Originally Posted by
Gargleblaster
Your GPS may be indicating your height ? Your transponder might also still be sensing and emitting altitude, so ask ATC what they're seeing ?
This would probably be one of the worst things you can do. Your transponder does not have an independent altitude sensor and will be using the same suspect numbers as the rest of the aircraft.
This is what doomed the Aeroperu 757 - they were trying to fly using the transponder altitude (as relayed by ATC) and hit the ocean when they thought they were (IIRC) 6,000ft.
Radio altitude would be your best bet unless GPS altitude is available.