Theory question 202
Surveying(a) Describe the Prismatic Compass. (b) Highlight four basic precautions that should be taken when making a compass traverse.
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(a) A circular magnetic compass used in surveying to take bearings; has a magnetic needle attached to a card graduated 0°-360°; a prism with a sighting slit on one side and a sight vane with a vertical hair on the opposite side; the prism reflects the reading on the card so the surveyor can read the bearing while sighting; often has a lid/glass cover and a lock/brake for the card.
(b) Keep away from magnetic materials (iron sheets, railway lines, power lines, vehicles) and remove metal objects from pockets; hold the compass level so the card swings freely; let the needle settle before reading; take a back bearing from the other end, which should differ by exactly 180°; record readings carefully; check the compass is not faulty.
Precautions are about avoiding errors in bearings.