Hummer 2004 H2 - Manual (Page 116)

The compass can be calibrated by driving the vehicle in
circles at five mph (8 km/h) or less until the display
reads a direction.
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.
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.
To adjust for compass variance, do the following:
1. Find your current location and variance zone
number on the 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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