GMC 2008 Envoy Denali Owner's Manual page 119

Compass Operation
Press the COMP button once briefly to turn the compass
on or off.
When the ignition and the compass feature are on, the
compass will show two character boxes for approximately
two seconds. After two seconds, the mirror will display
the compass heading.
Compass Calibration
The compass may need calibration if one of the
following occurs:
•
If CAL is displayed while driving in the vehicle.
•
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, press
and hold the COMP button until CAL is displayed.
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.
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.
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