Prismatic compass is a simple instrument comprising an aluminum ring graduated to 30 minutes held at the top of a broad magnetic needle set in balance on a hard steel pivot. This compass is fitted in a circular metal box with a transparent glass cover. The glass – top is further covered with a hinged metal lid. On one side of the circular box there is a hinged eye- vane with a prism having convex vertical and horizontal face in order to magnify the marking of degree on the aluminium ring as reflected from the hypotensual side of the prism. Two coloured sunglasses are set at a hinge to avoid direct sun rays at the eye vane.
On the other side of the circular box, there is a sighting vane consisting of a hinged metal frame with a horse hair at the centre. This horse hair in reality may be a fine silk thread or a metal wire. A hinged adjustable mirror is set on the outer side of the sighting vane in order to locate the too high or too low object through reflection.
There is a brake pin or knob which brings the oscillating circular ring to standstill.
The metal box has a threaded bottom which can be screwed in by another threaded knob at the top of a ball fitted and adjustable in a socket of the compass- stand.
Prismatic compass measures the horizontal angle with reference to the magnetic north- line. The bearing thus obtained is a magnetic bearing which is measured as a horizontal angular distance of any line of the traverse from magnetic north in clock wise direction.
The corners of plot or building, bends of a road can be measured directly by prismatic compass. The bends of a meandering canal, if divided into segments, can be favourably determined by a prismatic compass.
Observe the prismatic compass. Note the name and function of each and every part.