ATC computer converts the pressure altitude with local altimeter
setting. If ATC knows everyone is using 3100 as their altimeter setting
(can be by regulation in high pressure 31" situation, such as in
Canada), the computer can do similar converson, so the IFR separation
would not be unaffected.

Wade Hasbrouck wrote:


I thought the Mode C transponder reports Pressure Altitude and is basically
fixed at 29.92?