Buick 2001 Park Avenue Owner's Manual page 120

Compass Variance
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 you live outside zone eight.
Under certain circumstances, as during a long distance
-
cross country trip, it will be necessary to adjust for
compass variance. Compass variance is the difference
between earth's magnetic north and true geographic
north. If not adjusted to account for compass variance,
your compass could give false readings.
To adjust for compass variance:
1. Find your current location and variance zone number
on the following zone map.
2. Press and hold COMPASS (or the on/off button for
mirrors equipped with OnStar controls) until a zone
number appears in the display.
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3. Once a zone number appears in the display, press the
COMPASS button (or the on/off button for mirrors
equipped with OnStar controls) quickly until the
correct zone number appears in the display. Stop
pressing the button and the mirror will return to
normal operation. If "C" appears in the compass
window, the compass may need calibration. See
"Compass Calibration" explained previously.