On Sep 26, 7:52*am, Peter Purdie wrote:
Transponders give Flight Level (assuming they are working properly), so
will give the altitude ATC are intereted in. *That's how they know what
FL you are at when not talking to them.

The controller see your transponder altitude CORRECTED for the local
pressure. So don't give them the number off the transponder display
unless the current pressure is 29.92

That's why the altimeter must be calibrated when you calibrate the
transponder. You must be able to set the altimeter to the current
(not what it was several hours or hundreds of miles away) before
talking to ATC, and that is why they will give you an altimeter
setting on initial contact unless you indicate you have current ATIS
info.

-Tom