Originally Posted by
DaveReidUK
Doesn't GPS actually measure relative, rather than absolute, altitude - that's to say relative to a reference ellipsoid that approximates the earth's surface ?
I can't speak for specific applications, but I have a GPS tracker system that I use in my high power rocketry. When I power it up, it reads that altitude of the launch site relative to MSL. There is a button that I then push that resets that altitude to zero so that subsequent readings are relative to the altitude of the launch site. The GPS can then be used to determine things like max altitude of the flight (and speeds - although during the ascent the speeds change to rapidly that the update rate of the GPS makes it less than completely useful).
As MechEngr notes, if that's how the system normally works, it would require a pretty detailed terrain map to reliably determine altitude to the local ground level.