How is AGL determined?

AGL, or Above Ground Level, is determined by calculating the height of an object or point above the surface of the earth. The correct method to determine AGL involves subtracting the terrain elevation from the true altitude. True altitude refers to the actual altitude of an aircraft or object above sea level, while terrain elevation is the height of the ground or surface where the measurement is taken. By subtracting terrain elevation from true altitude, you effectively obtain the height of the object above the immediate ground level, which is necessary for understanding and navigating obstacles and terrain in a flying context.

Other options may involve incorrect methodologies in associating altitude with terrain and do not accurately depict how AGL is properly calculated. For instance, adding terrain elevation to true altitude or averaging terrain elevation does not yield the necessary information needed to determine how high an object is relative to the ground. Additionally, using the altimeter setting at sea level would not provide the necessary ground reference to compute AGL accurately.