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CONT: Equipment and instruments ... for 90° angle setting-out and slope measurement: the optical square [prism; cross-staff; box sextant] ...
CONT: The cross-staff - This is a long-established instrument but is less precise than the optical square ... it consists of a brass box, octagonal in shape, about 75 mm deep and 60 mm broad. Down the centre of each face is cut a slot which contains a fine wire strained vertically. The staff can be fitted on the end of a ranging pole and be erected vertically. More properly, it should be attached to a tripod so that it can be placed exactly over a point on the ground by using a plumb-bob from the head of the tripod.