mike regish schrieb:
Well, what exactly are their reference datum(s?).

GPS altitude and pressure altitude are completely different and, most
important, *incompatible* systems.

GPS altitude gives you a geometrically derived value above some
reference surface. Mostly it's the WGS84 geoid. Of course this can be
converted to MSL altitude if you wish. But this isn't the point.

The point is that the pressure altimeter measures, well, a pressure, not
an altitude. It displays a value in feet, but actually, this is wrong.
It may be true in very specific conditions, but the real atmosphere very
seldom matches the theoretical ISA atmosphere model on which the
altimeter is based. And, most important, there is no way to calculate
the real (geometric) altitude from the displayed pressure altitude. (Of
course it could, if you knew the complete atmosphere profile, but you
usually don't.)

So the point is: GPS gives you the true altitude in reference to the
earth, but this doesn't help you, because the whole aviatic system
(airspace boundaries, ATC clearances, traffic separation) is based on
pressure altitude, and there is no practical way to convert one into the
other. If you are given an ATC clearance for a certain pressure altitude
but fly GPS altitude instead, then you act exactly like that bozo who
drives on the wrong side of the road.

The only use for GPS altitude in aviation is to calculate the final
glide of a glider. (Or an IFR approach, but I don't know enough about
IFR flight to comment this.)

But I'm sure you knew all this before, because after all, you've passed
a written.

Stefan