(a) The description of prismatic compass. This is a circular magnetic compass used in surveying or for taking bearing. Prismatic compass is a prism with a slit in it and has a slight vane with a vertical line opposite the prism. Most often, it contains a circular card and graduated in degrees (0° — 360°). The plane containing the prism slit and the sight vane hair also contain the pivot of the compass card. The prism reflects the figure of the card immediately below it and allow the surveyor to read. A prismatic compass contains a magnetic needle, pointing to the magnetic north.
(b) The precaution to be taken when making a compass traverse are as follows:
(i) All magnetic materials should be removed from the body and pockets of the reader before the bearing is taken.
(ii) The bearing should not be observed from a position close to strongly, magnetic materials eg. corrugated iron sheets, railway lines or power lines.
(iii) The compass must be held horizontally so that the edge of the card does not rub against the top of the compass.
(iv) The "back" bearing should be taken from the other end of the line and should differ from the on/forward bearing by exactly 180°.