"David Megginson" wrote in message
news
As far as I understand, your mode C will be reporting pressure altitude to
ATC, and their computers will adjust that based on the altimeter setting
in
use for the sector. So the difference between the mode C and indicated
altitude can be huge, but I don't think that's what you're asking.

You are correct that if you have a transponder which displays its reported
pressure altitude, that can be significantly different than field elevation.
Just turn your altimeter to 29.92 and note the difference between that
pressure altitude and the true altitude with a proper altimeter setting.

When ATC reports your Mode C altitude back to you, their computer has
adjusted the pressure altitude for the proper altimeter setting. The
allowed tolerance is 200 feet; if your altitude is off by more than this
then you should have your altimeter and static system checked.


--------------------
Richard Kaplan, CFII

www.flyimc.com