Chevrolet 2007 TrailBlazer Owner's Manua page 136

If the display reads CAL, you will need to calibrate
the compass. For more information on calibration,
see below.
To adjust between Fahrenheit and Celsius, do the
following:
1. Press and hold the on/off button until either a
flashing F or C appears.
2. Press the button again to change the display
to the desired unit of measurement. After
approximately four seconds of inactivity, the
new unit will be locked in and the display
will return.
If an abnormal temperature reading is displayed
for an extended period of time, please consult your
dealer. Under certain circumstances, a delay in
updating the temperature is normal.
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.
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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, do the following:
1. Find your current location and variance
zone number on the following zone map.
2. Press and hold the on/off button until a Z and
a zone number appear in the display.
The compass is now in zone mode.