What is the purpose of calibrating an altimeter before flight?

Calibrating an altimeter before flight primarily serves to ensure that the altitude readings are accurate relative to the standard altimeter setting. This calibration process typically involves setting the altimeter to the local atmospheric pressure at sea level, which is obtained from weather sources or ATIS (Automatic Terminal Information Service) broadcasts.

This step is crucial because altimeters measure altitude based on atmospheric pressure, which can vary with weather conditions. By calibrating the altimeter to the current pressure setting, pilots can maintain an accurate understanding of their true altitude above sea level or the terrain below, which is vital for safe navigation and avoiding collisions.

Accurate altitude information is vital for several aspects of flight, including adherence to safety regulations, effective communication with air traffic control, and ensuring that the aircraft is flying safely above terrain and obstacles. Hence, the calibration is a fundamental part of pre-flight checks to guarantee this accuracy.