Neil Gould writes:

I'm not sure why you think that "GPS is far less accurate than an
altimiter..." ...

Because I studied GPS for a long time and was using it before anyone
else knew what it was, and I know how badly it measures altitude. GPS
was designed to accurately measure longitude and latitude; at best, it
gives only a rough estimate of altitude (ships and troops, the
original users of GPS, don't need to know their altitudes).

... as an altimeter only need be accurate to 75' to be legal.

GPS is routinely off by hundreds of feet.

GPS can do much better than that ...

No, it cannot ... not vertically. It's actually much worse.

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