Have I got this right? We use barometric pressure to
measure altitude, difference in height, whatever. It
actually measures pressure which has to be converted
to height. We know that this 'measurement' will be
inaccurate dependent on temperature and the pressure
situation at the location, the amount of the accuracy
is completely unknown. Two height diamonds gained on
the same day at different locations will have to 'gain'
different amounts of height.
GPS measures the height above a known datum, with error
correction the height readouts are the same everywhere,
GPS measures the altitude in the units we use (Metres
or feet), there is no conversion required.
The question I am asking is why are we clinging on
to and outdated and inaccurate system when a much more
accurate system is available. We measure distance over
the globe in feet or metres and happily use GPS for
that, why do we not use it to measure the vertical
directly distance as well?

Perhaps we should revert to measuring in cubits

At 09:54 04 June 2004, Paul Bart wrote:

'Andy Durbin' wrote in message
. com...
'Paul Bart' wrote in message news:
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