Compass survey part 1

compass survey

1. COMPASS SURVEYING Arun Ravi. K. R Instructor, Draughtsman (Civil ) PART - I

2. LEARNING OBJECTIVES Compass surveying  The prismatic compass Surveyors Compass Setting of the Prismatic Compass

3. Compass surveying   Compass surveying is a type of surveying in which the directions of survey lines are determined with a magnetic compass, The length of the surveying lines are measured with a tape or chain. The compass is generally used to run a traverse line.

4. Principal of Compass surveying   A freely suspended magnetic needle always lies in North- South direction

5. Uses of Compass surveying   It is used where speed is more important than accuracy, hence it is used for preliminary survey works such as road, railway, military etc. Accuracy of readings depends on Local attraction of that site

6. There are two forms of compass in common use The prismatic compass The surveying or surveyor’s compass. Types of Compass

7. The prismatic compass The prismatic compass consists of cylindrical metal box of 8cm to 12 cm diameter. METAL BOX

8. The prismatic compass In the centre of the compass consists of a pivot which carrying a magnetic needle . Magnetic needle attached to the graduated aluminium ring with the help of an agate cap P IVOT AGATE CAP MAGNATIC NEEDLE GRADUATED RING

9. The ring is graduated to half a degree and is ready by a reflecting prism. prism cap protect the prism from dust, moisture etc. The opposite side to the prism is the object vane and it carrying a horse hair , with which an object is bisected. The eye is applied through eye vane . P RISM CAP PRISM OBJECT VANE HORES HAIR EYE VANE PRISM

10. The angular readings can be observed directly by the eye through the prism . The readings can be made clearly by adjusting the prism to the eye sight by the focusing screw PRISM OBJECT VANE HORES HAIR EYE VANE FOCUSING SCREW

11. The prismatic compass is protected by a glass cover. The needle is lifting buy using lifting lever by pressing lifting pin. The oscillations of the needle stop by using brake pin GLASS COVER LIFTING LEVER LIFTING PIN BREAK PIN

12. Reflecting mirror helps to take the bearings of very high (or) very low objects. A pair of sun glasses helps to take readings during heavy sun glare. SUN GLASS REFLECTING MIRROR A metal cover fits over the glass cover as well as the object vane when the compass is not in use.

13. The compass is placed over a tripoded

14. In the Prismatic compass, graduations are marked on the ring in a clockwise direction with 0°or 360° at south end of the needle. 90° is marked at the West 180° at the North and 270° at the East. The figures are written upside down

15. The greatest advantages of prismatic compass is, both sighting the object as well as reading circle can be done simultaneously without changing the position of the eyes.

16. The bearing shows 330° at the observer’s end under the prism ( ie at the south end)

17. Surveyors Compass The graduated ring is directly attached to the circular box and not with the magnetic needle. The magnetic needle floats freely over the pivot. No prism is attached to the eye vane and it is having a narrow vertical slit. It is similar to prismatic compass except with a following modifications.

18. Readings are taken directly with naked eye against the north end of the needle. The angle is graduated in quadranted system of having 0° at north and south ends 90° at East and west ends. Surveyors Compass

19. The prismatic compass is required to be temporarily set over the station at which the bearing of survey line required to be measured. It is basically a two-step procedure. 1. Centering 2. Levelling This is also called the temporary adjustments of compass. Setting of the Prismatic Compass

20. The compass is set so that its centre lies exactly above the station by suspending a plumb bob from. If the conical end of plumb bob lie exactly over the station, the compass is considered to be exactly centered. If not, the legs of the tripod are adjusted in position by moving one leg first and then simultaneously moving other two legs in perpendicular direction to first movement. If the plumb bob is not available, a small piece of stone or pebble can be used for centering. 1. Centering

21. 2. Levelling The compass is required to be levelled so that the aluminium ring is in horizontal plane. The levelling can be checked by a spirit level or by rolling a pin on compass box. If not, the level can be adjusted by moving the legs of tripod.

22. ARUN RAVI.K.R Instructor, Draughtsman (Civil)