Correction  for Sag

Tapes are suspended at shorter length and horizontal distances are measured whereas measuring on unevenly sloping ground. This technique eliminates errors due to measurement along slopes, but necessitates correction for sag. Therefore, measured length is more than real length. Therefore the correction is -ve. The correction, which is difference between the length of catenary and true length is given by

Where, W = the weight of the tape of span length

P = the pull applied and          L = measured length

It can be noted that if pull is more than standard pull, the correction for pull is +positive, while correction for sag is always negative. The pull for which these 2 corrections neutralise each other is called 'normal tension'. Therefore normal tension Pn can be found as,

                                                               Cp = Cs

The value of Pn is to be resolute by trial and error method.