Originally Posted by Dubdub
Altitude is not the strong point of GPS and is only an approximation which is generally derived using GPS telemetry and the map database which you are using. Plus it is not corrected for pressure. I don't know if there is a true spec for it or not as pressure variations can not be predicted with much accuracy very far in advance.
I know very little about GPS, but I was under the impression that it allows calculation of your position relative to the center of the Earth's mass. That combined with a map or model of the Earth would be enough to figure out altitude above mean sea level, yes? I can see how barometric altimeters would depend on pressure, but not GPS.