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Tape correction is the application of a formula to tape measurements in surveying to smooth out errors and yield accurate results. Manufacturers of survey tapes design durable products and test them extensively, but in the field, conditions are not controlled, and a number of problems can create errors. When a high degree of accuracy is critical for a project, it is necessary to apply tape correction formulas to correct these issues and return accurate survey results. These are noted in the documentation that accompanies the survey for the benefit of anyone who wishes to review the results.
A certain amount of tension must be maintained on survey tapes while in use, and they can sag if they are not supported in the middle, reading long; a length of tape 10 units long might, with a sag, generate a reading of 10 units when the distance is really 9.8 units, for example. Temperature, slope, and other conditions on a site can also impact the accuracy of survey tape. Manufacturers test their products and develop certificates with information about the errors and tolerances for ease of tape correction.
In tape correction, the surveyor takes the measurements, notes the conditions, and then corrects for any errors that may be present. This can be done by subtracting or adding to the measured distance to yield a correct result. Tape correction is also necessary when tapes are spliced or repaired, as these activities can change the length of the tape and create problems with measurements. The surveyor may confirm results with other measuring instruments to make sure the finished survey is complete.
Survey tapes are made from very strong, durable materials with minimal contraction and expansion within a range of normal temperatures, and some natural tension and rigidity to support them. These measures reduce the risk of errors, but each tape can be slightly different, and a surveyor may need to periodically recalibrate tapes for tape correction purposes. A seemingly small error could become quite large on a complex project, and might result in inaccurate data that could come back to haunt the surveyor later.
Questions about tape correction can be addressed by looking up reports on the survey. Surveyors take detailed notes on their activities in the field, including discussions of conditions, any necessary corrections, and other issues. If a measurement appears inaccurate or seems to have a problem, the documentation can be reviewed to determine if there was an error on site or with the measurements that was not appropriately corrected.