2008 GMC Acadia Owner's Manual - Page 123

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. 3. Scroll through the zone numbers that appear in the window on the mirror by pressing the on/off button. Once you find your zone number, release the button. After about four seconds, the mirror will return to the compass display, and the new zone number will be set. If C or CAL appears in the compass window, the compass may need calibration. See "Compass Calibration" listed previously. 2-41