How can a pilot determine altitude during flight?

A pilot can accurately determine altitude during flight primarily by using an altimeter calibrated to local atmospheric pressure. The altimeter works by measuring the air pressure surrounding the aircraft. As the aircraft ascends or descends, the change in air pressure is translated into altitude readings. For this device to provide accurate information, it needs to be calibrated to the local pressure setting, often referred to as the altimeter setting, which is obtained from the nearest weather station or air traffic control.

The other methods mentioned, while helpful in certain contexts, do not provide the same degree of precision as a properly calibrated altimeter. Observing landmarks can offer a visual reference but is not consistent or reliable for measuring altitude, particularly at higher elevations or in poor visibility conditions. Using GPS coordinates can give position data which can be used to assess altitude indirectly; however, it is not as direct or reliable for altitude management as a barometric altimeter. Radar indications can be useful from an air traffic control perspective but are not tools that pilots rely on for directly determining their own altitude in-flight. Thus, the calibrated altimeter is the most effective and customary tool for altitude determination in aviation.