On Feb 24, 7:07*pm, Marc Ramsey wrote:

The easiest way to see this is in an IGC file from an approved flight
recorder. *During summer months, note the relative difference between
the pressure and GPS altitude at the lower and higher altitudes. *As
altitude increases, GPS altitude will increase relative to pressure
altitude, as pressure altitude is reading too low at higher altitudes.
Here in the western US, we can see this clearly near mountain peaks, as
a correctly set altimeter will be reading as much as 1000 feet too low,
whereas GPS altitude matches the known elevations of the peaks.

OK, I'm a layman, late middle age, and little slow. What am I missing?
The pressure altitude (per the altimeter, at least) is less accurate
than the GPS altitude? By up to 1000 feet at Western USA soaring
altitudes? In the old days, we used a start gate that evaluated
optically how high we were above the ground. Assuming no one tripped
over the guy wires, that actual altitude stayed the same during a
contest. Now we're evaluated using a pressure-altitude-recording
device that may or may not reflect how high we really are?

Do the experts maintain that GPS altitude is bad because (a) it
DOESN'T have the errors inherent in pressure altitude or (b) because
its precision isn't good enough? It seems like I've seen both
positions on this forum.

Chip Bearden
ASW 24 "JB"
USA