Chain Surveying Lecture notes very important 2.pptx

Advance survey

1. CHAIN SURYEYING

2. Introduction Chain surveying is used to conduct small surveys in open ground with simple details. It is simple and quite useful for land surveying. It is used to secure data for making a plan for establishing boundaries of a piece of land. It is unsuitable for larger and crowded areas.

3. Only linear measurements are taken in the field. Suitable for surveys of small extent to describe the boundaries of land to locate the existing features on them In this method area is divided into network of triangles and sides are measured directly in the field with chain or tape and no angular measurements are taken. Contd....

4. Principles of Chain Surveying Principle of chain surveying is to divide the area to be surveyed into a network of connected triangles. Triangle is the only simple figure that can be plotted from the lengths of its sides measured in the field.

5. Selection of Survey Station The stations should be mutually visible... The principle of working from whole to part should be strictly observed. The station should be selected in such a way that well conditioned triangles may be formed. The tie stations should be suitably selected to fix the direction of adjacent sides. The subsidiary stations should be suitably selected for taking check lines. The survey lines should be as few as possible.

6. Conventional Symbols

7. Instruments used in Chaining The following instruments are used while chain surveying :- Metric Chain Tapes Ranging rod Arrows Pegs Plumb bob Cross staff Optical square

8. Metric Chain

9. Details of Metric Chain

10. The chain length is 20m or 30m measured from one extreme of handle to other. The chain is divided into 100 or 150 links each of 20 cm length. The links are connected with each other by means of three rings to afford flexibility. The length of link is from the centre of middle rings on either side of it. The length of first link is inclusive of the length of handle. To enable easy reading small brass rings are provided at every metre length and tallies are fixed at every five metre length. A groove is cut in the middle on the outside surface of the handle so that arrow can be placed in position.

11. Measuring Tapes Tapes are used for more accurate measurements. The tapes are classified based on the material of which they are made of such as:- Cloth or linen Tape Metallic tape Steel tape Invar tape

12. Steel Tape

13. Ranging rod Ranging rods are used for ranging some intermediate points on survey line. Ranging rods are generally 2-3m in length and are painted with alternate bands of black and white or red and white colour in length of 20 cm. The location of any survey station can be known from long distances.

14. CHAINING OPERATIONS Chain Surveying evolves following operations :- Ranging Chaining Offsetting

15. Ranging Definition:- The process of establishing intermediate points on a straight line between two end points is known as ranging. Purpose:- a) Obstruction b) Distance greater than chain length

16. Types of Ranging There are two types of ranging:- 1.Direct Ranging 2. Indirect Ranging

17. CHAINING The operation of measuring linear distances with the help of a chain or tape is known as chaining. For works of ordinary precision a chain is used but where greater accuracy is required, a steel tape is used.

18. 1. Perpendicular Offsets 2. Oblique offsets Types of Offsets

19. C D E

20. Open Cross staff It is used for setting out right angles at a given point on a chain line or for dropping the perpendicular from a given point on a chain line.

21. Cross staff Metal cross Staff Wooden Cross Staff

22. Optical Square This instrument is more accurate than an open cross staff. This is used for accurate work specially for long offsets. This is based upon principle of reflection .

23. Chaining Obstacles in

24. B -

25. AB= ACxBB1/AC1

26. B

27. CHAINING MISTAKES IN

28. The mistakes are generally avoidable and cannot be classed under any law of probability.  The following mistakes are commonly made by inexperienced chainmen:- Displacement of arrows Failure to observe the position of the zero point of the tape Adding or omitting a full chain or tape length Reading from the wrong end of the chain Reading numbers incorrectly Calling number wrongly Reading wrong metre marks Wrong booking

29. 1)Displacement of arrows : - When the arrow is displaced, it may not be replaced accurately.  To guard against this mistake, the end of each chain length should be marked both by the arrow and by a cross (+) scratched on the ground. 2) Failure to observe the position of the zero point of the tape : - The chainmen should see whether it is at the end of the ring or on the tape. 3) Adding or omitting a full chain or tape length  (due to wrong counting or loss of arrows): - This is the most serious mistake and should be guarded against.  This is not likely to occur, if the leader has the full number (ten) of arrows at the commencement of chaining and both the leader and follower count them at each transfer.  A whole tape length may be dropped, if the follower fails to pick up the arrow at the point of beginning.

