Casio 3069 Manual page 81

Altitude Differential
You also can use the Altimeter Mode to measure the altitude differential
from a specific reference altitude. Specifying the point is as simple as
pressing a single button to reset the current reference altitude value to zero.
Altitude differential measurement is performed each time the watch performs
an altitude measurement.
• You must perform altitude differential measurement whenever you select
format number 3 or 4 as the Altimeter Mode display format.
• The range of the altitude differential value is –3,000 meters to 3,000
meters.
• "-- -- --" is displayed in place of the altitude differential value whenever the
measured value is outside the allowable range.
• The watch will assume that the reference altitude setting is zero for the first
measurement after you exit the Altimeter Mode setting screen (page E-77).
• See "Using Altitude Differential Measurement While Mountain Climbing or
Hiking" (page E-80) for some real-life examples of how to use this feature.
E-79