mike regish writes:

Well, what exactly are their reference datum(s?).

GPS uses a theoretical model of the earth's surface--a geoid. It may
or may not coincide with sea level.

However, that's not the main source of inaccuracy in GPS altitudes.
The main source is the difficulty of measuring altitude when the
position to be measured involves so many very oblique angles from the
reference satellites. It's easier to measure lateral position looking
straight down than it is to measure vertical position looking at a
shallow angle from 1000 miles away.

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