30. 4)Reading from the wrong end of the chain : - e.g. reading 10 m for 20 m in a 30 m chain, or reading in the wrong direction from a tally, e.g. reading 9.6 m for 10.4 m.  The common mistake in reading a chain is to confuse 10 m tag with 20 m tag.  It should be avoided by noticing the 15 m tag. 5) Reading numbers incorrectly : - Transposing figures e.g.37.24 for 37.42 or reading tape upside down, e.g. 6 for 9, or 36 for 98. 6) Calling number wrongly : - e.g. calling 40.2 as “forty two”. 7) Reading wrong metre marks : - e.g. 58.29 for 57.29. 8) Wrong booking : - e.g. 345 for 354. To guard against this mistake, the chainmen should call out the measurements loudly and distinctly, and the surveyor should repeat them as he books them.

31. Sources of Error Instrumental Error: The error due to imperfect in construction, incorrect graduation, improper adjustment of the plate level Personal error: The error due to lack of perfection in human sight, lack of perfection in setting the instrument. Natural Error: The error due to variation in temperature, humidity, gravity, refraction and magnetic declination.

32. Errors can be classified into two categories:- Compensating Error: Cumulative Error Errors in Chaining

33. Compensating Error The errors those are likely to change their signs from + to – and vice versa and thus tend to compensate each other. In chaining they are as follows: 1. Variation in the Pull Applied : Sometimes the pull applied may be more during stretching and sometimes it may be less, thus changing the length differently

34. 2. Incorrect Holding of Chain : The handle of chain may not be on the arrow. It may have moved ahead of it due to stretching or it may be short of it, thus chaining the length differently. 3. Due to Combined Use of Chain or Tape : Errors may be in the measurement or in making chain angles.

35. Cumulative Errors These errors do not change their signs and thus tend to add up or accumulate. Erroneous Length of Chain(+ ve or – ve ): The chain may either be too long or too short, due corrections should be applied. To eliminate it the chain should be compared with a standard chain and its correct length ascertained. After measurements the necessary correction is applied to get the correct measured length. Measurement on a Wrongly Ranged Line(+ ve ): The measured distance along the wrongly ranged will always be more than correct distance. Its effect are more serious when offsets are taken from this line as then all such points get displaced from their true position.

36. 3. Change in Temperature : Due to change in temperature, the length of the chain varies. It is not very serious in chaining but for correct measurements with a steel tape, due corrections should be applied. This is positive or negative. 4. Measurement along Slope: When distances are measured on sloping ground and no corrections are applied to reduce it to the horizontal distance. This is always negative.

37. Chain /TAPE Correction In

38. Correction for absolute length The absolute length or true length may be different from the designated length i.e. from 20m, 30m, 50m etc.while the absolute length refer to what it actually is. The length is checked with a standard length of chain or tape under standard condition so that error can be ascertained. The correction for absolute length is given by : C a = (L x c)/l Where C a = Correction for absolute length c = Correction for chain/tape length l = designated length of chain/tape L = Measured length by chain/tape The sign will depend upon the sign of ‘c’

39. True distance = (L’/L) * measured distance of line Where L’ = Actual Length of Chain/tape L = Designated length of chain/tape

40. Correction for Temperature The chain/tape length changes due to variation in temperature while taking the measurements. The temperature correction C t  which therefore needs to be made is given by: C t  = α (T m  – T o )L Where T m  = Mean temperature during measurement T o = temperature at standardisation α = Co- eff of thermal expansion of material L = Measured length in m The correction is + ve if the temp during measurement is more than standard temp and – ve if temp is less than standard temp at which the tape was standardized.

41. Correction for Pull (or Tension) The correction is necessary when the pull used during measurement is different from that at which the tape is standardized.  It is given by the formula, C p  = (P-P o )L / AE where C p  = the correction for pull in metres.                            P = the pull applied during measurement, in newtons (N).                            P o = the pull under which the tape is standardized in newtons (N).                            L = the measured length in metres.                            A = the cross-sectional area of the tape, in sq.cm.                            E = the modulus of elasticity of steel.

42. Sag Correction When a tape is stretched over points of support, it takes the form of a catenary .  In actual practice, however, the catenary curve is assumed to be a parabola.  The correction for sag (or sag correction) is the difference in length between the arc and the subtending chord (i.e., the difference between the horizontal distance between supports and the length measured along the curve).  It is required only when the tape is suspended during measurement.  Since the effect of the set on the tapes is to make the measured length too great this correction is always subtractive.  It is given by the formula, C s  = l( wl ) 2  / 24P 2  (- ve ) = l(W) 2  / 24P 2  (- ve ) Where, C s  = the sag correction for a single span, in metres.                              l = the distance between supports in metres.                              w = the mass of the tape/chain, in kilograms per metre.                              W = Total mass of the tape/chain in kilograms.

