Every bridge on the Lancaster Canal seemed to have a benchmark. Then the penny dropped. Benchmarks were used for linear surveys - ideal for a canal. The canal needed to be level to retain the water so surveying must have gone from bridge to bridge. But does that imply that the bridges were built as each section was dug? I suppose I'd assumed they dug the whole canal and then added the bridges but actually that wouldn't make sense in terms of access for local residents ...
