In Traversing, when compass is use for making angular measurements, it is refer as compass traversing or compass surveying. In compass surveying, the compass is use to determine the direction of survey lines of the frame work of the traverse by measuring the angles which these lines make with magnetic meridian. The process of chaining and offsetting is the same as in chain surveying and running of check lines is not necessary.
The compass surveying is suitable :
- When the survey details are require along the long narrow strips e.g. a road, railway, stream etc.
- The area cannot be divide into triangles such as in towns.
- The ground is a large open tract with a few isolate noticeable features.
- Long straight lines and convenient tie lines cannot be drawn on account of obstacles.
- Surveying is complete in a jungle or in a dense forest.
- When the country is hilly where chaining is tedious and liable to error.
A compass is small instrument which consists essentially of a magnetic needle, a graduate circle and a line of sight. When the line of sight is direct 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 graduate circle. The compass cannot measure the angle between the two lines directly.
If it desire to discover out angle between two lines, firstly their angles with the magnetic meridian are determine separately. Then the difference of the two values is identify which is equal to angle between the lines.
There are two forms of the compass in common use :-
- The Prismatic Compass.
- The Surveyor’s Compass.
The Prismatic Compass
It is very valuable instrument and is commonly use for rough surveys where speed and not the accuracy is main consideration. It was invent by Captain Kater in 1814.
Prismatic compass consists of cylinderical metal box (1) of about 8 cm to 12 cm diameter, in the centre of which is a pivot (2) carrying a magnetic needle (3) which is already attach to the graduate aluminium ring (4) with the help of an agate cap.
(5) The ring is graduate to half a degree and is read by a reflecting prism (7) which is protect from dust, moisture etc. by the prism cap.
(8) Diametrically opposite to the prism is the object vane (12) hinge to the box side an caring a horse hair (13) with which can object in the field is bisect. The eye is apply at the eye-hole below the sighting slit.
(9) The graduations on the ring can be observe directly by the eye after they are reflect from the diagonal of the prism. The graduations can be make clearly visible by adjusting the prism to the eye sight by the focusing screw (11) Both the horizontal and vertical side faces of the prism are make 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 (6) which presses against a lifting pin (17) and the needle is then automatically lift off the pivot by the lifting lever.
(18) To damp the oscillations of the needle before taking a attach to the inside of the box is brought in contact with the edge of the ring by gently pressing inward the brake pin.
(15) If the bearings of very high or very low objects are to be taken, The reflecting mirror (14) which slides on the object vane is tilt and image obtain in it is bisect by the horse hair. A pair of colour glasses (10) shall have to be interpose between the silt and the object vane when the Sun or some other luminous object is to bisect. 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
This can be use while holding it in hand, but for better accuracy, it is usually mount on a light tripod which carries a vertical spindle in the ball and socket arrangement, to which the compass is screw. By means of this arrangement, the compass can be place in position easily. Its working involves the following three steps.
- Observing the bearing.
The center of the compass is place 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.
By means of ball and socket arrangement, the compass is than level so that the graduate ring swings quiet freely. It may be test by rolling a round pencil on the compass box.
Observing the bearing
Having center the instrument over the station and level it, raise or lower the prism until the graduations on the ring are clearly visible when look 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 graduate ring. Readings are usually estimate up to nearest 15 minutes.
The Surveyor’s Compass
It was formerly much used for land surveys but now-a-days it is little use. 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 in place of broad form needle.
What is leveling survey https://civilcrews.com/leveling-instruments-types/