On Mon, 13 Nov 2006 18:07:31 -0500, LWG wrote:
I have always found good agreement between GPS altitude and my
altimeter. And I've read all about the derivation of GPS altitude, and I
wouldn't think it would be so close.

Part of the problem of comparing the GPS receiver output to the altimeter
output is that the GPS receiver output is digital and you end up seeing
the digits constantly changing... If you displayed the output of the GPS
receiver on an analog gauge, it might be a bit better... If you averaged
the altitude over multiple data point, it might even be even more
stable... The non-aviation GPS receivers usually update their position
every second... The Garmin GPSMAP 496 updates 5 times per second... From
what I understand, that 5 Hz update rate is considered the minimum for an
accurate aviation guidance system... I've seen others that operated at 10
Hz... The more messages that you can get per second, the more you can
average out the calculations to keep the altitude from radically jumping
around...