A benchmark

I spotted this etched into a wall in Saltaire. It amazes me how I can walk past the same places a thousand times and then suddenly spot something I've never consciously noticed before. I took it for some kind of mason's mark but my learned (engineer) friend told me it is a benchmark. It's obviously been there for many years, though I don't think the benchmark network existed when the village was built; I think it dates back to the early 20th century. 

Dr Google tells me that: 'Bench marks are the visible manifestation of Ordnance Datum Newlyn (ODN), which is the national height system for mainland Great Britain and forms the reference frame for heights above mean sea level.'  So, in Britain, they tell you how high above sea level you are relative to the base level in Newlyn, Cornwall. 'ODN is realised on the ground by a network of approximately 190 fundamental bench marks (FBMs). From these FBMs tens of thousands of lower-order BMs were established. The network has had little maintenance for 30 years, and in some areas (mining areas for example), subsidence has affected the levelling values. In these regions the BMs cannot be relied upon to accurately define ODN. Most commonly, the BMs are found on buildings or other semi-permanent features. Although the main network is no longer being updated, the record is still in existence and the markers will remain until they are eventually destroyed by redevelopment or erosion.'

They were used in building projects, for levelling, though I suppose there are more modern technologies these days. But the network remains. 

There's an exciting map on the Ordnance Survey website that shows the locations of all of them in the UK. Look at the map for Saltaire: 

There are lots more for me to find!

I see from the data that, in moving house, I've risen from about 83m above sea level to about 115m. Going up in the world, literally!