Error in problem 171 Tree Height Measurement
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For some reason all angles given in the example and input data are obtuse. A right triangle cannot have an obtuse angle. There are also errors in the description:
"So the angle B is 90 degrees when line of sight is horizontal and is 120 degrees when the line of sight is raised 30 degrees above horizont" -No, if the line of sight is raised 30 degrees above the horizon (angle A) then the angle B is 60 degrees (180-90-30=60).
So the B angle is the one outside of the triangle. Weird way to bloat an assignment, but whatever.
This is, however, how things seem to be done in the real world; an example.
I'm curious what would be the reason for it.
The reason is that establishing the downward direction is much easier and more precise (gravity goes there!) than the horizontal direction. A plumb line is a much better tool than, say, a spirit level.