"Jim Macklin" writes:

they give altitude, accurate to within a few feet.

Unfortunately, no, they do not. GPS is accurate for lateral
navigation, not vertical navigation. GPS altitudes can easily be off
by as much as 200 feet at ground level in comparison to a correctly
set altimeter, and at altitude the disparity can reach 500 feet.

The reason for this is that the angles used for triangulation of
lateral positions are large and permit a high level of precision, but
the angles for triangulation of altitude are very small and it's very
easy to be off by a wide margin. GPS was designed to measure lateral
positions accurately, but it performs poorly for altitude. It is
typically much less accurate than an altimeter for altitude, and the
computed altitude constantly changes (significantly) as the satellites
move, in a way that is, for aviation purposes, practically random.

... just set the altimeter to read the same.

This is a good way to fly into a mountain, or another aircraft, or the
runway.

... then you know adjusted
pressure. considering the legal requirements, it is
perfectly adequate.

No, it is not. It's especially dangerous for RVSM flight, but it's so
inaccurate that it should never be used for anything, except as a last
resort (if the altimeters disintegrate, or whatever).

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