Simply put, absolute altitude is the height above ground level (AGL) – or to put it another way, it is the vertical distance of an aircraft above the landscape or terrain that it is flying over. Therefore, when on a runway, an altimeter will read 0ft.
How to Calculate Absolute Altitude
Absolute altitude can be determined through the use of a radio or radar altimeter, which work by timing how long it takes a beam of radio waves to reflect from the ground and return to the aircraft.
Generally, radio or radar altimeters have a limited range of operation, giving readings up to a maximum of 2,500 feet AGL.
Absolute altitude can also be computed from the pressure altitude readings if the position of the aircraft is known, though this can often lead to inaccurate results.
What is Absolute Altitude Used For
Absolute altitude is used in pressure pattern navigation, terrain clearance, or as a backup for the pressure altitude.
However, keep in mind that absolute altitude doesn’t account for things like trees and other structures that stick up above the earth’s surface. The ground is also uneven, which makes maintaining constant absolute altitude very tricky.
Absolute altitude may therefore not always be as useful or accurate as it may first seem, though it is particularly useful in pressure pattern navigation.
Absolute Altitude vs. True Altitude
Whereas absolute altitude is the height above ground level (AGL), true altitude refers to the height above mean sea level (MSL).
True altitude is used far more often and can be seen for expressing airways, obstacles, airspace altitudes, and terrain figures. Instrument approach charts, on the other hand, are expressed in feet above ground level.
Related: How High Do Planes Fly?