Correction for Sag:
Correction for sag is applied while the tape is stretched on supports among two points; it takes the form of a horizontal catenary. The horizontal distance will be less than the distance along the curve. The difference among horizontal distance and the measured length along catenary is known as sag correction and it is always negative. This correction (Cs) is given through:
Cs = l (wl )2/24P2
where, l = The length of the tape (in m) suspended between the supports,
P = Pull applied in kg or N, and
w = Weight of the tape in kg or N per m run.