Timex W-268 Mode D'emploi page 273

Press B (FLYBACK/RESET) to reset (zero) the chronograph.
When using distances other than one mile or kilometer,
multiply the result by the distance. For example, if you
are using two marks one half mile apart, and the 4th
center hand points to 80, the speed is 40 miles per hour
(1/2 x 80).
COMPASS CALIBRATION
Before use, the compass must be calibrated.
Since a compass uses the earth's magnetic field, you
must be aware of large metallic objects, such as vehicles
or bridges, or objects that give off electromagnetic
radiation, such as televisions or computers, which
could affect the magnetic field around the watch. If the
compass is calibrated near such objects, it will only
work properly in that same location. If the compass
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is calibrated away from such objects, it will not work
properly near them. For best results, calibrate in the area
of intended use.
In calibrating and taking headings, it is important, as with
all compasses, to keep the compass level. Calibrating or
taking a heading when the watch is not level can result
in large errors. Also avoid areas that may exhibit higher
than normal concentrations of hard and soft iron, as they
may cause inaccurate headings. Whenever a heading is
suspect, recalibrate.
Do not store the watch near any source of magnetism,
such as a computer, appliance, or television, as the
watch can take on a magnetic charge, yielding inaccurate
headings. Typical accuracy of wrist-worn instruments is
within +/- 10 degrees.
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