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. Aircraft Instrumentation Pressure altimeter A pressure altimeter (also called barometric altimeter) is the altimeter found in most aircraft. In it, an barometer measures the atmospheric pressure from a static port outside the aircraft. Air pressure decreases with an increase of… (More on Altimeter)