One thing though is it appears to be using a different datum from whatever Google earth is using because its consistently out by a hundred feet in both directions, not that it matters right now.
I'm not sure which datum Apple uses for measuring GPS Altitude. Their documentation states only:
Positive values indicate altitudes above sea level.
Now I'm assuming if they have said "Sea Level" then they are using the WGS84 ellipsoid approximation of Earth, which most GPS units report altitude at. It
probably doesn't apply the correction model to determine that
actual height above sea level though, since the ellipsoid model can be out by 100ft at various parts of Earth.
I'd be interested to see if the GPS alt reported by Apple is different/similar to the GPS altitude on airliners especially. We may then be able to apply the corrected model to get a more accurate representation of AMSL.
EDIT: Found an answer on the Apple developer forums. An Apple representative says:
- iPhone 4 & iPad use the WGS84 ellipsoid
Corrected for local variations in elevation. They don't say, but this most likely uses the WGS84 + EGM96 model (as used by Google Earth).
- iPhone 3GS and iPhone3G only reference compared to the WGS84 ellipsoid, much like your airline/handheld GPS does. This means your true altitude above the ocean may be out by about 100ft, but will probably be the same as your FMS GPS output.
Unfortunately you can't tell in code whether you're running iPhone4 or iPhone 3GS, so I can't apply a correction to the iPhone 3GS GPS altitude to make it
true (local) AMSL.
Edit again - Yeah I'm working on Track-up now