Of course a COTS could easily output an ICAO-ISA altitude, it's just a
matter of using the formula and unselect any other corrections. The
manufacturer could thus implement this feature, it is much easier than
correcting and auto-calibrating GPS altitude.

This is true, but the pressure sensors need to have rather good
temperature compensation and long term stability, which may not be the
case with the sensors in consumer grade GPS receivers.



Sure, I forgot. The sensor of course need to be not only calibrated but also
compensated against temperature.
I think that latest Garmin have a temperature sensor for this, but not old
ones.

However, I feel that nowadays "consumer grade GPS" sold in dozen of
thousands of units at a price of 500-600$ (in europe much more) cannot be
called cheap and represent the state-of-the-art in terms of technology.
There are also low cost gps units, and you get what you pay for, exactly as
with LCD screens and computers, or a pair of glasses made in china.
So I would'nt bet that a good Garmin prices at 500$ is inferior to any
"professional" altimeter or gps ot both. Au contraire, it probably is better
being more recent.
Let's not forget that the cost of an Interseema sensor , used in GP941 and
many other I guess, is below 20$ to the common user.

In the end, I feel that IGC will not change the Code to make COTS usable for
badges. They should change the rule about what is altitude, abandoning
ICAO-ISA which is a standard.

Maybe with a petition?

Thanks again Mark! Paolo