"Andy Durbin" wrote in message
om...
"Paul Bart" wrote in message news:x2Gvc.4102

however given that GPS
can *potentially* reduce an error, by up to 1000 ft it should be
seriously
considered. I wonder if the resistance to change is mainly due to the
high
average age of the gliding fraternity?

Paul


You seem to have missed the frequently stated point that the
difference is not an error. An error free measurement of pressure
altitude will not be equal to an error free gps (geometric) altitude
except under rare conditions.

No I did not. You are correct, I have incorrectly used the word error if
one considers it's meaning in a relation to the output of the measuring
device, however that was not my intention.


Recognition of this fact may have something to do with age, but the
real issues are recognizing what is to be measured,



Height above ground I would have thought. If I understand the preceding
discussion correctly, pressure altitude was used because there were no other
viable options, not because it was a good measure of height above ground.



why it is being measured,



To either establish benchmark, or to fulfill some requirements. For each of
these I would rather know the actual distance above ground, not a measure
that depends on prevailing meteorological conditions. Unless, of course,
you consider Martin's observation, that the effort to climb to a particular
pressure altitude takes about the same effort regardless of the geometric
altitude and also assuming that it is the effort that is important, rather
then the actual height above ground.



and then determining whether it is reasonable to change to
measuring something else.





Thank you for your observation.



Paul