What is the function of an altimeter?

The primary function of an altimeter is to measure altitude above sea level, which is crucial for pilots to maintain safe flying levels, especially when navigating through varying terrain and avoiding obstacles. Altimeters work by calculating the atmospheric pressure outside the aircraft and converting that measurement into an altitude reading based on the standard atmospheric pressure at sea level. This allows pilots to determine their vertical position relative to sea level, which is essential for altitude management, adherence to air traffic control instructions, and ensuring safe separation from other aircraft.

Other options describe different instruments or measurements; for example, measuring the rate of climb pertains to a variometer, while indicating airspeed relates to an airspeed indicator, and showing the aircraft's heading refers to a heading indicator. Each of these functions plays an important role in flying and navigation but does not pertain to the primary purpose of an altimeter.