Angle of elevation and depression

Answer 1

If you have a device that measures angle above or below level (a bubble sextant does or a transit as used by surveyors) you can figure out how far something is above or below you.
For example if you are standing on the ground and looking at the top of a building 100 meters away and you device says the angle between horizontal and the top is 30 degrees that means the angle of elevation is 30 degrees.
That means that the tangent of 30 degrees = height of building above your eye / 100 meters horizontal to building from you

depression angle is the same thing but BELOW horizontal

Answer 2

Here is a picture
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