… and Christmas trees were tall … one of the odd things we learned at primary school was the purpose of bench marks. The school was split between two locations with only one canteen so, every lunch time, we were marched in pairs down to the lower school; a hungry chattering snake. Carved into a wall near the end of the route was a bench mark – I have never been good with remembering the abstract but because there was a tangible example nearby, the lesson stuck.
With the advent of more sophisticated mapping techniques these old marks have fallen into disuse. This explanation is from the Ordnance Survey website:
Ordnance Survey Bench marks (BMs) are survey marks made by Ordnance Survey to record height above Ordnance Datum. If the exact height of one BM is known then the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling.
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 Although the main network is no longer being updated, the record is still in existence and the markers will remain until they are eventually destroyed by redevelopment or erosion.
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. 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.
When outdoors for a walk along the local lanes yesterday I came across this example on a local farm gate:
Following some extensive online time-wasting, I found there was a Bench Mark Database – a trainspotter’s delight! Even more exciting, this bench mark was not registered but, it is now – Beaufront, Gatepost 3 – not exactly my own star but an acceptable, humble alternative 🙂