Errorsandmistakesinchaining 170810071200

Errors and mistakes in chaining

1. Lect#11 QUEST, CAMPUS Larkana Errors and Mistakes in chaining

2. Errors Chain is standardized at 20 c temperature and 8 kg tension(pull). So the error in chain may occur due to variation of temperature and pull from standard value. Errors may also be due to defective instrument. So errors are related to instrument and its faults and methods of measurement. There commonly two types of errors (1) Cumulative Error (2) Compensating error

3. (1) Cumulative Error Cumulative errors are said to be systematic errors or instrumental errors as they are one-directional hence keep on accumulating as the survey progresses. If not checked they have serious implications to the accuracy of the survey. Errors in this class include incorrect length of the tape, page of the tape or the tape not being in line. Since the sources of these errors are known, they can be eliminated. They are proptional to L

4. They can either be positive or negative errors. Positive error These errors make the measured length more than the actual length which results into wrong calculations by the Surveyor. The following are some of the positive errors: The length of chain is shorter than the standard length due to bending of links, removal of connecting rings and knots in links. The temperature is lower than at which the tape was calibrated. Not applying sag correction. Sag takes place due to self weight of the chain. Incorrect alignment

5. Negative errors These errors make the measured length less than the actual length. Following are some of the negative errors: Length of chain or tape greater than its standard length due to flattening of rings, opening of ring joints and temperature being higher than at which it was standardized. Wearing of connecting rings Elongation of links

6. (2) Compensating error Compensating errors are said to be accidental errors hence cancel out and does not pose serious problem to the accuracy of the survey. They occur both direction positive and negative. These errors are proportional to square root of L(length of chain )

7. Following are some of the compensating errors: 1) Incorrect marking of the end of chain 2) Fractional parts of the chain may not be correct when the chain is corrected by adding or removing a ring. 3) Graduation in the tape may not be exactly of same length throughout 4) In the method of stepping horizontality and verticality not properly maintained. Angles not accurately measured.

8. Mistakes: Mistakes are errors which arise from inattention, inexperience, carelessness and poor judgment or confusion in the mind of the observer. They do not follow any mathematical rule (law of probability) and may be large or small, positive or negative. They cannot be measured. However, they can be detected by repeating the whole operation.

9. If a mistake is undetected, it produces a serious effect upon the final result. Hence, every value to be recorded in the field must be checked by some independent field observation. The following are the examples of mistakes: Erroneous recording, e.g. writing 69 in place of 96 Counting 8 for 3 Forgetting once chain length Making mistakes in using a calculator. So mistake is related to human errors not instrumental error.

10. Corrections in chaining

11. Corrections in chaining Whenever errors occur in chaining it becomes necessary to apply correction. Remember that for every negative error correction is positive and vice versa. Following corrections are applied in a chain CORRECTION FOR SLOPE CORRECTION FOR TEMPERATURE CORRECTION FOR PULL CORRECTION OF SAG

12. CORRECTION FOR SLOPE If length measured ‘L’ and the difference in the levels of first and last point ‘h’ are given then correction for slope is, Chl =h2 /2L If θ and L are given, Chl =L (1-cosθ) This correction is always subtractive.

13. CORRECTION FOR TEMPERATURE Temperature correction is necessary because the length of tape or chain may increase or decrease due to variation of temperature. Let α- Coefficient of thermal expansion of the material of tape Tm – Mean temperature during measurement To - Temperature at which tape is standardized, and L – Measured length Then temperature correction Ct is given by Ct= Lα (Tm-To)

14. CORRECTION FOR PULL Chain is standardized at 8 kg tension but at field pull may be more or less than the standard pull and due to elastic property of chain strain will vary according to pull applied , for this pull correction are necessary. Let, E – Young’s modulus of the material of tape A – Cross – sectional area of the tape P – Pull applied during measurement P0 - Standard pull, and L – Measured length of chain Then, the correction for pull Cp is given by Cp= (P-P0) L/AE The above expression takes care of signs of the correction also

15. CORRECTION OF SAG While taking reading, if the tape is suspended between two supports, the tape sags under its own weight. Hence, measured length is more than the actual length. Hence, this correction is subtractive. This correction is given by