chainage
An imaginary line used to measure distance, often corresponding to the centre of a straight road.
chainage: measured distance along a survey line from a fixed start
Chainage is the system of recording linear distance along a surveyed line, measured cumulatively from a defined zero point. On road projects, chainage typically runs along the centreline, with distances marked at fixed intervals, usually every 20 metres or 100 feet depending on local convention. A chainage of 1250 means 1250 metres from the project datum, making it simple to locate any point on the alignment without referring back to coordinates.
The method originated in chain surveying, where the actual measuring tool was a surveyor's chain, typically 66 feet long in imperial systems or 20 metres in metric. A chain was divided into 100 links, allowing fractional distances to be recorded precisely. The term persisted even after chains gave way to measuring tapes and electronic distance measurement. Chainages are now set out using GPS, total stations, or laser rangefinders, but the nomenclature remains.
Use in design and construction
Chainage anchors design data to physical location. Road design sheets specify culvert positions, curve tangent points, and gradient changes by chainage rather than by absolute coordinates. A note reading vertical curve from 2450 to 2650 communicates instantly to site engineers where work begins and ends. This linear referencing system is far more intuitive on site than grid references, particularly when working along a route.
Chainage works equally well for railways, pipelines, and utility corridors. Each asset records its longitudinal position this way. Disputes over responsibility for damage, or confusion about which section requires maintenance, are resolved by comparing chainages. The system also allows parallel works; a service trench running beneath a road will be logged using the same chainage as the pavement above it.
Common pitfalls include cumulative errors if chainages are set out incrementally without periodic verification against long measurements. Chainage values can also undergo adjustment if the design alignment is revised; new chainages must then be published clearly to avoid crews working at the wrong location. On long projects, surveyors typically establish regular chainage reference marks, usually metal pegs or witness marks, so that loss of intermediate marks does not corrupt the whole survey.