Because the altimeter setting you dial into the Kolsman window on the
altimeter is based on the pressure on the ground. The altimeter assumes a
standard pressure lapse rate as you go higher. The further you get from the
ground and the more non-standard the lapse rate really is, the less the
altimiter reading corresponds to the actual MSL altitude.

At airports that don't report altimeter settings I set my altimeter
according to my GPS altitude and this has given me a reasonable ground MSL
according to the published altitude of the airport.

--
Best Regards,
Mike
http://photoshow.comcast.net/mikenoel
"Pablo" wrote in message
oups.com...
Thanks for the input. I understand that true altitude is what the GPS
is reading. But why then would the elevation at airport reference
points be so highly accurate, while the reading at a few thousand feet
above the surface varies so much against the altimeter??