There are two forms of the compass in common use: 1. The Prismatic Compass 2. The Surveyor’s Compass.
A compass is a small instrument which consists essentially of a magnetic needle, a graduated circle and a line of sight. When the line of sight is directed towards a line, the magnetic needle points towards magnetic meridian and the angle which the line makes with the magnetic meridian is read at the graduated circle.
The compass cannot measure the angle between the two lines directly. If it is desired to find out angle between two lines, firstly their angles with the magnetic meridian are determined separately and then the difference of the two values is found which is equal to angle between the lines.
Form # 1. The Prismatic Compass:
It is very valuable instrument and is commonly used for rough surveys where speed and not the accuracy is main consideration. It was invented by Captain Kater in 1814. Fig. 5.1 shows its sectional elevation.
ADVERTISEMENTS:
It consists of cylindrical metal box of about 8 cm to 12 cm. diameter, in the centre of which is a pivot carrying a magnetic needle which is already attached to the graduated aluminium ring with the help of an agate cap.
The ring is graduated to half a degree and is read by a reflecting prism which is protected from dust moisture etc. by the prism cap. Diametrically opposite to the prism is the object vane hinged to the box side and carrying a horse hair with which an object in the field is bisected.
The eye is applied at the eye-hole below the sighting slit. The graduations on the ring can be observed directly by the eye after they are reflected from the diagonal of the prism. The graduations can be made clearly visible by adjusting the prism to the eye sight by the focussing screw. Both the horizontal and vertical side faces of the prism are made convex to give magnified readings.
To prevent undue wear of the pivot point, the object vane is brought down on the face of the glass cover which presses against a lifting pin and the needle is then automatically lifted off the pivot by the lifting lever.
ADVERTISEMENTS:
To damp the oscillations of the needle before taking a reading and to bring it to rest quickly, the light spring brake attached to the inside of the box is brought in contact with the edge of the ring by gently pressing inward the brake pin.
If the bearings of very high or very low objects are to be taken, the reflecting mirror which slides on the object vane is tilted and image obtained in it is bisected by the horse hair. A pair of the object glasses shell have to be interposed between the slit and the coloured vane when the Sun or some other luminous object is to be bisected. A metal cover fits over the glass cover as well as the object vane when the compass is not in use.
Working of the Prismatic Compass:
ADVERTISEMENTS:
This can be used while holding it in hand, but for better accuracy, it is usually mounted on a light tripod which carries a vertical spindle in the ball and socket arrangement to which the compass is screwed. By means of this arrangement the compass can be placed in position easily.
Its working involves the following three steps.
(i) Centering
(ii) Levelling, and
ADVERTISEMENTS:
(iii) Observing the bearing.
(i) Centering:
The centre of the compass is placed vertically over the station-point by dropping a small piece of stone below the centre of the compass so that it falls on the top of the peg marking that station.
(ii) Levelling:
ADVERTISEMENTS:
By means of ball and socket arrangement, the compass is then levelled so that the graduated ring swings quite freely. It may be tested by rolling a round pencil on the compass box.
(iii) Observing the bearing:
Having centered the instrument over the station and levelled it, raise or lower the prism until the graduations- on the ring are clearly visible when looked through the prism. Turn the compass-box until the ranging rod at the forward station is clearly visible. Use the brake-pin and bring the ring at rest and then take the reading at which the hair line appears to cut the graduated ring. Readings are usually estimated up to nearest 15 minutes.
It may be noted that with this compass, the sighting, of the object and reading of the graduated ring are done simultaneously.
Form # 2. The Surveyor’s Compass:
It was formerly much used for land surveys but now-a-days, it is little used. It is similar to a prismatic compass except that it has another plain sight having a narrow vertical slit in place of the prism and that it carries an edge bar needle.(Fig. 5.2. (a)] in place of broad form needle [Fig. 5.2. (b)].
Note:
(i) Why Zero is marked at South in the Prismatic Compass. Since the bearing of the North direction is zero, therefore when the North end of the needle and object vane point towards North, the reading under the prism should be zero.
But since the prism is placed exactly opposite to the object vane, the south end will be under the prism. Hence, the zero graduation of the ring must be placed at South end of the needle. In this way bearings are obtained clockwise from North.
(ii) Why East and West are interchanged in the Surveyor’s Compass. The letters E and W are interchanged from their true positions in order to read the bearings in the proper quadrants. Supposing, the bearing of a line is N 30°E.
Since the graduated ring is attached to the box, therefore, it moves with the sights when the box is rotated while the magnetic needle remains stationary. The N and S points on the ring and the sights move through 30° from left to right when the point P is bisected (Fig. 5.5).
Thus the actual East of the compass moves away from the North end of the needle while the actual west comes near it. Therefore, if the letters E and W are interchanged from their natural positions, only then the reading N 30°E can be read otherwise the reading observed will be N 30° W, which is wrong.
