Chain (unit) - Contemporary Use

Contemporary Use

In Britain, the chain is no longer used for practical survey work. However it survives on the railways of the United Kingdom as a location identifier. When railways were designed the location of features such as bridges and stations was indicated by a cumulative longitudinal "mileage", using miles and chains, from a zero point at the origin or headquarters of the railway, or the originating junction of a new branch line. Since railways are entirely linear in topology, the "mileage" is sufficient to identify a place uniquely on any given route. Thus a certain bridge may be said to be "at" 112m 63ch, meaning that it is at the location 112 miles and 63 chains (181.51 km) from the origin. In the case of the photograph the bridge is near Keynsham, that distance from Paddington station. The indication "MLN" after the mileage is the Engineers line reference describing the route as the Great Western Main Line, so that visiting engineers can uniquely describe the bridge they are inspecting, as there may be bridges at 112m 63ch on other routes.

The chain is not taught in British schools, but has survived for these reasons:

• Railways need to keep permanent records of as-built drawings of structures, and of the topography of routes and junctions;
• Chains and links are in many survey and real estate records;
• Miles and chains remain values familiar to many people.