Altitude has never been an accurate part of GPS, as it also depends on barometric data which GPS doesn't have. I assume that the map application uses the GPS position to place you on a map and then uses that spot to interpolate the elevation. Not real accurate, but better than nothing I guess.
Not sure if it is still the case, but in the "early days" of GPS, it assumed a standard day setting of 29.92 inches to determine altitude. There are very few standard days so the number never was real accurate. Military versions were much better however due to the usage and the need for more accurate data.
Not sure what your issue might be, but I would never trust an altitude readout from GPS.
Last edited by Dubdub : 04-08-2009 at 07:39 AM.