Mxsmanic wrote:
mike regish writes:

My point is that they both are, basically, the same frame of
reference-height above sea level.

Nope. GPS is height above the mean surface of the geoid, altimeter is
height above mean sea level. They can be hundreds of feet apart.

You're thinking of height above the ellipsoid, which can be hundreds of
feet different from height above the geoid. But the geoid does
represent the mean sea level height - including in places that are far
from the sea. Internally GPS receivers generally initially calculate
height relative to the ellipsoid model of the earth's shape (using the
WGS-84 model parameters). However, recent models with which I'm
familiar then apply a correction term based on an internal lookup table
to convert the ellipsoid height to the geoid height (equivalent to
height above MSL) at that particular location. See:
http://www.esri.com/news/arcuser/0703/geoid1of3.html
The altitudes that end up being displayed by the GPS after its internal
correction are therefore based on elevation above MSL with some
measurement uncertainty that's dependent on the current satellite
geometry.