Prismatic Compass Surveyor’s Compass The graduated ring is attached to the magnetic needle. The graduated ring and needle are free to move independently w.r.t. each other. Graduated ring remains stationary while box with prism and object vane rotates as the ring attached with needle is not attached with the box. Graduated ring rotates with rotation of box, eye vane and object vane as the ring is attached with the box of the compass and only needle remains stationary. Prism is provided to take reading and thus, the graduated ring is marked with figures of mirror image. (as the values are reflected and erected through mirror) The graduated ring is graduated with ejected figures and no prism is provided to take reading. Graduation are marked 0° and 360° in clockwise direction. Graduations are marked 0° to 90° in each quadrant. 0° is marked at South, 180° at North, 90° at West and 270° at East. In this compass East and West are interchanged. Tripod may or may not be provided. The instrument cannot be used without tripod. It measures or gives WCB of a line. It measures or gives QB of a line.
