(a) Describe the Prismatic Compass. (b) Highlight four basic precautions that should be taken when making a compass traverse.

Answer Details

(a) The Prismatic Compass

The prismatic compass is a small, portable hand instrument used for measuring the bearing (direction) of objects and for carrying out compass traverses in surveying.

• It consists of a circular metal box (case) containing a magnetic needle fixed to a graduated circular card (dial) marked from 0 to 360 degrees.
• The needle always points to the magnetic north, so bearings are read as whole-circle bearings from north in a clockwise direction.
• It has a sighting vane (with a fine vertical hair or slit) on one side and a prism with an eye-hole on the opposite side. The prism reflects and magnifies the reading on the card so that the observer can sight the object and read the bearing at the same time.
• It is fitted with a lifting/brake device to steady the swinging card and a lid that also acts as a sighting slit, and it can be mounted on a tripod or held in the hand.

(b) Four precautions when making a compass traverse

• Keep the compass away from metal objects, electric wires and magnetic materials, which deflect the needle and give wrong bearings.
• Hold the compass steady, level and horizontal, and allow the needle to come to rest before taking each reading.
• Take both fore-bearings and back-bearings at each station to check and correct errors.
• Record all readings, station names and distances carefully and immediately in a field notebook.
• Sight the object accurately through the sighting vane and read the bearing through the prism at eye level to avoid parallax error.