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Chain Surveying

Faculty of Applied Engineering and Urban Planning. Surveying. Civil Engineering Department. 2 nd Semester 2007/2008. Chain Surveying. Content. Chain Surveying Sources of Errors Types of Errors. Accuracy and precision.

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Chain Surveying

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

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

  3. 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.

  4. Content • Chaining Obstacles • Vision is obscured, Chaining is Possible • Vision Possible, Chaining is Obscured • Both of Vision and Chaining are Obscured

  5. Vision is obscured, Chaining is Possible

  6. Vision is Possible, Chaining is Obscured AB=AC+EF+DB =AG+KL+HB

  7. Vision is Possible, Chaining is Obscured From the similar triangles EDF, FGH A’

  8. Vision is Possible, Chaining is Obscured DE is set out on near bank and bisect at C Line FCG is constructed s.t FC=CG Rod at H and on line AB a rod can be set at J on intersection of lines EG, HC with double backwards ranging process. Unknown dist. FH=JG

  9. Vision is Possible, Chaining is Obscured AB crosses on the skew Poles are placed on AB, at E, G Line DF is set out so that GF is perpendicular on DF Perpendicular from D is constructed to meet AB at C Lines CE, EF, ED is measured.

  10. Both of Vision and Chaining are Obscured Random Line

  11. Both of Vision and Chaining are Obscured Prolonged Line

  12. Both of Vision and Chaining are Obscured A-Method

  13. Errors in Chaining • Types of Errors: • Blunders • Systematic • Random Errors

  14. 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.

  15. Blunders • Mistakes caused by human carelessness • Omitting Measurement • Misreading the chainage (14 m 20 cm) • Erroneous Booking (32.14 >> 23.14)

  16. 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.

  17. Systematic Errors • Their source and effect are known • Temperature Ct • Sag Cs • Tension Cp • Length Errors due to Wear and Tear Cl • Cc = Ct + Cs + Cp + Cl

  18. Systematic Errors Temperature Correction Ct Ct = 0.0000116 (T1 – To) L (0.0000116) is thermal expansion coeff. for steel per 1oC T1 Field Temp. To Temp. under which tape is calibrated LLength of Line

  19. Systematic Errors Sag Correction

  20. 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

  21. Systematic Errors Sag Correction WTotal weight between supports wweight per meter LInterval between supports P Tension on the tape

  22. Systematic Errors Calculate the sag correction for a 100 ft steel tape weighing 2 Ib and supported at the ends only with a 12 Ib pull.

  23. Systematic Errors Calculate the sag correction for a 30 ft steel tape weighing 0.0112 kg/m and supported at 0, 15 and 30 points under a tension of 5 kg.

  24. Systematic Errors Tension Correction Cp Elongation of tape P1 Applied Tension Po Calibrated Tension A Cross-Sectional Area E Modulus of Elasticity

  25. Systematic Errors Length Correction 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. ClLength Correction la Actual Length of Tape lo Nominal Length of Tape L Length of Measured Line

  26. Systematic Errors Length Correction

  27. Random Errors Error can be minimized by making several measurements and then calculating the average

  28. Example The tape has a mass of 0.026 kg/m and a cross-sectional area of 3.24 mm2. It was standardized on the flat at 20°C under a pull of 89 N. The coefficient of linear expansion for the material of the tape is 0.000011/oC, and Young's modulus is 20.7 x 104 MN/m2. Station length (m) Temp. (oC) Tension (N) I 29.899 18.0 178 2 29.901 18.0 178 3 29.882 18.1 178 4 29.950 17.9 178 Determine the absolute length of the survey line.

  29. Example Station L2 L3

  30. Example

  31. Example = 119.636 m

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