GPS Altitude
GPS Altitude
Gearpins is pretty close. GPS uses a spheroidal model of the earth (WGS 84) as a reference which is based on an equipotential surface worldwide. This averages all the peaks and troughs of the real earth's surface in terms of their gravitational effect. The altitude calculated by a GPS receiver will be the distance above this theoretical surface and may disagree with your real altitude above mean sea level. Neither can be equated to the reading on an altimeter (set to QNH) but it is probably close enough for most purposes. Just don't be surprised when you ditch in the Pacific and find your GPS reads -180ft even at high tide.
As far as GPS altitude variation with latitude is concerned, the satellites are not polar orbiting so there may be some reduction of accuracy (increased dilution of precision) in altitude near the poles. That is why you have triple IRS to iron out the wrinkles.
Hope that isn't too much information.
Cheers Chart8R