Types of Compass - Compass Traversing, Surveying and Levelling | Surveying and Levelling Notes- Agricultural Engg - Agricultural Engineering PDF Download

TYPES OF COMPASS

There are two types of compass:

1. The prismatic compass, and
2. The surveyor’s compass.

1. The prismatic compass In this compass, the readings are taken with the help of a prism. The following are the essential parts of this compass:

(a) Compass Box  The compass box is a circular metallic box (the metal should  be non-magnetic) of diameter 8 to 10 cm. A pivot with a sharp point is provided at the  centre of the box.

(b) Magnetic Needle and Graduated Ring  The magnetic needle is made of a broad, magnetised iron bar. The bar is pointed at both ends. The magnetic needle is attached to a graduated aluminium ring.

The ring is graduated from 0⁰ to 360⁰ clockwise, and the graduations begin from the south end of the needle. Thus 0⁰ is marked at the south, 90⁰ at the west, 180⁰ at north and 270⁰ at the east. The degrees are again subdivided into half-degrees. The figures are written upside down. The arrangement of the needle and ring contains an agate cap pivoted on the central pivot point. A rider of brass or silver coil is provided with the needle to counterbalance its dip.

(c) Sight Vane and Prism The sight vane  and the reflecting prism are fixed diametrically opposite to the box. The sight vane is hinged with the metal box and consists of a horsehair at the centre. The prism consists of a sighting slit at the top and two small circular holes, one at bottom of the prism and the other at  the side of the observer’s eye.

(d) Dark Glasses Two dark glasses are provided with the prism. The red glass is meant for sighting luminous objects at night and the blue glass for reducing the strain on the observer’s eye in bright daylight.

(e) Adjustable Mirror A mirror is provided with the sight vane. The mirror can be lowered or raised, and can also be inclined. If any object is too low or too high with respect to the line of sight, the mirror can be adjusted to observe it through reflection.

(f) Brake Pin A brake pin is provided just at the base of the sight vane. If pressed gently, it stops the oscillations of the ring.

(g) Lifting Pin A lifting pin is provided just below the sight vane. When the sight vane is folded, it presses the lifting pin. The lifting pin then lifts the magnetic needle out of the pivot point to prevent damage to the pivot head.

(h)   Glass Cover A glass cover is provided on top of the box to protect the aluminium ring from dust

2. The Surveyor’s compass The surveyor’s compass is similar to the prismatic compass except for the following points.

(a) There is no prism on it. Readings are taken with naked eye.

(b) It consists of an eye-vane (in place of prism) with a fine sight slit.

(c) The graduated aluminium ring is attached to the circular box. It is not fixed to the magnetic needle.

(d) The magnetic needle moves freely over the pivot. The needle shows the reading on the graduated ring.

(e) The ring is graduated from 0⁰ to 90⁰ in four quadrants. 0⁰ is marked at the north and south, and 90⁰ at the east and west. The letters E (east) and W (west) are interchanged from their true positions. The figures are written the right way up.

(f) No mirror is attached to the object vane.