Originally Posted by
aviationluver
Does anyone know why there is usually a difference of a few hundred feet between the Indicated Altitude read on the Altimeter versus the GPS Altitude? The GPS Altitude is usually a few hundred feet higher.
The difference will vary depending on where you are in the world and on the prevailing air pressure.
GPS altitude is relative to a reference ellipsoid which may or may not coincide with mean sea level depending on where you are in the world. What you see on the altimeter will also differ from height AMSL by an amount depending on the difference (if any) between the prevailing QNH and the subscale setting.