Which instrument is used to measure an aircraft's altitude?

The altimeter is the correct instrument for measuring an aircraft's altitude. It operates by sensing changes in atmospheric pressure as the aircraft ascends or descends. The altimeter measures the external air pressure and translates this information into altitude above sea level, allowing pilots to maintain safe flight levels and navigate effectively.

In contrast, the speed indicator measures the aircraft's speed through the air, providing critical information for controlling flight dynamics but not altitude. The vertical speed indicator, while useful for showing the rate of ascent or descent, does not provide a specific altitude reading; instead, it indicates how fast the aircraft is climbing or descending. The magnetic compass serves a different purpose entirely, as it provides directional information based on the Earth's magnetic field but does not relate to altitude at all. Thus, the altimeter is uniquely tasked with providing altitude readings essential for safe flight operations.