Prismatic Compass : The prismatic compass is a magnetic compass in which there is a prism for taking observations.

•  The prismatic compass is generally smaller in size than a surveyor’s compass.

•  The prismatic compass consists of a circular box, about 85 to 100 mm diameter.

•  The magnetic needle used in a prismatic compass is of broad in shape.

•  An aluminum ring graduated in degrees and half degrees is directly attached with the needle.

•  The graduations on the aluminium ring increase clockwise from 0º to 360º, with the zero of the graduations co-inciding with the south end of the needle, 90º graduation is at the west, 180º graduation at the north and 270º gradua-tion at the east as shown below in the figure.

•  The prismatic compass is used for the determination of the whole circle bearings (W.C.B.) of the lines.

•   Readings are taken through a prism attached to the box.

•   It may be noted that in a prismatic compass, the sighting of the object and the reading of the bearing are done simultaneously