Questions on high altitude pressures

es330td writes:

This brings up an interesting wrinkle then as GPS altitude info is not
dependent on external pressure so pilots must be careful to ignore
that info if available. As stated before, in Class A everyone sets
their altimeter to 29.92 so that as long as everyone is wrong together
everything is okay. Adding GPS info into the mix splits the groups
into two; one that is wrong together at 29.92 and another that is
right at actual altitude.

GPS is too inaccurate for most purposes in vertical positioning, anyway. It
is not designed to determine altitude with a high degree of accuracy, and can
easily be hundreds of feet off.