GPS is most accurate when your position is surrounded by satellites. In
the horizontal plane, that is usually true. In the vertical direction,
you always have satellites above you, but none below you. This also
makes altitude slightly more difficult to resolve.

http://gpsinformation.net/main/altitude.htm
--
Gene Seibel
Gene & Sue's Aeroplanes - http://pad39a.com/gene/planes.html
Because we fly, we envy no one.