Suppose that you are looking at an object in the distance.
If the object is above you, then the angle of elevation is the angle your eyes look up.
If the object is below you, the angle of depression is the angle your eyes look down.
Angles of elevation and depression are measured from the horizontal.
It is common mistake not to measure the angle of depression from the horizontal.
Using the angle of depression or elevation to an object, and knowing how far away the object is, enables us to find the height of the object using trigonometry.