Contrary to what its name might lead you to believe, an altimeter does not directly measure altitude. It is a pressure measuring instrument used to estimate an altitude.

Its utilization requires calibration.

This animation shows an altimeter used in aviation. It is graduated in feet (1000 feet = 304.8 meters), and the pressures are in mb (millibars) or in inches (in) of Mercury.

An adjustment enables one to make an altitude correction.  See the Student Sheet for a simulation of its utilization in navigation by an aircraft.