To measure the magnetic bearing of a line with the help of a prismatic compass.

Object: To measure the magnetic bearing of a line with the help of a prismatic compass.

Theory:

The prismatic compass calculates the bearings of lines with respect to the magnetic needle. The name Prismatic compass is given to it because it essentially consists of a prism which is used for taking observations more accurately. Some of the important components of this compass include:

Object vane (Hair vane): The object vane is diametrically opposite to the prism and eye vane. The object vane carries horse hair or black thin wire to sight objects in line with eyesight.

Eye vane: The eye vane is a fine slit provided with an eyepiece at the bottom to bisect the object from the slit. It also contains glasses, which are used when some luminous objects are to be bisected.

Graduated scale: This is an aluminum graduated ring marked with 0o to 360o to measure all possible bearings of lines, and attached with the magnetic needle.

The ring is graduated to half a degree.

The horizontal angle that a line makes with the magnetic meridian measured from the Magnetic North line is called magnetic bearing. The prismatic compass calculates the bearings in the whole circle bearing system which determines the angle that the survey line makes with the magnetic north in the clockwise direction.

Instruments required:

1. 5 ranging rods
2. Prismatic compass
3. Tripod

Procedure:

• Let AB be the survey line as shown in Figure (a), which is to be the magnetic north.
• The compass is centered over the station A on a tripod and levelled as shown in Figure (b) and a ranging rod is fixed at B.
• The compass is turned so that line of sight is aligned in the direction of AB by making eye slit of observation vane, vertical hair of object vane and ranging rod at B in same horizontal line.

• The viewing prism is focused by moving it vertically with the help of focusing stud.
• The reading of the image of hair line, as observed through prism, is adjusted to zero.
• The other four rods C, D, E and F are fixed at consecutively increasing angles from each other on one side of AB.
• The bearings of these rods are taken by turning and aligning the compass in the same way as mentioned above for each rod.
• The readings of the image of hair line as observed through prism are noted, indicating the whole circle bearings of the lines AC, AD, AE and AF.

Results:

Observation in the field: