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Compass surveying - The prismatic compass
Prismatic compass is the most convenient and portable form of magnetic compass which can either be used as a hand instrument or can be fitted on a tripod. Prismatic compass having a magnetic needle is attached to the circular ring or compass card made up of aluminium a non magnetic substance. When the needle is on the pivot it will orient it self in the magnetic meridian. The line of sight is defined by the object vane and the eye slit both attached to the compass box. The object vane consists of a vertical hair attached to a suitable frame while the eye slit consists of a vertical slit cut into the upper assembly of the prism unit, both being hinged to the box. When an object is sighted the sight vanes will rotate with respect to the NS end of ring through an angle which the line makes with magnetic meridian. A triangular prism is fitted below the eyes slit, having suitable arrangement for focusing to suit different eye slits. The prism has both horizontal and vertical faces convex, so that magnified image of the ring graduation is formed. The 00 or 3600 reading is therefore engraved on the south end of the ring, so that bearing of the magnetic meridian is read as ‘0’ with help of prism which is vertically above south end in this particular position. The readings increase in clockwise direction from 00 at south end to 900 at the west end, 1800 at north and 2700 at east end. If the instrument is not uses, the object vane can be folded on the glass lid which covers the top of the box. To sight the objects which are too high or too low to be sighted directly a hinged mirror capable of sliding over the object vane is provided and the objects sighted by reflection when bright objects are sighted dark glasses may be interposed into the line of sight.
The main advantage of prismatic compass is that both sighting the object as well as reading circle can be done simultaneously without changing the position of the eye.
Last modified: Thursday, 12 May 2011, 5:45 AM