Thanks Marc,

is it correct to say that the Pressure Altitude is an altitude calculated
starting from a pressure value, following a sort-of a rule as for ICAO-ISA ?
I guess official IGC loggers read the exact pressure as garmins and suunto
watches (!) and then they apply some calculations and name this result as
"altitude".
Do they do this without looking at what the GPS say?? Not even for an hint?

What is the formula used by all IGC loggers for doing this, then?

It's beyond my comprehension why if we are talking about pressure which is
always measured in the same way (right?) then this value has different
meanings and cannot be simply converted like with QNE-QNH-QFE.
ICAO-ISA is sort of a more complicated QNE, right? (question!)

On garmins you have a pressure sensor just like on a Colibrì, then this
sensor is used to compensate the gps and vice-versa, according to the patent
they have registered. By the way Marc could you understand anything useful
out of it?

After 4 years there are again the same questions on this matter so I guess
it's not very clear to everybody (me too).

thanks!
Paolo



"Marc Ramsey" ha scritto nel messaggio
et...

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