Chevrolet 2006 TrailBlazer EXT Owner's Manual page 122

Compass Calibration
Press and hold the on/off button down for nine seconds
to activate the compass calibration mode. CAL will
be displayed in the compass window on the mirror.
The compass can be calibrated by driving the vehicle in
circles at 5 mph (8 km/h) or less until the display
reads a direction.
Compass Variance
Compass variance is the difference between earth's
magnetic north and true geographic north. The mirror is
set to zone eight upon leaving the factory. It will be
necessary to adjust the compass to compensate
for compass variance if you live outside zone eight.
Under certain circumstances, such as during a
long distance cross-country trip, it will be necessary
to adjust for compass variance. If not adjusted to
account for compass variance, your compass could give
false readings.
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To adjust for compass variance, do the following:
1. Find your current location and variance zone
number on the map.
2. Press and hold the on/off button for six seconds.
Release the button when ZONE is displayed.
The number shown is the current zone number.