ERRORS IN TAPING
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
Thus even for ordinary precision measurement, the temperature effect cannot be ignored. Even if the tape temperature is measured there may be an index error in the thermometer used, part of the tape may be in shade and part in the sun, or the thermometer may record ground or air temperature which may not be the same as the tape temperature.
Although the use of an invar tape would resolve the problem, this is rarely, if ever, a solution applied on site. This is due to the high cost of such tapes and their fragility. The effect of an error in temperature measurement can be assessed by differentiating equation
If L = 50 m and the error in temperature is 2◦C then δCt = 1.1 mm. However, if this error remained constant the total error in the measured line would be proportional to the number of tape lengths. Every effort should therefore be made to obtain an accurate value for tape temperature using calibrated thermometers.