The instrument used to measure the height of mountains is called an altimeter. An altimeter can determine the altitude of an object above a fixed level, often sea level. There are different types of altimeters, such as barometric and GPS altimeters. Barometric altimeters measure pressure changes to estimate altitude based on the atmospheric pressure, while GPS altimeters use satellite signals to provide precise height measurements. In mountaineering and aviation, altimeters are crucial for navigation and safety, helping climbers and pilots understand their altitude relative to surrounding terrain.