chain and compass traversing.pptx

basic surveying

1. Department of Civil Engineering Created By Er.A.B . Jadhav Mob.No 9075009500 Basic Surveying Presented By Er . Ashish B.Jadhav Unit No 3:-Chain And Compass Traverse Surveying CO:-Conduct Traversing in the field using Chain &Compass

2. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .1 Compass Traversing :Open Open traverse: A traverse is said to be open traverse when the traverse starts at one point and terminates at another point as shown in the figure. Open traverse is also called as unclosed traverse. It is suitable for surveying of roads, coastal lines, etc . An open traverse is a traverse in which the sides of traverse do not form a closed polygon.

3. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .1 Compass Traversing :Closed Closed traverse: A traverse is said to be closed traverse when the traverse formed a closed circuit as shown in the figure. In this case, both starting and terminating points of the traverse coincide with each other. It is suitable for the survey of boundaries of ponds, sports grounds, forests, etc .. A closed traverse is a traverse in which the sides of a traverse form a closed polygon.

4. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .2 Concepts of Meridian Meridian: Meridian is a standard direction from which, the bearings of survey lines are measured . There are three types of meridians . True meridian Magnetic meridian Arbitrary meridian.

5. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .2 Concepts of Meridian True Meridian: The line on a plane passing through the geographical North Pole or geographical South Pole and any point on the surface of the earth is known as true meridian. It is also called as geographical meridian . The angle between true meridian and line is known as true bearing of the line. It is also known as azimuth .  

6. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .2 Concepts of Meridian Magnetic Meridian: When magnetic needle is suspended freely and balance properly , unaffected by magnetic substance it indicate a direction this direction is known as magnetic meridian. The angle between magnetic meridian and line is known as magnetic bearing of the line .  

7. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .2 Concepts of Meridian Arbitrary Meridian: S ometime survey of a small area a convenient direction is assume as a meridian known as Arbitrary meridian . The angle between Arbitrary meridian and line is known as Arbitrary meridian of the line.  

8. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .2 Concepts of Bearing Whole Circle Bearing : In this system, the bearing of a line is always measured clock wise from the direction of the North of the meridian towards the line around the circle. Whole circle bearings of lines have been shown in figure.   If the survey line falls between the  first quadrant  then the   Whole Circle Bearing  lies between the  0° to 90 °.  If it lies between the  second quadrant  then the  Whole Circle Bearing  of that survey line lies between  90° to 180°.  If it lies in the  third quadrant  then the Whole circle bearing will be between the  180° to 270 °. And in the  fourth quadrant , the Whole Circle Bearing values range between 270° to 360 °.

9. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .2 Concepts of Bearing Quadratic Bearing/Reduced Bearing : The  Quadratic bearing  is also known as a  Reduced   bearing . Quadratic bearings are generally measured from the North or South direction towards the East or West direction . The quadratic bearing or reduced bearing can be measured either in a clockwise or anticlockwise direction . The  quadratic bearing  varies from  0° to 90 ° . In the quadratic bearing or reduced bearing system, the bearings are taken either from the magnetic North or the magnetic south direction. It will depend on which one is nearer to that line . In the Quadratic bearing system, magnetic North and magnetic South lines are considered as a reference line.

10. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .2 Concepts of Bearing Quadratic Bearing/Reduced Bearing :   In the quadratic bearing system, you can take both clockwise as well as the anticlockwise angle from the reference line . It is necessary to state the particular quadrant in which that line lies. The letters N(north), S(south), E(east), and W( west) are used to represent the quadrant . The quadrants are represented are as follows 1 st quadrant = N – E, 2 nd quadrant = S – E 3 rd quadrant = S – W, 4 th  quadrant = N – W The example of the  quadrantal bearing  are as follows N35°E,S49°E,N65°W,S25°W etc.

11. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .2 Concepts of Bearing Difference Between Whole Circle Bearing and Quadrantal Bearing Sr. No . Whole Circle Bearing Quadratic Bearing 1 The horizontal angle which is made by the survey line, with the magnetic north in a clockwise direction is known as the Whole circle bearing. The horizontal angle which is made by a survey line with the magnetic North or South whichever is near the line in the eastward or westward direction is known as quadratic wearing or reduced bearing. 2 In the whole circle bearing, the magnetic North line is considered as the reference line. In the quadratic Bearing, both magnetic North as well as South lines are considered as a reference line. 3 In the whole circle bearing only clockwise angle is taken from the reference survey line In the quadratic bearing both clockwise, as well as the anticlockwise angle from the reference line, is taken 4 The value of the whole circle bearing ranges from 0°  to 360° The value of the quadratic wearing for reduced bearing ranges from 0° to 90° 5 The example of a whole circle bearing are 30°,45°,80°,120°,230°, and 320°, etc Example of quadratic bearing or reduced bearing are N35°E, S49°E, N65°W, S25°W, etc.

12. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .2 Concepts of Bearing Conversion of Whole Circle bearings into Quadratic Bearing or Reduced Bearing.:- Sr. No. W.C.B QUADRANT RULE 1 Between 0⁰ to 90⁰ NE RB = WCB 2 Between 90⁰ to 180⁰ SE RB = 180⁰ -WCB 3 Between 180⁰ to 270⁰ SW RB = WCB-180⁰ 4 Between 270⁰ to 360⁰ NW RB = 360⁰-WCB

13. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .2 Concepts of Bearing Fore Bearing : Bearings measured in the direction of progress of the survey are known as fore bearing . Back Bearing: Bearings measured opposite to the direction of the survey are known as Back bearing.

14. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .2 Concepts of Bearing Dip of magnetic needle : The magnetic dip is defined as the angle made with the horizontal by the earth’s magnetic field lines . It is also known as dip angle or magnetic inclination. Magnetic Declination: Magnetic declination is defined as the angle between magnetic north and true north on the horizontal plane, 

15. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .3 Components of Prismatic Compass Metal Box : The compass is enclosed in a cylindrical metallic box. The diameter of the box usually varies from 8 to 12cm. It serves as a protective casing and protects against dust, rain etc . Pivot : Pivot is the centrally located part that provides support to the freely suspended magnetic needle.

16. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .3 Components of Prismatic Compass Lifting Pin and Lifting Lever : Lifting pin is provided right below the sight vane. The lifting pin gets pressed as the sight vane is folded . The arrangement of lifting pin and lever help to lift the magnetic needle from the pivot point . This prevents the damage to the pivot head.

17. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .3 Components of Prismatic Compass Magnetic Needle : The magnetic needle is the main part of a prismatic compass . It measures the angle of a line from the magnetic meridian as the needle always points towards north and south pole at the two ends of the needle when freely suspended. It is regarded as the heart of a prismatic compass .

18. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .3 Components of Prismatic Compass Ring or Graduated Circle : The graduated circle consists of an aluminum ring that measures the bearing. It is marked from 0 degrees to 360 degrees and is attached to the magnetic needle . Least count of Prismatic Compass is 30’

19. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .3 Components of Prismatic Compass Prism : Prism is used to take the exact readings and is placed exactly opposite to the object vane. The hole of the prism is protected from dust and rain by a prism cap . Object Vane : The object vane is placed diametrically opposite to the prism and eye vane . The main purpose of object vane is to sight the object in line with the eyesight. It consists of horsehair or black wire.

20. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .3 Components of Prismatic Compass Eye Vane : Eye vane is a fine silt-like part provided to bisect the object from silt. It consists of an eye-hole at the bottom . Glass Cover : The glass cover is provided to cover the instrument box. A provided glass cover protects the instrument and is transparent which helps in taking the readings. Sunglasses: Sunglasses can be used when a luminous object has to be bisected.

21. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .3 Components of Prismatic Compass Reflecting Mirror : Reflecting mirror is directly placed on the object vane . It is used to get the image of an object located below or above the instrument level . Spring Break : It is also known as the break pin. Spring Break is provided on the compass to damp the oscillation before a reading is taken . Sunglasses: Sunglasses can be used when a luminous object has to be bisected.

22. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .3 Temporary adjustments of Prismatic Compass Fixing the compass to the tripod The box of prismatic compass is fixed to a spindle of ball and socket joint. By the ball and socket arrangement, this can be quickly levelled and rotated in any direction . Centring the compass The prismatic compass is centered over a survey station correctly by means of a plumb bob or by dropping a pebble from the center of the instrument. Levelling the compass The compass is quickly levelled by ball and socket arrangement by eye judgement. It should be levelled in such a way that dial moves freely and does not touch the rim of the bob .

23. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .3 Temporary adjustments of Prismatic Compass Sighting the object The object is sighted with the help of eye vane and object vane in the compass. The surveyor views through the eye vane and rotate the box until the ranging rod at a station is bisected. Observation of Bearing After citing the object correctly, the bearing of the survey lines are noted through prism at which the line of sight and object cuts the image of the graduation on the dial.

24. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .4 Local Attraction Local Attraction: T he magnetic needle is sometimes disturbed from its normal position under the influence of external attractive forces. Such a disturbing influence is called as  local attraction . This is because that these magnetic compass is influenced by other magnetic objects at that locality such as wires carrying electric current, rails, steel and iron structures, steel tapes etc . Local attraction at a place can be detected by observing bearings from both ends of the line in the area. If fore bearing and back bearing of a line differ exactly by 180°, there is no local attraction at either station. But if this difference is not equal to 180°, then local attraction exists there either at one or both ends of the line

25. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .4 Local Attraction Local Attraction Correction: Method 1:- This method is based on the difference of fore and back bearings. We already know that the difference between fore and back bearing of a line will be 180˚ if there is no errors in measurement . So based on this error free observation of bearings, corrections for other lines can be calculated . However if there is no two bearing has a difference of 180˚, we can calculate the correction from the mean value of that bearings which may have least error.

26. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .5 Methods of Plotting a Traverse Method 1. By Parallel Meridians through Each Station : The position of the starting point say A, draw a line representing the magnetic meridian. Then with a protractor plot the bearing of line AB (θ 1 ), and cut AB according to the scale. Then at B, draw a line parallel to the previous line representing the meridian and plot the bearing of BC (θ 2 ) and measure its length with the scale.

27. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .5 Methods of Plotting a Traverse Method 1. By Parallel Meridians through Each Station :   Repeat the whole process until all the lines are drawn . If the traverse is a closed one, last line should end at the starting point. If it does not, discrepancy is said to be the closing error . This method is defective because the error in plotting the direction of one line is earned forward in whole of the traverse.

28. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .5 Methods of Plotting a Traverse Method 2 . By Included Angles:  Draw the meridian at the starting station A and plot the bearing of line AB (θ 1 ) and measure length AB to the scale. Then at B, draw the angle ABC with the help of a protractor and cut off length BC to the scale. Repeat the process at each of the succeeding stations .

29. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .5 Adjustment of Error Angular Error : The theoretic sum of the interior angles of a traverse should equal (2N-4) right angles, and that of the exterior angles should equal (2N+4) right angles, where N is the number of sides of a closed traverse. The difference between the theoretic sum and the sum of the measured angles in a closed traverse is called the angular error of closure . When all angles are measured with equal care and under similar conditions, this error is distributed equally among all the angles . However, if the accuracy of some angle or angles is suspected due to irregular field conditions, the whole or the most of the angular error may be assigned to that angle or angles.

30. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .5 Adjustment of Error Closing Error in Bearings: If traversing is done by taking bearings of the lines, the closing error in bearing may be determined by comparing the back and fore bearings of the last line of the closed traverse as observed at the first and last stations of the traverse respectively. When the traverse ends on a line of known bearing, the error in bearing may be determined by finding the difference between its observed bearing and known bearing.

31. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .5 Adjustment of Error Graphical Adjustment of Closing Error: This method is the graphical application of Bowditch’s rule . In this method, the correction is applied to the lengths as well us to the bearings of the lines in proportion to their lengths Therefore, this method is also known as proportionate method. Here each station is shifted propor­tionately according to the length and direction of the closing error. This method is used when the angular and linear measurements are equally precise.

32. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .5 Adjustment of Error Graphical Adjustment of Closing Error:   The scale need not be the same as that of the plan but is usually kept much smaller, At A’, draw a line A’ a parallel and equal to the closing error A’A. Join Aa and from B’, C’, D’ and E’ draw lines B’b , C’c , D’d and E’e parallel to A’ A meeting the line Aa at b, c, d and e respectively . In this case, it will be noticed, that the stations will have to be shifted downwards . To do this, draw lines parallel to the closing error at each of the stations B’, C, D’, E’ and set off along them the respective intercepts on the proper side . Joining the points having shifted positions is obtained an adjusted traverse ABCDEA.

33. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .5 Adjustment of Error Graphical Adjustment of Closing Error: For example, AB’C’D’E’A’ is a traverse as plotted from the bearings and lengths of the lines, where AA’ is the amount of closing error which is to be adjusted .(Fig. a) To adjust it, draw a line AA’ equal in length to the perimeter of the traverse to any convenient scale and set off along it the distances AB’, B’C’, C’D’, D’E’ and E’A’ equal to the lengths of the sides of the traverse.(Fig. b)

34. Department of Civil Engineering Created By Er.A.B. Jadhav Mob.No 9075009500 3 .5 Adjustment of Error Graphical Adjustment of Closing Error: For example, AB’C’D’E’A’ is a traverse as plotted from the bearings and lengths of the lines, where AA’ is the amount of closing error which is to be adjusted .(Fig. a) To adjust it, draw a line AA’ equal in length to the perimeter of the traverse to any convenient scale and set off along it the distances AB’, B’C’, C’D’, D’E’ and E’A’ equal to the lengths of the sides of the traverse.(Fig. b)