Bob Gardner wrote:

AIM 1-1-19(a)(8) tells pilots not to use GPS altitude: "GPS altitude should
not be relied upon to determine aircraft altitude since the vertical error
can be quite large."

Main reason is that other planes will be using pressure-based altimeters
so maintaining separation can only be done when everyone uses the same
method with the same errors.

GPS altitude is measured above the GPS sphere, which is not sea level.

No, GPS altitude is measured above the ellipsoid defined by WGS-84, not
any spherical surface. Furthermore, the altitude is then corrected
using a lookup table to account for the difference between the geoid
(which represents the sea-level surface) and the ellipsoid. The
altitude figure displayed by the GPS receiver is therefore measured from
the hypothetical sea-level surface.

Bob Gardner

"Aluckyguess" wrote in message
news
How come the GPS reads a different altitude than the Altimeter?

As others have mentioned, the pressure-based altimeter assumes a
standard atmosphere model for the temperature lapse rate and can be
quite far off if the actual conditions don't match the model - even when
the altimeter is corrected for the current ground-level pressure.