The main problem is that QNH measures relative altitude and GPS measures absolute altitude, so the comparison really should be between the rad altimeter and the GPS, but over uneven terrain that rad alt reading isn't going to be so helpful unless the plane has a detailed terrain map to convert the latitude and longitude into an expected distance reading. The one place that QNH and GPS would coincide is on the runway, but then it's too late for circumstances like this.
To be more clear - the difference in altitude detected by the barometric system is dependent on the density of the air, so the comparison is inexact, but it won't matter as the control is based on eventually matching the pressure reported for the runway and the pressure detected by the barometric system. In the air the difference in pressure is a measure of the relative pressure altitude, not the absolute actual difference.
Last edited by MechEngr; 18th Oct 2022 at 16:08.