Casio WSD-F10 User Manual page 61

Displayed Altitude Values (Relative Altitude)
The two ways to express altitude are described below.
Relative Altitude:
This is the difference in the altitudes between two locations.
Example: Height measured from the ground to the top of a building (A)
Sea Level Altitude (Elevation):
Altitude measured from sea level.
Example: Height measured from sea level to the top of a building (B)
The altitude readings produced by your watch are relative altitudes, based
on the fact that barometric pressure drops as altitude increases. The altitude
values displayed by your watch are based on International Standard
Atmosphere (ISA) relative altitude data defined by the International Civil
Aviation Organization (ICAO).
①
Sea level
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②