What does a correctly set altimeter ensure during flight?

A correctly set altimeter ensures accurate altitude readings and terrain clearance, which is vital for safe flying operations. The altimeter indicates the aircraft's altitude above sea level based on the air pressure, which can fluctuate with weather changes. When the altimeter is set correctly to the local altimeter setting, it allows pilots to maintain required altitudes, avoid obstacles, and comply with air traffic control instructions.

Maintaining proper altitude becomes especially important when flying in proximity to terrain and other aircraft. Accurate altitude readings help prevent accidents related to controlled flight into terrain (CFIT) and ensure that pilots are clear of any obstructions or other aircraft flying at different altitudes. By ensuring that the altimeter is properly set, pilots can verify their altitude with respect to sea level, aiding in safe navigation and adherence to altitude restrictions within controlled airspaces.

Other options, while they may have some relevance to flying, do not directly relate to the primary function of an altimeter in maintaining safety and operational standards regarding altitude.