GMC 2007 Acadia Owner's Manual page 141

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.
If after a few 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, note pad
holder, or similar object. If the letter C or CAL
appears in the compass window, the compass may
need to be reset or calibrated.
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:
1. Find your current location and variance zone
number on the following zone map.
2. Press and hold the on/off button until the zone
number is displayed. The number shown is
the current zone number.
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