instrument used to measure the altitude of an object above a fixed level

An altimeter is an tool that can measure the height of an object, above a reference plane. A common reference plane is meters above sea level. There are different ways that this measurement can be done. The most common one uses atmospheric pressure. Other ways include ultra sound, laser light, or microwaves. Altimeters in planes often use lasers or microwaves.

Altimeter in an aircraft.