Altimeter

An altimeter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth underwater.

Altitude with analog and digital altimeters is determined based on the measurement of atmospheric pressure (atmospheric pressure is the force exerted against a surface by the weight of air molecules above that surface). The greater the altitude, the fewer the air molecules and therefore the lower the pressure exerted against the altimeter surface.

GPS (Global Positioning System) units can also be used to measure altitude with the help of four satellites, but this system is less accurate and in some places unavailable.