Obstacles in Chaining:

Horizontality of distance measurements can be maintained easily on a comparatively level ground. Ground undulations tend to introduce errors within measurements as the natural tendency of chain and/or tape will be to follow ground level. A distance so measured will be larger than the actual horizontal distances between two stations. Larger the ground slope and larger will be the error introduced. To overcome this obstruction, chain is stretched horizontally between two stations, as tightly as possible as shown in Figure (a). The stretched chain will form a catenary. Through measuring the sag at mid point, horizontal distance AB could be mathematically associated to length of chain S and the sag δ.

Figure (a) : Distance Measurement on Undulating Ground

A better method will be to divide the entire chain into several smaller segments, particularly when the ground slope is larger, the sags between the segments will be negligibly small and the segment of the chain/tape will require much smaller physical effort in stretching (Figure (b)). Hence, the total length L between A (A₀) and B (A_n) will be L=  δl_i.

Figure (b) : Ground with Large Slope