Something the meaning of which eludes both me and a friend is an odd sign which we have spotted on things as diverse as old buildings - colleges, churches, and mileposts.
They are Ordnance Survey "bench marks" which are precisely surveyed for their height above the zero level for the contours seen on ordnance survey maps. Their height is measured using precise methods and is accurate to about a cetimetre.
Like the "Trigonometrical Points" that you see on the tops of many hills, which are accurately surveyed in plan position (latitude and longitude, and/or grid reference), they are largely being supplanted by Global Positioning Systems (GPS) that use the same orbiting satellites as sat-nav systems in some expensive cars - and motorhomes!
Hope this helps!
Tony
I agree with this having lived in a 10 year old cottage 30 years ago. I remember when the ordanance survey man came around looking for the bench mark on the cottage. Every building marked on an ordanance survey map has a benchmark on it somewhere. This shows it is marked on the map.
