An angle of depression is an angle with one horizontal arm, and one arm below horizontal.

Usually an angle of depression is less than or equal to 90°.

An angle of depression of one location relative to another is always congruent (equal in measure) to the angle of elevation of the first location relative to the second.

In the illustration below, the angle of depression of the cat relative to the bird is equal in measure to the angle of elevation of the bird relative to the cat.

The "line of sight" forms a transversal of the two horizontal parallel lines. As a result the angles of elevation and depression are alternate (interior) angles.

Since the transversal intersects two parallel lines, and alternate interior angles are congruent, the angle of depression and angle of elevation must also be congruent.