Copied from the owners manual Section 2-108 & 2-109:
http://www.vadengmpp.com/owners-manual/buick/2002-Buick-Rendezvous.pdf
Automatic Compass Calibration
The compass is self-calibrating, which eliminates the
need to manually set the compass. When the vehicle is
new, the calibration process may not be complete. In
these cases, the calibration symbol C will be displayed
where the compass reading is normally displayed.
To calibrate the compass, in an area free from large
metal objects, make three 360 degree turns. The
calibration symbol will turn off and the compass reading
will be displayed.
Manual Compass Calibration
If the compass appears erratic and the calibration
symbol does not appear, you must manually put the
compass into the calibration mode. To get into this
mode, do the following:
1. Turn the ignition on and cycle the DIC to the
compass/temperature mode.
2. Press and hold the SET button for longer than
three seconds.
ZONE # PRESS & HOLD SET TO CHANGE:
This display mode will be displayed when you are manually
calibrating the system. The current zone number for
compass variance will be displayed in place of the
# symbol. Use the SET button to select the zone number
from the graphic shown to select the current area of the
country that you are driving in.
PRESS SET TO CALIBRATE COMPASS:
After selecting your zone, press the MODE button and this
will be displayed on the DIC. Press the SET button and
complete three 360 degree turns in an area free from
large metal objects.
The following two messages will toggle in the display
until you calibrate the compass:
DRIVE VEHICLE IN CIRCLE
CALIBRATING COMPASS
When calibration is complete, the display will return to
its normal mode.