At 00:00 29 December 2004, Eric Greenwell wrote:
My example was for GPS distance, and pressure altitude,
to indicate that
a distance measurement wasn't a problem. That's why
a I later referred
to flying off at least 1000 feet if GPS altitude was
used.

What I don't know is how much error change one can
expect in GPS
altitudes taken 5 or 10 minutes apart. The difference
(GPS start height
minus GPS finish height) might have a much smaller
error than the
altitude itself, which would allow shorter glides
(500 foot loss if the
differential error was only 5 feet, for example).

Clarification noted - but distance measurement is a
problem with GPS with respect to polar calculations.

Without knowing the technique Dick Johnson uses, or
the specs on a specific pressure transducer, it's hard
to know if measuring pressure altitude through a digital
transducer is more or less accurate than the traditional
method. I'd guess it's a close call, but that has nothing
to do with GPS.

The main source of error, is being able to turn GPS
ground speed (or distance) into IAS reliably by subtracting
wind speed and adjust for altitude. An even greater
source of error is trying to use fixes from a typical
soaring day with airmass movements and pilot control
inputs, airspeed changes and flightpath deviations.

The empirical evidence is that there is way too much
randomness from the above noted effects to tease out
a anything much beyond just how much randomness there
in fact is on a typical flight.

Maybe if you did fifty 10-mile runs on a dead calm
day across five different airspeeds, you'd get less
scatter - but I think that's more or less what Dick
does, except he measures IAS directly, rather than
having to figure it out from ground speed.

9B