Wed Mar 12, 2008 6:37 pm
#575950
[quote='Anonymous'][My PDA GPS often reports altitudes that are 1-200ft different from my altimeter [/quote]
This would be expected behaviour, even if the GPS were 100% accurate in reporting height, because GPSs and altimeters measure different things.
Altitude on the altimeter is an air pressure measurement which, for practical purposes, is reported to the pilot as feet or metres. This is obvious when you think about re-setting the altimeter for almost every flight, pressure having changed. Pressure does not increase uniformly with height, so if you could measure being exactly 2,000 ft above sea level with a long tape measure, your altimeter might report a figure which differs noticeably.
The GPS attempts to measure actual height above a notional sea level datum.
Airspace altitude boundaries are expressed in terms of pressure altitude, not actual height.
Thus to derive the pressure altitude from a GPS trace, one would need to know the actual sea-level pressure and the pressure gradient for the particular airmass. This would be somewhat difficult!
However, if your GPS says that you are 1,000 ft vertically inside controlled airspace, that's probably pretty good evidence that you are 500 ft or more inside. If it says that you are 100ft inside, it would be very difficult to use the trace to prove that you did infringe.
There's a recently concluded discussion among glider pilots (see rec.aviation.soaring via Google Groups) about whether GPS traces alone could be used to claim badges and records. Currently the technology we use is a secure logger (to record the GPS data) which also contains a pressure altimeter, and it is this pressure altimeter data which is used to validate height gain claims and to reject claims which infringe airspace.