What is a prismatic compass and how is it used?

A prismatic compass is a surveying instrument that measures magnetic bearings of survey lines. It has a magnetic needle, graduated circle (0-360 degrees), prism for simultaneous viewing of object and readings, and sighting vane for targeting. The compass is held in hand while sighting the object, and the bearing is read through the prism. Used for reconnaissance surveys, rough traversing, and determining magnetic meridian. Accuracy is limited to about 30 minutes due to magnetic variations.