Image source: http://www.polarfocus.com/graphics/elevman.gif
Only an angle and measured distance along the ground are needed to determine
the height of objects far from the ground. Two possible angles can be measured
to help determine this information: the angle of elevation from the ground or
the angle of depression from the top of the object. The angle of elevation is
the angle from the ground that is aimed towards the top of the of the object.
The angle of depression is the angle from the horizontal line of sight at the
top of the object directed towards a spot, or an observer, on the ground. Each
of these angles has the same measure because they are congruent alternate interior
angles that share a transversal that intersects parallel lines (the ground and
horizontal view from the top of the building).