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COMPASS AND PLANE TABLE SURVEYING

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COMPASS AND PLANE TABLE SURVEYING

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  3. 3. A compass consist of A magnetic needle A graduated circle The line of site Box house the above The two forms of compass that all used commonly for angle measurement ◦1.Prismaticcompass ◦2.Surveyor’s compass 3
  4. 4. Prismatic Compass: 4
  5. 5. Difference between prismatic compass & surveyor's compass. Item Prismatic compass Surveyor's compass Graduated card The graduation circle is fixed to broad needle. It does not rotate with line of sight. The graduation circle is fixed to the box and rotates with line of sight There is a prism at viewing end. No prism. Only slit The graduations are in WCB system. The graduation are in Q.B system. The graduations are marked inverted. The graduations are marked directly. Magnetic Needle Magnetic needle does not act as index. Magnetic needle acts as index. Tripod Tripod may not be provided, the instrument can be used even by holding suitably in hand The instrument can’t be used without tripod. Reading The reading is taken with the help of a prism provided at the eye slit Taken by directly seeing through the top of the glass 5
  6. 6. Compass traversing: Important Definition  True meridian: Line or plane passing through geographical north pole and geographical south pole  Magnetic meridian: When the magnetic needle is suspended freely and balanced properly, unaffected by magnetic substances, it indicates a direction. This direction is known as magnetic meridian. The angle between the magnetic meridian and a line is known as magnetic bearing or simple bearing of the line. North pole True meridian Magnetic meridian True bearing Magnetic bearing 6
  7. 7. Designation of magnetic bearing ◦ Whole circle bearing (WCB) ◦ Quadrantal bearing (QB) WCB: The magnetic bearing of a line measured clockwise from the North Pole towards the line is known as WCB. Varies 0-360° Quadrantal Bearing: The magnetic bearing of a line measured clockwise or anticlockwise from NP or SP (whichever is nearer to the line) towards the east or west is known as QB. This system consists of 4-quadrants NE, SE, NW, SW. The values lie between 0-90° ◦ QB of OA = N a E 7
  8. 8. Reduced Bearing: When the whole circle bearing of a line is converted to quadrantal bearing it is termed as reduced bearing. Fore and Back Bearing: ◦ In WCB the difference between FB and BB should be exactly 180° ◦ BB=FB+/-180° ◦ Use the +ve sign when FB<180° ◦ Use the –ve sign when FB> 180° 8
  9. 9. Magnetic declination: The horizontal angle between the magnetic meridian and true meridian is known as magnetic declination. Dip of the magnetic needle: If the needle is perfectly balanced before magnetisation, it does not remain in the balanced position after it is magnetised. This is due to the magnetic influence of the earth. The needle is found to be inclined towards the pole. This inclination of the needle with the horizontal is known as dip of the magnetic needle. Local Attraction Method of correction for traverse: ◦ First method: Sum of the interior angle should be equal to (2n-4) x 90. if not than distribute the total error equally to all interior angles of the traverse. Then starting from unaffected line the bearings of all the lines are corrected using corrected interior angles. ◦ Second method: Unaffected line is first detected. Then, commencing from the unaffected line, the bearing of other affected lines are corrected by finding the amount of correction at each station. 9
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  11. 11. Plane Table Surveying 11
  12. 12. Plane Table Surveying is a graphical method of survey in which the field observations and plotting are done simultaneously. It is simple and cheaper, It is most suitable for small scale maps. The plan is drawn by the surveyor in the field, while the area to be surveyed is before his eyes. Therefore, there is no possibility of omitting the necessary measurements. 12
  13. 13. Equipments and Accessories for Plane Tabling The following instruments are used in plane table surveying. Equipments: Plane Table Tripod Alidade Trough Compass Spirit level U-Fork with Plumb bob Water proof cover Drawing paper Pins Drawing accessories 13
  14. 14. Plane Table 14
  15. 15. ALIDADE The alidade is useful for establishing a line of sight. Two Types of alidade are used. Simple alidade Telescopic alidade 15
  16. 16. SIMPLE ALIDADE 16
  17. 17. TELESCOPIC ALIDADE 17
  18. 18. TROUGH COMPASS 18
  19. 19. SPIRIT LEVEL 19
  20. 20. U-Fork With Plumb Bob 20
  21. 21. ADVANTAGES AND DISADVANTAGES OF PLANE TABLE SURVEYING 21
  22. 22. Advantages It is simple and cheaper than the theodolite survey. It is most suitable for small scale maps. No great skill is required to produce a satisfactory map and work may be entrusted to a subordinate. It is useful in magnetic areas where compass may not be used. The mistakes in writing field books are eliminated. 22
  23. 23. Disadvantages (Limitations) It is not intended for very accurate work. It is not suitable in monsoon. It is essentially a tropical instrument. Due to heaviness, it is inconvenient to transport. Since there are so many accessories, there is likelihood of them being lost. 23
  24. 24. Methods Of Plane Tabling There are four distinct methods of plane tabling: Method of Radiation Method of Intersection Method of Traversing Method of Resection 24
  25. 25. Radiation Method 25
  26. 26. Method Of Intersection 26
  27. 27. Method Of Traversing 27
  28. 28. Method of Resection 28
  29. 29. Two Point Problem In this problem, two well-defined points whose positions have already been plotted on the plan are selected. Then, by perfectly bisecting these points, a new station is established at the required position. 29
  30. 30. Two Point Problem 30
  31. 31. The Three Point Problem In this problem, three well defined points are selected, whose position have already been plotted on the map. Then, by perfectly bisecting these three well-defined points. A new station is established at the required position. No auxiliary station is required in order to solve this problem. This table is directly placed at the required position. The problem may be solved by following methods (a) Bessel’s method (b) Mechanical Method (c) The trial and error method 31
  32. 32. Bessel’s Method 32
  33. 33. The Mechanical Method 33
  34. 34. The method of Trial and Error 34
  35. 35. Error In Plane Tabling The various sources of error may be classified as : Instrumental errors Errors in manipulation and sighting Errors in plotting 35
  36. 36. Instrumental Errors The surface of drawing board is not plane The edge of alidade is not straight. The object vane and sight vane are not perpendicular to the alidade. The edge of alidade is not is not parallel to the line of sight. The fixing clamp is not proper. 36
  37. 37. Errors In Manipulation And Sighting Defective Leveling Defective Sighting Defective Orientation Defective Centering Movement of Board between sights 37
  38. 38. Errors in Plotting Defective scale of map Wrongly intersecting the rays drawn from two different stations. 38
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