Originally Posted by
RatherBeFlying
Harmonised Transition Altitude
In North America, it's 18,000 which gives time for ATC systems to spot a mismatch between cleared altitude and mode C readout. It seems more sensible to me to set the altimeter to ATIS once I receive it, than have an opportunity to forget to do it sometime later.
Mode C data are not corrected by the altimeter setting in the cockpit, but adjusted by computer at the ATC facility based on current altimeter setting - therefore the reading that was shown on their screens would be the actual altitude - ergo no excuse for the ATCO to not notice.