Bob Gardner wrote:

AIM 1-1-20 (a)(8) tells us not to rely upon GPS to determine aircraft
altitude.

Right -- ATC counts on the altimeter's temperature errors for traffic
separation (since every plane in the same area will share the same errors).
Our altimeters are pretty inaccurate at cruising altitudes, but as long as
we're all inaccurate in the same way, the system works.

In any case, I don't think that the poster was using his GPS for altitude --
he was just concerned that his altimeter might be wrong. I expect that this
will be a FAQ now that a lot of people have WAAS-enabled GPS's (they used to
think that the altimeter was right and the GPS altitude wildly inaccurate).


All the best,


David