GMC Sierra Classic 2007 Owner's Manual page 177

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 appear 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 next.
Compass Calibration
The compass may need calibration if one of the
following occurs:
•
After approximately five seconds, the display
does not show a compass heading (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.
•
The compass does not display the correct
heading and the compass zone variance
is set correctly.
In order to calibrate, CAL must be displayed in the
mirror compass windows. If CAL is not displayed,
push the on/off button for approximately
12 seconds or until CAL is displayed.
177