What is meant by true altitude?

True altitude is defined as the height of an aircraft above mean sea level (MSL). This measurement is crucial for navigation, safety, and ensuring proper separation between aircraft. It is the actual vertical distance of the aircraft from the standardized reference point of mean sea level, which is derived from the average sea level over a specified period across various locations. True altitude is used in aviation to ensure that pilots understand their position vertically in relation to the earth's surface, especially when flying in mountainous or varying terrain.

Understanding true altitude helps pilots make informed decisions about their flight paths and safety measures. The other options do not accurately convey this definition; for instance, the height relative to the nearest airport does not account for variations in elevation at different airports or locations, and altitude above the ground does not consider the broader reference of mean sea level, while a non-standard altitude based on atmospheric conditions introduces variables that are not part of the standard definition of true altitude.