Altimeter Calibration Height

In other words, an altimeter indicates altitude at the level of the
instrument itself.

That seems reasonable.

But what if the instrument itself is 100 ft tall? Or maybe just 30 inches
tall? Which point on the altimeter should one pick and why?

To answer that question, one must understand that an altimeter does not
sense altitude, it senses pressure. The pressure would be some sort of
average over the entire height of the sensing surface, whose exact value
would depend on the construction of the device. That pressure would
likely be equal to the ambient pressure at some altitude within that range.

By painting appropriate numbers next to the pointer, one could get the
altimeter to indicate altitude. It is the choice of numbers that is the
issue, as that is arbitrary, and not subject to the laws of physics,
only to the laws of the FAA.

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.