Knowing the difference between an angle of depression and an angle of elevation will help us in our work with trigonometry problems. It’ll help us solve problems such as finding the distance to a township from a plane. Let’s assume that I’m standing on level ground and I’m looking straight ahead, so that my line of sight is parallel with the ground. If I hear a plane above me than what I have to do to see that plane is to raise my head, or to move my line of sight from looking ahead to looking up at the plane. In other words am elevating or lifting my line of sight in order to look directly at the plane.
Can you see that when I do this, I create angle between my original line of sight, which was on a horizontal level, and my new line of sight to the plane? This angle is known as the angle of elevation because I had to lift up or elevate my eye, in order to see the object. Let’s have another look at the first situation I mentioned. This time I want you to imagine you’re the pilot in a plane and you see a town below you. We’ll say the plane is flying horizontally and parallel to the ground. In order to see the town which is ahead of you and below you, you’ll have to drop your line of sight. I’m sure you know that the word “depression” refers to something that’s down in some way, whether it’s your mood or a hole in the ground.
Here is your line of sight what goes downwards, so we call the angle we created from the horizontal when we look down, the angle of depression. Now we can say that whenever we talk about an angle of depression, we’re talking about the angle between horizontal line of sight and the line of sight we create when we look down onto something.