What is absolute altitude?

Absolute altitude refers to the height of an aircraft above the terrain directly beneath it. This is typically measured using a radar altimeter, which provides information about the distance between the aircraft and the underlying ground. The key aspect of absolute altitude is that it gives pilots a clear understanding of their height relative to the ground surface, which is crucial for safe flight operations, particularly when flying at low levels or on approach to landing.

The other options describe different types of altitude measurements. For instance, the first choice speaks to altitude as measured from sea level using GPS, which is actually referred to as true altitude or height above sea level. The third option refers to air density changes, which is related to altitude but is not a direct measure of how high an aircraft is above ground or sea level. Lastly, while the fourth choice discusses altitude concerning mean sea level (MSL), it describes true altitude rather than absolute altitude as defined in aviation terminology. Understanding these distinctions helps clarify the concept of absolute altitude and its importance in aviation.