Chains:

The primary instrument or equipment used in chain surveying is a chain or a tape. A survey chain is commonly composed of 100 or 150 links formed through pieces of galvanised mild steel wire of 4 mm diameter. The ends of every link are looped and connected together through means of three circular or oval shaped wire rings to gives flexibility to chain. The length of each link is measured as the distance among the centres of two consecutive middle rings. The joints of links are welded to prevent length changes because of stretching.

The ends of chain are given with brass handles along with swivel joints. This helps in turning the chain without twisting. The end link length involves the length of handle and is measured from the outside of the handle that is considered as zero point or the chain end. Tallies, that are metallic tags of various patterns, are provided at suitably specified points within the chain to facilitate quick and simple reading. A semi-circular grove is given in the centre on the outer periphery of handle of chain for fixing the mild steel arrow at the end of one chain length.

The number of links within a chain could be 100 in a 20 m chain and 150 in a 30 m chain. The details of a metric chain are as display in Figure.

Figure: Details of Metric Chain

The chain could be used conveniently in a rugged terrain and could be subjected to rough use under adverse site conditions. It can be read simply through even semi-literate persons. Therefore, the length is liable to be changed due to continued usage. Its comparatively heavy weight might cause sagging in the chain thus affecting the measurement accuracy.