What does 'true altitude' refer to?

True altitude specifically refers to the actual height of an aircraft above mean sea level. It represents the vertical distance of the aircraft from this reference point, regardless of atmospheric conditions such as pressure or temperature.

In aviation, various terms are used to describe altitude. Indicated altitude, for instance, is the altitude read directly from the altimeter, which can be affected by local air pressure settings. While adjustments for non-standard temperature can influence indicated altitude, they do not define true altitude itself.

True altitude is critical for navigational purposes, especially when flying over mountainous terrain or in controlled airspace, ensuring that the aircraft maintains a safe vertical clearance from the ground or obstacles. The use of GPS indicated altitude provides another reference but does not inherently capture the definition of true altitude, which is strictly about the relationship to mean sea level.