Casio 3145 Manual page 86

There are two standard methods of expressing altitude: Absolute altitude and
relative altitude. Absolute altitude expresses an absolute height above sea
level. Relative altitude expresses the difference between the height of two
different places.
E-84
Height of building 130 m
(relative altitude)
Rooftop at an altitude of
230 m above sea level
(absolute altitude)
Sea Level