Casio 3069 Manual page 106

Question: How does the altimeter work?
Answer: Generally, air pressure and temperature decrease as altitude
increases. This watch bases its altitude measurements on
International Standard Atmosphere (ISA) values stipulated by the
International Civil Aviation Organization (ICAO). These values define
relationships between altitude, air pressure, and temperature.
Altitude
4000 m
3500 m
3000 m
2500 m
2000 m
1500 m
1000 m
0 m
Source: International Civil Aviation Organization
E-104
616 hPa
701 hPa
795 hPa
899 hPa
500 m
1013 hPa
Air Pressure
About 8 hPa per 100 m
About 9 hPa per 100 m
About 10 hPa per 100 m
About 11 hPa per 100 m
About 12 hPa per 100 m
Temperature
–11°C
–4.5°C
About 6.5°C
per 1000 m
2°C
8.5°C
15°C