Altitude in aviation is expressed in relation to what?

Altitude in aviation is commonly expressed in relation to sea level or ground level because this standard helps ensure consistency and safety in navigation, flight operations, and air traffic control. When altitude is measured relative to sea level, it allows pilots and air traffic controllers to have a common reference point, which is essential for avoiding terrain and other aircraft. This measurement can be either true altitude, which is the actual height above sea level, or indicated altitude, which is what the altimeter records based on settings and atmospheric conditions.

Using sea level as a baseline is particularly vital in aviation, given that variations in atmospheric pressure can affect how altitude is perceived and recorded. Ground level measurements may also be used, particularly in specific contexts such as takeoff and landing, but sea level provides a universally recognized reference for operational procedures and safety protocols in the broader airspace.