43. The length of a line measured with 20m chain was found to be 500m. It was subsequently found that the chain was 0.04m too long. What is the length of line? Correct length of chain, L’ = 20 + 0.04 = 20.04m Length, L = 20m Measured length, = 500m True length = (L’/L) * measured length = (20.04/20) * 500 = 501m

44. True area = (L’/L)2*measured area True Volume = (L’/L)3 * measured volume Where, L’ = incorrect length of chain L = correct length of chain

45. EXCERCISE - 1 The distance between two stations was 1200m when measured with a 20 metre chain, the same distance when measured with a 30 metre chain was found to be 1195 m. if the 20 m chain was 0.05m too long what was the error in the 30 metre chain?

46. True distance = L’/L * Measured distance 20.05/20 * 1200 1.0025 * 1200 1203 m 1195 = L’/30 * 1200 L’ = 1195/1200 * 30 = 29.875 Error in Chain = 30 – 29.875 = .125 m short The distance between two stations was 1200m when measured with a 20 metre chain, the same distance when measured with a 30 metre chain was found to be 1195 m. if the 20 m chain was 0.05m too long what was the error in the 30 metre chain?

47. EXCERCISE – 2 A line AB was measured between the station A and B as 348.28 m using a 20 m tape too short by .05 m Determine the correct length of AB. Please solve this problem and give the answer right now.

48. ANSWER: Since the tape is too short by .05 m then actual length AB will be less than the measured length. Correction required to the measured length is C a = c/L * Measured length c = .05m, L = 20m and measured length is 348.28 m Then , C a = .05/20 * 348.28 = 0.87 m Correct length of line AB will be = 348.28 – 0.87 = 347.41 m.

49. EXCERCISE - 3 A 20 m chain was found to be 2 cm too long after chaining a distance of 1200 m . After chaining a total distance of 2500m it was tested again and was found to be 5cm too long. If the length of chain was correct at the beginning find the correct distance measured.

50. A 20 m chain was found to be 2 cm too long after chaining a distance of 1200 m . After chaining a total distance of 2500m it was tested again and was found to be 5cm too long. If the length of chain was correct at the beginning find the correct distance measured. ANSWER: As the chain was correct at the beginning but was 2cm too long after measuring a distance of 1200 m so an average error can be assumed for this section Average error = (0+.02)/2 = .01 m Therefore incorrect length of chain = 20 + .01 = 20.01 m T.D. = L’/L *M.D. = 20.01/20 * 1200 = 1200.6 m ..............(1) In the second part average error of the chain will be (.02+.05)/2 = .035 m Measured distance in second part = 2500-1200 = 1300 m T.D. = 20.035/20 * 1300 = 1302.275 m ...............(2) Total True Distance = (1) + (2) = 1200.6 + 1302.275 = 2502.875 m

51. EXCERCISE-4 To measure a line a 30m steel tape was standardized at 15 ° C under a pull of 10 kg was used. Find the correction per tape length if the field temperature 20 ° C and the pull exerted was 16 kg. Weight of the tape is 0.8kg. Area of cross section of tape is 0.03419 cm ² , E= 2.1x10 6 kg/cm ² and coefficient of expansion is 7.1x10 -6/ ° C

52. ANSWER: Temperature Correction Ct = α (Tm – To)L = 7.1x10-6 (20-15)* 30 = 0.001065 m (+ ve ) Pull Correction Cp = (P-Po)L / AE = (16 – 10)*30/.03419* 2.1 x106 = .002507 m (+ ve ) Sag Correction Cs = l(W)2  / 24P2 = 30 *(0.8)2 /24 * 16 2 = .003125 m (- ve ) Correction per tape length = (.001065 +.002507 - .003125) = 0.000447 m(+ ve ) = .447 x 10 -3 m (+ ve )

53. Link for area calculation of chain and cross staff survey https://www.youtube.com/watch?v=5O_cj8yK0Pc Link for Equipments used in chain survey https://www.youtube.com/watch?v=mnnQPTlyOIU Link for Ranging https://www.youtube.com/watch?v=zd_iJgECoYI