Originally Posted by
Lightning Mate
As a senior ATPL lecturer of aircraft instrumentation, may I comment?
An altimeter "measures" a vertical displacement from absolutely any pressure datum you wish to choose. It has nothing to do with a "notional ground level" or indeed "sea level".
An altimeter is merely an aneriod barometer, the measurement datum of which may be chosen as you wish.
Sorry, but when calibrated as an ALTImeter, it is using pressure to sense altitude i.e. height. The reference pressure does indeed correspond to a notional ground level, in the case where you set to achieve the correct reading at touchdown, or a notional sea level, which is the higher altitude approach, using the ISA that gives a notional height above notional sea level.
The measurement data used for aeronautical purposes are to enable an instrument to provide a relative height to a reference.
Otherwise, the scale would be in Pa or Mbar, not feet and metres.