TYPES OF COMPASS
There are two types of compasses generally used in surveyling
They are
(1) The prismatic compass and
(2) The surveyor’s compass
THE PRISMATIC COMPASS
In this compass, the readings are measured with the help of a prism. Hence, it is called prismatic compass. The main advantage of this compass is the sighting and measurement of bearing can be done at a time. Fig. 3.1. Shows the cross-section of a prismatic compass, with details of all components. Description of component parts of a prismatic compass with their functions are discussed as below.
PARTS OF A PRISMATIC COMPASS AND THEIR FUNCTIONS
1. Compass Box: It is a circular metallic box (Brass or Aluminium) of diameter 80 to 110 mm. A pivot with a sharp point is the provide at the centre of the box. The box is covered with plain glass at top.
Function: It holds a magnetic needle and graduated ring freely floating on a pivot located at the centre of the box.
2. Magnetic Needle: shows the location of Magnetic needle. The needle is made of a symmetrical bar of magnetised steel or Iron. It is hung from a conical Jewel bearing supported on a sharp, hardened pivot, and it is attached to the ‘Graduated ring’.
Function: Magnetic needle facilitates in measuring bearings of survey lines with reference to the magnetic north.
3. Graduated Ring: The readings are marked on the graduated ting from 0 to 360° clockwise, inverted. The reading on the graduated ring begin from south marked as 0°, 90° at the west, 180° at the north and 270° at the east. The least count of the graduated ring is hall degree or 30 min
Function: Graduated ring facilitates to measure the bearing of lines upto half degree accurately
4. Object Vane: The object vane is hinged with the metal box and consists of a horse hair at the centre. It also consists of a hinged mirror, to sight the objects located too high or too low with respect to the line of sight.
When the object vane is not under use or when the instrument is shifted from one station to another, the object vane is folded on to the glass cover. When it is folded on to the glass cover, it automatically presses the lifting pin, which in turn lifts the lifting lever. The lifting lever lifts the magnetic needle and graduated ring out of the pivot.
Function: To sight the object and to lift the magnetic needle out of pivot, to avoid the wear and tear of pivot.
5. Eye Vane & Prism: The prism box consists of a sighting slit at the top and two small circular holes, one at the bottom of the prism and the other at the side of the observer’s eye. The prism box can be moved up and down for focussing the graduated ring. It is hinged to the circular Box, so that it can be folded to the rim of the compass box.
Function: To sight the object in line with hair in the object vane and, to errect and magnify the image of inverted readings on the graduated ring.
6. Dark Colour Glasses: Two dark colour glasses are provided with the prism. One is red and another is blue.
Function: The red colour glass is provided for sighting luminious objects at night. The blue colour glass is provided for reducing the strain on the observer’s eye in bright day light.
7. Brake Pin and Spring Brake: A brake pin is provided just at the base of the object vane. If pressed gently, it moves the spring brake inside the compass box, which stops the oscillations of the ring.
Function: To stop the oscillations of the graduated ring.
8. Lifting Pin and Lifting Lever: A lifting pin is provided just below the object vane. When the object vane folded over to the glass cover, then the lifting pin automatically presses the lifting lever inside the compass box, which lifts the magnetic needle along with graduated ring out of the pivot point.
Function: To avoid undue wear and tear of pivot point.
TEMPORARY ADJUSTMENTS OF PRISMATIC COMPASS
The following procedure should be adopte while measuring the bearing by prismatic compass.
1.Fixing the Compass to The Tripod Stand:The tripod stand is at the required station with its legs placed apart. The prismatic compass is taken into left hand and fixed to the threaded top of the stand.
2. Centering:Generally, the compass is centred by dropping a piece of stone from the bottom of the compass box. Centering may also be done with the aid of a plumb-bob held centrally below the compass box.
3. Levelling:Levelling is done with the help of a ball-and-socket arrangement provided on the top of the tripod stand. The adjustment is done in such a way that the graduated ring rotates freely without touching either the bottom of the box or the glass cover on top of it.
4. Adjustment of Prism :The prism is moved up and down till the readings on the graduated ring are appear clear and sharp.
5. Observation of Bearing:After the centering and levelling the compass box over the station, the ranging rod at the required station is bisected perfectly by sighting through the slit of the prism and horsehair at the object vane.
While rotating the compass box the graduated ring may rotate rapidly. To stop the rotation of the graduated ring the break pin is pressed gently. When the ring comes to rest, the box is struck very lightly to verity the ring is freely pivoted or not. Then the reading is taken from the graduated ring through the hole in the prism box. This reading will be the magnetic bearing of the line.
SURVEYOR’S COMPASS:
It is similar to the prismatic compass, except for the following points
1. There is no prism on it. Hence reading should be taken with naked eye.
2. The graduated aluminium ring is attached to the circular box. It is not fixed to the magnetic needle.
3. The ring is graduate from 0° to 90° in four quadrants. 0° is marked at north and south, and 90° at the east and west.
4. It consists of an eye-vane (in place of prism) with a fine sighting slit.
5. The magnetic needle moves freely over the pivot. The needle indicates the reading on the graduated ring.
6. No mirror is attached to the object vane.