Chip Bearden wrote:
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?

Once again, pressure altitude and GPS altitude measure two different
things using the same units. Because we like to fly when there is a
non-standard temperature lapse rate, there is almost always noticeable
pressure altitude error above a few thousand feet AGL. If an optical
start gate is showing the actual heights, most gliders will appear to be
starting high, since the altimeters are reading low, and the validity of
the start is determined from the recorded pressure altitude. Those
pilots who are recording only GPS altitude (GPS handhelds, etc.) need to
be careful, as they start will be scored based on actual height, which
means they have to start lower. They need to be watching the GPS
display, rather then the altimeter, when they are flirting with the top
of the start cylinder.

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.

As you know, experts generally maintain whatever favors their position.
Look at it this way, GPS altitude is more accurate at measuring actual
height, pressure altitude is more accurate at measuring, well, pressure
altitude. Since one function of the flight recorder is to detect and
penalize airspace incursions, pressure altitude will continue to be a
consideration, no matter what else happens. Some of the other air
sports, like ballooning, have already switched over to using actual
height, they use sounding data and software to convert to/from pressure
altitude as needed.

Marc