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.

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.

If your GPS has WAAS enabled, you're in an area where the model of the
earth closely matches reality and you have a current altimeter setting
for the exact area, they should come very close to matching. If any of
the above isn't true, you'll see some degree of difference between
them.

Chris W wrote:
It is my understanding that even if a GPS is reading perfectly and your
standard altimeter is reading perfectly even on a standard day that the
2 values will not be the same. Does anyone know about how far off they
are and if there is a way to calculate what the difference should be?
My guess is that at different points around the earth, the correction
will be significantly different, making a generic formula difficult if
not impossible to write.

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