Buick Lucerne 2007 Owner's Manual page 134

Compass Variance
Compass variance is the difference between
earth's magnetic north and true geographic north.
If the mirror is not adjusted for compass
variance, the compass could give false readings.
The mirror is set in zone eight upon leaving
the factory. It will be necessary to adjust the
compass to compensate for compass variance if
the vehicle is driven outside zone eight. Under
certain circumstances, such as a long distance,
cross-country trip, it will be necessary to adjust the
compass variance.
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To adjust for compass variance, do the following:
1. Find your current location and variance
zone number on the zone map. See Automatic
Dimming Rearview Mirror with OnStar
Compass on page 130 for map information.
2. Press and hold the on/off button until a zone
number appears on the display.
3. Once the zone number appears on the display,
press the on/off button quickly until you reach
the correct zone number. If C or CAL appears
in the compass window, the compass may
need calibration. See "Compass Calibration"
listed previously.
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