GMC 2009 Sierra - Manual (Page 114)

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 of
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 do the following:
1. Find your current location and variance zone
number on the following zone map.
2. Press and hold the on/off button until a Z and a
zone number appears in the display. The compass
is now in zone mode.
3. Keep pressing the on/off button until the desired
zone number appears in the display. Release the
button. After approximately four seconds of
inactivity, the new zone number will be locked in
and the compass/temperature display will return.
4. Calibrate the compass as described below.
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