Altimetry
FFF,
You are quite correct about the airfield QNH. My example referred to a departure from a sea-level airfield where the altimeter had been zeroed before take-off and the pressure setting had not been changed. The temperature error in this case would have been zero before departure but would be as I quoted in my example. If you set Airfield QNH on the ground (no matter what is its elevation), your altimeter should read airfield elevation. If, however, you climb above the airfield, your altimeter will only indicate correctly if the temperature is ISA. If the temperature is below ISA, the altimeter will over-read.
Regards