What altitude measurement is typically used in broadcasting Flight Levels?

The correct answer is based on the standard practice in aviation where Flight Levels are measured above sea level using the International Standard Atmosphere (ISA) as a reference. This measurement is crucial because it provides a uniform reference point for aircraft operating at high altitudes, ensuring that all aircraft are operating at distinct vertical levels regardless of geographical features such as mountains or valleys.

Using above sea level as the baseline helps in maintaining safe vertical separation between aircraft, especially in controlled airspace where multiple aircraft may be operating at similar altitudes. This standardization is essential for collision avoidance and the safe management of air traffic.

Other altitude references like above ground level or height above terrain can lead to dangerous situations, especially in areas with significant elevation changes. Flights operating at higher altitudes must rely on the consistent, universally understood reference of above sea level to ensure safety and efficiency in flight operations.