What is the purpose of an altimeter?

The purpose of an altimeter is to measure altitude above sea level. An altimeter functions by determining the atmospheric pressure at a given altitude and converting that pressure reading into a measurement of height above a predetermined reference level, typically sea level. This is crucial for pilots to maintain safe flight levels, adhere to air traffic control instructions, and manage terrain avoidance.

Other options do not align with the function of an altimeter. Measuring airspeed is done with aairspeed indicator, while vertical speed is monitored using a vertical speed indicator (VSI). Fuel quantity relates to a fuel gauge. Each of these instruments serves a distinct purpose in flight, but the altimeter specifically addresses altitude related to sea level, making it vital for navigation and safety in aviation.