An altimeter's an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude's called altimetry, which's related to the term bathymetry, the measurement of depth underwater. Aircraft instrumentation Pressure altimeter A pressure altimeter (also called barometric altimeter)'s the altimeter found in most aircraft. In it, an aneroid barometer measures the atmospheric pressure from a static port outside the aircraft. Air pressure decreases with an increase… (More on Altimeter)