What are the Errors in Taping | Taping in Surveying
Methods of measuring with a tape have been dealt with, although it must be said that training in the methods is best undertaken in the field. The quality of the end results, however, can only be appreciated by an understanding of the errors involved. Of all the methods of measuring, taping is probably the least automated and therefore most susceptible to personal and natural errors.
The majority of errors affecting taping are systematic, not random, and their effect will therefore increase with the number of bays measured. The errors arise due to defects in the equipment used; natural errors due to weather conditions and human errors resulting in tape-reading errors, etc. They will now be dealt with individually
Taping cannot be more accurate than the accuracy to which the tape is standardized. It should therefore be routine practice to have one tape standardized by the appropriate authority. This is done on payment of a small fee; the tape is returned with a certificate of standardization quoting the ‘true’ length of the tape and standard conditions of temperature and tension.
This tape is then kept purely as a standard with which to compare working tapes. Alternatively a base line may be established on site and its length obtained by repeated measurements using, say, an invar tape hired purely for that purpose. The calibration base should be then checked at regular intervals to confirm its stability.
When measuring with a steel tape, neglecting temperature effects could be the main source of error. For example, in winter conditions in the UK, with temperatures at 0◦C, a 50 m tape, standardized at 20◦C, would contract by
11.2 × 10−6 × 50 × 20 = 11.2 mm per 50 m
Thus even for ordinary precision measurement, the temperature effect cannot be ignored.
If the tension in the tape is greater or less than standard the tape will stretch or become shorter. Tension applied without the aid of a spring balance or tension handle may vary from length to length, resulting in random error. Tensioning equipment containing error would produce a systematic error proportional to the number of tape lengths. The effect of this error is greater on a light tape having a small cross-sectional area than on a heavy tape
The correction for sag is equal to the difference in length between the arc and its subtended chord and is always negative. As the sag correction is a function of the weight of the tape, it will be greater for heavy tapes than light ones. Correct tension is also very important
The last errors in taping is misalignment. If the tape is not in a straight line between the two points whose distance apart is being measured, then the error in the horizontal plane can be calculated in a similar manner to the error due to slope in the vertical plane. If the amount by which the end of the tape is off line is e, then the resultant error is e2/2L.
A 50 m tape, off line at one end by 0.500 m (an excessive amount), would lead to an error of 2.5 mm in the measured distance. The error is systematic and will obviously result in a recorded distance longer than the actual distance. If we consider a more realistic error in misalignment of, say, 0.05 m, the resulting error is 0.025 mm and completely negligible. Thus for the majority of taping, alignment by eye is quite adequate.