The angle of elevation of an object as seen by an observer is the angle between the horizontal and the line from the object to the observer's eye (the line of sight). This angle is seen outside of the triangle. If the object is below the level of the observer, then the angle between the horizontal and the observer's line of sight is called the angle of depression. This angle is seen inside the triangle. The angle of depression equals the angle of elevation because they from alternate interior angles or congruent angles located on different parallel lines cut by a transversal line.
