"Alan White" wrote in message
...

Your GPS will display altitude above sea level. Above 'transition
altitude' aircraft fly at a 'pressure altitude' which may not coincide
with altitude above sea level.

Even with an altimeter set to the correct sea level pressure (strictly the
"QNH"), the barometric altitude on the pilot's altimeter may not correspond
to the true altitude -- the altimeter makes standard assumptions about the
temperature of the air between its level and its reference level (usually
sea level) in order to turn a pressure difference into an altitude.

For air traffic control purposes, this is a Good Thing. It doesn't matter
if you fly around with 10000 ft on your altimeter when the true altitude is
9700 ft, as long as everyone flying at that level agrees to use the same
standard!

Julian Scarfe