What does true altitude measure?

True altitude specifically refers to the actual altitude of an aircraft above Mean Sea Level (MSL). This measurement is critical for ensuring safe separation from terrain and obstacles, as well as for accurate navigation and flight planning. True altitude provides pilots with a reliable reference point, which is essential for proper altitude assignments, especially when flying in mountainous or varied terrain.

While other options may pertain to different altitude measurements—such as height above a specific location like an airport or altitude calculations under varying atmospheric conditions—none of these provide the comprehensive and universally applicable understanding that true altitude does in relation to sea level. Hence, recognizing true altitude as the actual elevation above MSL is paramount for aviation safety and navigation.