In chain surveying using chain / tape, the sag due to its self - weight takes the form of a catenary. While applying sag correction the catenary is approximated as:

Explanation:

Sag correction is given by:

\({C_s} = - \frac{{{{\left( {w L} \right)}^2}L}}{{24\;{P^2}}}\)

Where,

l = the length of the tape (in metres) suspended between supports

P = Pull applied in kg or N

W = weight of the tape in kg or N per metre run

Cs = Sag correction in metres for length (l)

wL = weight of tape suspended between the supports.

i) Sag correction is directly proportional to the square of the weight of the tape (kg per metre run)

ii) Sag correction is inversely proportional to the pull applied.

iii) While applying sag correction the catenary is approximated as parabola.