Chevrolet TrailBlazer 2007 Owner's Manual page 134

Compass Calibration
Press and hold the on/off button 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.
To adjust for compass variance, do the following:
1. Find your current location and variance
zone number on the following zone map.
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2. Press and hold the on/off button for
six seconds. Release the button when
(FOR ZONE) is displayed. The number
shown is the current zone number.
3. Scroll through the zone numbers that appear in
the window on the mirror by pressing the on/off
button. Once you find your zone number,
release the button. After about four seconds,
the mirror will return to the compass display,
and the new zone number will be set.