Barometric altitude is relative to a notional ground level or sea level and not necessarily relative to the local terrain.
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.
