Chain Surveying

1. Faculty of Applied Engineering and Urban Planning Surveying Civil Engineering Department 2nd Semester 2007/2008 Chain Surveying Lecture 4

2. Content • Chain Surveying • Sources of Errors • Types of Errors

3. Errors • - No measurement (except count) can be free of error. • True value is determined statistically (mean) to calculate error. • Systematic error: error whose magnitude and algebraic sign can be determined and eliminated (temp. error). • Random Error: • - Error due to surveyor skill. • - Tend to cancel each other. • - Little significance except for high precision survey. • - Unskilled or careless surveyor can make problem. • - Large random error doesn’t result in accurate work even if they cancel.

4. Mistakes There are many mistakes that cold be happened to surveyors. - Blunders made by survey personnel e.g. 68 instead of 86. - Miscounting tape length, measuring from wrong point. - Mistakes will occur and must be discovered and eliminated by verifying the measurement (Repeat Geometry analysis, etc.). - Every measurement should be repeated to eliminate mistakes and improved precision.

5. Accuracy and precision Accuracy: relationship between measure & true value of measurement. Precision: Degree of refinement with which the measurement is made. Example: True Distance Measured Distance Error Cloth tape 157.22 157.3 0.08 Steel tape 157.22 157.23 0.01 - More precise method resulted in more accurate. - More precise method may result in less accurate measurement. Example: Repaired tape.

6. Contents • Types of Distance Measurement • Measurement Methods • Direct (tapes) • Indirect: (EDM, Stadia) • Errors and Corrections for Tape Measurement • EDM

7. Introduction • One of the fundamentals of surveying is the need to measure distance. Distances are not necessarily linear, especially if they occur on the spherical earth. In the present subject we will deal with distances in Euclidean (geometric) space, which we can consider a straight line from one point or feature to another.

8. Common Methods for Measuring Distances • Pacing • Taping • Tachometry (required theodolite, and graduated rod) • EDM (Electronic Distance Measurement)

9. Equipment Used in Chain Surveying • Measurement of Lines • Chain • Tapes • Invar Tapes (more accurate, 65% steel, 35 Nicke) • Making Right Angles • Optical Prism • Others • Ranging Rods • Arrows • Pegs • Plumb bobs

10. Types of Distance Measurement

11. Types of Distance Measurement

12. Types of Distance Measurement

13. Examples: When use which • 1. If you are intending to draw a map or area, horizontal distance and height difference (vertical distance) should be used to enable plan and height information to be drawn. • 2. If you are to locate points such as a corner of a building or centre line of a road, slope distance and vertical distance are required to enable pigs be located at correct points on site (Layinging Out).

14. Methods of Measurement • Pacing • Accuracy 1 : 100 • Taping • Accuracy 1 : 10,000 • Electronic Distance Measurement (EDM) • Accuracy 1 : 10,000 to 1:100,000

15. Pacing • Practical measure of distance. • Don't try to pace out one meter with every step. Walk casually over 100 m counting the number of steps. Work out the length of a casual step and use this instead. • Varies with uphill, downhill, and your age. • Low accuracy • No equipment needed

16. Taping (or chaining) • Chaining is applied to measurement with a steel tape or synthetic tape (plastic or fiberglass). All standard in lengths • 100 m, 50m, 30 m, 20 m. • It is fairly quick, easy and cheap, and hence is the most common form of distance measurement. • Unfortunately, Chaining is prone to errors and mistakes. • For high accuracy, steel tape should be used which is graduated in mm and calibrated under standard temp (20 degree) and tension (5kg). Be careful, easily break. • Synthetic tape is more flexible graduated in 10mm

17. Equipment Used in Chain Surveying

19. Measuring wheels Tapes in lengths up to 100 ft Some Taping Instruments

20. Taping Procedures • ranging rods set up between points A and B • from A to B, set zero of tape at A • tape unwound towards B • A third range rod is “ranged” in at C • Tape straightened, held tight and read at rod C • C marked with a pin • for next bay, tape moved from A and zero set at C and so on

21. Accessories

22. Processes in Chain Surveying • Ranging and Measurement of Lines • Level Ground • Uniformly Sloping Ground • Uneven Ground

23. Level Ground Rod Rod A A1 A2 A3 A4 B

24. Stationing Along Baseline: stations At right angle: offset dist. 0+00 2+50 4+96 5+43.12

25. 25

26. 26 Length AB = 4 x Full tape distance + 1 Short section REMEMBER ! It works only on smooth ground or uniform slope surfaces

27. Example 27

28. Example How about this very uneven case or high accuracy is required? 28

29. Example Sag curve Sag curve measurement is not common nowadays and is restricted to steel tape only. 29

30. Processes in Chain Surveying

31. Uniformly Sloping Ground

32. Uneven Ground

34. In Previous • Ranging and Measurement of Lines • Level Ground • Uniformly Sloping Ground • Clinometers • Abney Level • Uneven Ground

35. Homework • Solve Problems: • 1.5, 1.6, 1.7 • 2.1, 2.2, 2.5

36. Lecture 5

37. Setting Out Right Angles Method 1

38. Setting Out Right Angles Method 2

39. Setting Out Right Angles Method 3

40. Setting Out Right Angles Method 4 Prove Method 4 provided in the text book

41. Setting Out a Line at Right Angles Method 1

42. Setting Out a Line at Right Angles Method 2

43. Setting Out a Line at Right Angles Method 3 5 m 3 m 4 m

44. Taping: Corrections Once a line is being measured, it is necessary to convert the measured length into a horizontal length. Series corrections have to be applied. Five possible corrections have to be considered. These are • Erroneous Tape Length • Slope • Tension • Temperature • Sag

45. Taping: Corrections • For synthetic tapes, only Erroneous Tape Length and slope corrections will be applied • The best accuracy that can be achieved is the order of 1:1000 • When using steel tapes, if only Erroneous Tape Length and slope corrections are considered, the best possible accuracy that can be obtained in the range 1:5000 If tension and temperature are added into consideration, accuracy can be increased to better than 1:10000 ~ 1: 20000 • Sag only applies if tape is supported only at ends

46. standardisation length (actual tape length) nominal length (assumed tape length) Erroneous Tape Length • tape has a nominal length under certain conditions, a tape stretches with time. • standardisation needs to be carried out frequently by using reference tape or baseline.

47. For a 30m Nominal Length Tape Tape used When comparing to a standard tape, the tape used indeed has a length 30 m + ∆l For every 30m measurement, the small elongated amount should be added for correction.

48. Slope Correction • All plan distances are always quoted as horizontal distances L, therefore any distance not measured on the horizontal will need to be corrected for slope. Slope correction must ALWAYS be considered, and either eliminated in the field or mathematically compensated. Angle may be measured by Theodolites

49. Standardisation tension Cross section Area Modulus of Elasticity of tape material For steel, E = 200,000 N/mm2 Tension Correction • A tape is a given length when pulled with a certain tension. If the tension changes then so does the tape length. Tension applied

50. Length error due to Temperature change Temperature change Measured length Coefficient of linear expansion Temperature Correction • Most materials expand and contract with temperature change, and this effects taped distances. If a tape has stretched due to heat it will read shorter than it would at its normal (or standard) temperature.