What is the purpose of the altimeter?

The purpose of the altimeter is fundamentally to measure the aircraft's altitude above sea level. Altimeters achieve this by detecting changes in atmospheric pressure, which decreases as altitude increases. As the aircraft climbs, the pressure outside reduces, and the altimeter translates this pressure difference into altitude readings. This information is critical for pilots to maintain safe flight levels, ensure proper separation from terrain, and adhere to air traffic control instructions. Understanding altitude is vital for navigating and operating within controlled airspace, especially during takeoff and landing when proximity to the ground is a major concern. Thus, the correct response accurately identifies the altimeter's key function in aviation.