Chainage

The term 'chainage' is used in surveying to refer to a distance measured in metres along an imaginary line, such as the centre line of a road or railway.

The term originated in 1620, when 66 feet chains (Gunter’s chains, named after mathematician Edmund Gunter) were first used to for the measurement of linear features such as roads or railways. The chain typically included 100 links, and 10 square chains was 1 acre. Cricket pitches are also generally 1 chain long, and a mile is 80 chains.

Ramsden's chains were longer chains of 100 feet developed in the 18th century.

Whilst this technique is no longer used, the name has remained, in particular in relation to railways, where it may be used to define the location of bridges and stations. A datum will be set as 0 at one point along the railway, and cumulative longitudinal distances measured using a device such as an odometer and then quoted along the length of the railway from that datum. This is generally sufficient to uniquely identify features such as bridges and stations.