With constant, regular use, a tape will stretch or distort. Therefore, when you are about to take some measurements, the tape should first be compared against a standard tape. The standard tape is simply a tape which is only used for standardisation of other tapes. It thus recieves no heavy usage, and should not have deformed.
Taking l as the nominal length of the tape (eg. 30m), l' as the standardised length, (eg. 30.011m), and L as the measured lenght, then:-
|Corrected length = L (multiplied by) ( l' / l )|
Subtract L from the corrected length to give the correction to be applied.
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