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Topic Date User

Altimeter / Barometer 20-Jan-2007 At 3:59:09 PM Joe

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GPS will not give you good heights for two reasons:
1. The heights being measured are in reference to an ellipsoid which approximates the shape of the earth. AHD heights are referenced to a gravitational surface (the geoid - roughly mean sea level) and because there are variations in gravity all over the world the difference between the ellipsoidal height and orthometric height (AHD) will also change. Basically, what you get from GPS is an ellipsoidal height which will differ from AHD depending on the geoid-ellipsoid separation at that point.
2. The satellite geometry provides better horizontal accuracy than vertical accuracy as the earth blocks signals from satellites that would improve vertical accuracy (e.g. on the other side of the world).

These are in addition to other errors that affect GPS (e.g. ionospheric and tropospheric interactions).