Citizen Eco-Drive BN4021-02E Instruction Manual page 32

Calibrating altitude indication
30
3
Press button
Choose this when the actual altitude is higher than
the measured one.
Choose this when the actual altitude is lower than
the measured one.
Each time you press the button, choice of "+" and "‒"
changes alternately.
Repeat steps 3 and 4 to control the altitude value after
calibration.
to choose + or –.