Day

September 21, 2017

21
Sep
2017
0

Benchmark or trig pillar: what’s in a name?

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.

The benchmark

Most weeks we’ll see a Twitter conversation where someone is asking what this mark is:

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. Read More