Carl Kunath wrote an excellent historical article on the use of the old Brunton compass in cave surveying. Unfortunately, the major source of error was not clearly noted.
That error is not having the Brunton as near perfectly level as possible when you make the sighting. A tripod helps if you pay attention, but it is possible to make accurate sights hand-held as well. I taught Herb Conn how to survey with a Brunton in 1959, and in many miles of hand-held Jewel Cave survey loops our error was consistently on the order of ½ of 1%. Keeping it level was the key. Considerable contortions were sometimes required of the surveyor, however. DirtDoc
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