Prismatic  compass is a light and simple instrument and is used for rough surveys where too much of accuracy is not necessary.

Components and Adjustment of Prismatic Compass

This compass is made into a box which is circular and 85 to 110 mm in diameter.  In the center of the compass box, is a pointed steel pivot is used.

A magnetic needle of broad form is balanced over the pivot, and to this is fixed a light aluminum ring.

The aluminum ring is calibrated in degrees and half degrees. The calibrations  are marked such that zero is at the south, 90 degrees at the west, 180 degrees at the north and 270 degrees at the east, i.e., the aluminum ring is calibrated  in a clockwise direction with the zero at the south.

The figures showing the readings on the aluminum ring are written inverted, and a glass lid covers the top of the compass box.

Dioganally  opposite the compass box are fixed the object or sight vane and the eye vane. The latter carries a reflecting prism.

The eye vane is fixed at the top of the prism. The reflecting prism can be raised or lowered to suit the eyesight of the observer by means of the focusing stud.

A hinge is provided so that the prism may be folded over the edge of the box when it is not in use.

When the eye is brought near the eye vane, the graduations on the ring are reflected to the eye and the same can be read.

The horizontal and vertical sides of the prism are made convex so that the readings on the ring are magnified.

The object vane consists of a metal frame hinged to the box. In the center of this metal frame is fixed a vertical horse hair or a tine wire or silk thread.

When the instrument is not in use the object vane should be folded over the glass lid, covering the top of the box.

When the object vane is folded over the glass lid, it presses a lifting pin which lifts the magnetic needle above the pivot and keeps it against the glass lid.

Thus the wear and tear of the pivot are reduced by keeping the magnetic needle away from the same when the instrument is not in use.

A spring brake is provided inside the box to damp the oscillations of the magnetic needle, i.e., to bring the magnetic needle to rest quickly before taking a reading.

By pressing the brake pin inward, the Spring may be made to come in contact with the magnetic needle thereby arresting the oscillations.

The object vane may be provided with a hinged mirror which may be slided to any position on the object vane, and the mirror can be made to incline on any angle.

This enables the surveyor to take the readings to objects which are too high or too low.

Sun glasses are provided to sight luminous objects. They are simply interposed in the line of sight and reduce the strain on the eyes of the observer.

A metal lid covers the glass lid and the sighting vane when not in use.

In the prismatic compass, the magnetic needle and the graduated rings are attached together and, therefore, they remain always along the north-south line when the box is rotated.

Procedure of Survey on Prismatic Compass.
For taking readings with a prismatic compass, instrument may be held in hand and turned till the ranging rod on the next station is bisected by the horse hair when seen through the eye-vane.

The instrument should be held as nearly level as can be judged. If the needle does not oscillate, it means that it is touching the glass lid and is not being held vertical.

It should be held properly, and the brake knob pressed lightly. Then the reading is taken. This gives the bearing of the line joining the point vertically below the compass and the point on which the ranging rod is held.

Least Count

Readings may be accurately taken up to 30 minutes and estimated up to the nearest 15 minutes.

Least count of Prismatic Compass is 30′ minute.

A Video About the Instrument

Here is a helpful video about correcting mistakes by Bowditch’s rule

Source(s) and Link(s):

Civil Mentor