vehicle altitude AGL = Vehicle altitude AMSL - Terrain elevation AMSL at vehicle location