Compass surveying

COMPASS SURVEYING

Compass Surveying: In a traverse surveying, when the direction of survey lines measured with compass and the length of survey lines measured with tape or chain, then it is said to be Compass Surveying. When the area to be surveyed is large, chain and compass surveying is preferable.

Bearing: Bearing is the horizontal angle between the reference meridian and the survey line. The angular measurement is done indirectly from the difference between the bearings of directions of two survey lines.

A compass is used to measure the bearings of directions of survey lines. There are two types of compass available, namely, 1. Prismatic Compass and 2. Surveyor's Compass.

1. PRISMATIC COMPASS

Description: See Fig. 23. Prismatic Compass consists of a Circular Metal Box. The box has a Pivot fixed at its center. The pivot carries a Magnetic Needle. A circular Graduated Aluminium Ring (non-magnetic) is attached to the needle. The needle can turn freely about its center.

A Glass Lid is provided to view the graduations on the ring. Aluminum ring is graduated from 0° to 360° clockwise.

An Eye Vane and an Object Vane are fixed diametrically opposite. The Eye Vane consists of an Eye Hole and a Triangular Reflecting Prism, which can be raised or lowered. The Line of Sight is defined by the object vane and the eye hole. A Pair of Sun Glasses are provided between the eye hole and object vane to sight luminous objects.

The prism has openings on the horizontal and vertical faces to read the angles on the ring through the eye hole. The prism has both horizontal and vertical faces convex, so that a magnified image of the ring graduation is formed.

The Object Vane has Horse Hair to bisect the field objects. It is provided with an Adjustable Reflecting Mirror to bisect very nearer objects and objects in the sky. This enables bearing of very low or high objects to be taken.

A Brake Pin at the base of the object vane damps the oscillations of the needle and brings it to rest before taking a reading.

Lifting Lever lifts the aluminium ring with magnetic needle off its support and holds it firmly against the glass lid. In this way, undue wear of the supporting pivot point is prevented.

Procedure to measure the Bearing of Station B from Station A

Prismatic Compass is used as a portable instrument or fitted on a Tripod.

1. Centering: Compass is centered over the station A, where bearing has to be taken. A plumb bob is used to judge centering.

2. Levelling: Levelling is the process of making the graduated ring to swing freely without touching the glass lid. The compass is levelled by adjusting the ball and socket support till the top of the box is horizontal.

3. Focusing the Prism: The eye vane and the prism are raised or lowered, till the readings on the ring are seen sharp and clear. When an object is sighted, the eye vane will rotate with respect to the NS end of the ring through an angle which the line makes with the magnetic meridian.

4. Observing the Bearing: The compass box is turned about its vertical axis so that a ranging rod held over the station B and the horse hair in the object vane are in one line. Now the reading is taken. The reading indicates the bearing of the line AB.

See Fig. 24. When a reading is taken through the prism, the needle is pointed towards North while South end of the needle is under the prism. Hence, the zero mark is provided at South end. Accordingly, the 90°, 180°, 270° marks are provided at West, North and East extremities. The readings taken are bearings measured

Advantage of Prismatic Compass

Both sighting the object as well as reading the circle can be done simultaneously without changing the position of the eye. The circle is read at the reading at which the hair line appears to cut the graduated ring.

2. SURVEYOR'S COMPASS (Fig. 25)

Surveyor's Compass was used in earlier days for land surveying. It is not in use now.

It consists of a Graduated Ring attached to the Metal Box. The ring is not oriented in the magnetic meridian, as was the case in Prismatic Compass. The object vane is similar to that of prismatic compass.

The ring is divided on to four quadrants. Each quadrant is marked with the graduation from 0° to 90°. 0° is marked with the letters N (North) and S (South).

Similarly, 90° is marked with the letters E (East) and W (West). E and W are interchanged from their true positions as shown in Fig. 26. With this, it is possible to read only the reduced bearings.

A Magnetic Needle is placed on the Pivot at the center of the metal box. One side of the needle is indicated with red mark showing the North.