CryptWolf wrote:

"Aluckyguess" wrote in message

news How come the GPS reads a different altitude than the Altimeter?


GPS altitudes are calculated in a way that is similar to how latitude and
longitude is calculated. Your GPS altitude is what is called a geopotential
altitude and it is based on a sphere. If you don't already know, the Earth
is an imperfect sphere. That is where wide area augmentation system
(WAAS) signals kick in with corrections to the latitude, longitude
and altitude. Even without WAAS, altitude is usually pretty close.

Yes it is, which makes it very clear that your previous statement about
GPS altitudes being based on a spherical earth is false. The earth's
polar and equatorial radii differ by about 13 miles, so altitudes based
on a spherical model would be off by miles. But my measurements with my
non-WAAS GPS show the altitude measurements to be within 40' well over
95% of the time.

In fact, the GPS measurements are based on the WGS-84 defined
ellipsoidal shape of the earth and then corrected for the difference
between the geoid (the hypothetical mean sea-level surface) and the
ellipsoid.

Don't forget the altimeter in the airplane can be off also.

If the GPS has a good skyview of the satellites then this is the most
likely source of the discrepancy. Pressure-based altitudes depend on an
assumed standard model of the atmosphere. The actual conditions can be
quite different and result in considerable variation of the reported
altitude.