Question

Why is zero degree marked at south in prismatic compass?

Answer 1

The graduations on this prismatic compass are situated on a light aluminium ring fastened to the needle, and the zero of the graduations coincides with the south point of the needle. The graduations therefore remain stationary with the needle, and the index turns with the sighting vanes. Since the circle is read at the observer's (rather than the target's) end, the graduations run clockwise from the south end of the needle (0� to 360�),