We’ve had a few questions recently about benchmarks and trig pillars and what they are and how they differ, so we thought we’d clear it up.
Most weeks we’ll see a Twitter conversation where someone is asking what this mark is:
A #TBT to the OS benchmark, spotted here by @770.92. These survey marks can still be found on walls and buildings across Britain and were a way of recording height. Today, our surveyors use GNSS technology and it takes just seconds to do a task which could take days in our past. There are around 500,000 benchmarks in various formats – have you ever spotted one?
Many think it is War Office-related, but it is in fact an OS benchmark (BM) and a means of marking a height above sea level. Surveyors in our history made these marks to record height above Ordnance Datum Newlyn (ODN – mean sea level determined at Newlyn in Cornwall). If the exact height of one BM was known, the exact height of the next could be found by measuring the difference in heights, through a process of spirit levelling. They can be found cut into houses, churches, bridges and many other structures. There are hundreds of thousands of them dotted across Great Britain, although we no longer use them today.