Chevrolet TrailBlazer Owner's Manual page 115

Compass Calibration
When on, the compass automatically calibrates as the
vehicle is driven. If, after two seconds, the display
does not show a compass direction, (N for North, for
example), there may be a strong magnetic field
interfering with the compass. Such interference may be
caused by a magnetic antenna mount, magnetic
note pad holder, or a similar magnetic item. If the
letter C should ever appear in the compass window, the
compass may need calibration.
Press and hold the on/off button for nine seconds to
activate the compass calibration mode. CAL will
be displayed in the compass window on the mirror.
The mirror can be calibrated by driving the vehicle in
circles at 5 mph (8 km/h) or less until the display shows
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 conditions, 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, the compass could give false readings. To
adjust for compass variance do the following:
1. Find the current location and variance zone number
on the following zone map.
2. Press and hold the on/off button until a zone
number appears in the display.
3. Once a zone number appears in the display, press
the on/off button quickly until the correct zone
number appears in the display. If C appears in the
compass window, the compass may need
calibration. See "Compass Calibration"
explained previously.
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