On 16 Apr 2006 13:52:58 -0700, "peter" wrote:

Grumman 236 wrote:
The altimeter is a barometer and the GPS is a computerized stopwatch.
Considering that they measure such different things, amazing that
there's any realationship at all, huh?

The mathematical model of the earth the the GPS software uses is
idealized somewhat and provides only an approximation of the bulging of
the surface of the earth relative to its geometric center point
(points, actually!). So, at any particular location you're over, your
actual distance from the ground can be different from what the GPS's
software thinks it is. So point A is that the GPS altitude can be
inaccurate.

But note that most consumer GPS receivers do not report altitudes based
only on the idealized mathematical model of the earth's shape
(generally the WGS-84 ellipsoid). They also include correction tables
that reflect the difference between the geoid shape and the ellipsoid
as a function of your position over the earth's surface. So the
reported altitude is with respect to sealevel, not relative to the
idealized ellipsoid. Naturally there will still be measurement
uncertainties and the correction table is also limited in precision,
but in terms of aircraft altitudes the errors are pretty small.

Point B is that your altimeter is only as accurate as the setting you
have in the Kollsman window. If it's based on old data or data for a
distant location or if rapid weather change is occurring, it won't
exactly reflect the pressure corrected for MSL for the location you're
above.

True, but then there's also Point C. Even if you've entered the
correct current value into the Kollsman window for your location, it
still just reflects the sealevel-adjusted pressure at ground level
below you and your altimeter then uses an idealized standard atmosphere
model to determine your altitude. If the temperature and lapse rate of
the actual atmosphere doesn't match the model then the reported
altitude can be significantly different from reality; hundreds or even
a thousand feet or more of variation.

But, as mentioned, aircraft vertical separations are based on
barometric altitudes and as long as everyone in a given area is off by
the same amount the separations will still be maintained.

Have a look at the following url:
http://sps.unavco.org/geoid/

David