GMC 2003 Sierra 1500 Pickup Owner's Manual page 143

In order to calibrate, CAL must be displayed in the
mirror compass windows. If CAL is not displayed, push
in the COMP button for approximately eight seconds
or until CAL is displayed.
The compass can be calibrated in one of two ways:
Drive the vehicle in circles at five mph (8 km/h)
or less until the display reads a direction, or
0
drive the vehicle on your everyday routine and after
several turns the compass will become calibrated
and will display 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
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:
1. Find your current location and variance zone
number on the following zone map.
2. Press and hold the COMP button for five seconds
until a zone number appears in the display.
3. Press the COMP button on the bottom of the mirror
until the new zone number appears in the display.
After you stop pressing the button, the display
will show a compass direction within a few seconds.
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