How come GPS reads different

Thanks
"Peter" wrote in message
...
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.