Absolute Altitude

Absolute altitude is the vertical distance between an aircraft and the terrain directly below it — also called height above ground level (AGL). Unlike indicated altitude, which is referenced to sea level, absolute altitude changes constantly as the terrain rises and falls beneath the aircraft.

Why It Matters

Absolute altitude is the number that determines whether you clear an obstacle or a ridge. On a cross-country flight at a constant indicated altitude, your absolute altitude can vary by thousands of feet depending on terrain. This is particularly relevant in mountainous areas of Canada, where terrain can rise rapidly and without prominent visual cues.

How It Is Measured

A standard barometric altimeter does not measure absolute altitude — it measures indicated altitude (referenced to sea level). Absolute altitude is measured by a radar altimeter, which sends a radio pulse downward and measures the time it takes to return. Radar altimeters are standard equipment on commercial transport aircraft and are used on approach for ground proximity awareness.

On the TC Written Exam

Exam questions on altitude types test your ability to distinguish between indicated, absolute, true, pressure, and density altitude. Absolute altitude = AGL. It is measured by a radar altimeter, not a standard barometric altimeter.

Related Terms

See also: Indicated Altitude, True Altitude, Pressure Altitude, Density Altitude.