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

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.

The fundamental benchmark

Less common to spot is a fundamental benchmark (FBM), which as the name suggests, is one of our high-accuracy benchmarks. There are around 190 of these which are still maintained and used by us at OS. They form our primary height network and, as such, are our link to the Ordnance Datum at Newlyn and are still crucial in defining this reference system today.

The height of each FBM relative to ODN was determined by a network of precise levelling lines across the country. The levelling network was then densified with lower order benchmarks, using less precise levelling.

The visible section of an FBM pillar is usually about nine by eleven inches and around a foot tall (to use the units of measurement from the Imperial era in which they were built), with a brass bolt set into the top and a name plate declaring it to be an ‘Ordnance Survey BM’. There is also an underground chamber containing the “master” precise reference marks for the point.

The flush bracket

The flush bracket is another way of accurately defining a height above sea level and used for the more important level control points. You will find flush brackets in most trig pillars – although not all – as well as set into walls and buildings (‘flush’ with the building).

The trig pillar

Identified on an OS Explorer map the trig point map symbol is a small blue triangle with a dot in the middle, a trig pillar (triangulation pillar) is a familiar sight when you’re out and about exploring Britain. Between 1936 and 1962 around 6,500 were built to form a state-of-the-art network to re-map Britain. The trig pillar provided a solid base for the theodolites used by the survey teams engaged in the retriangulation of the country, the mathematical process that made accurate map-making possible. In the same way that BMs mark an accurate height, trig pillars mark a point with an accurate horizontal position (eastings and northings coordinates). In addition, most, but not all, trig pillars also have a flush bracket, to define their height above sea level.

How we survey Britain today

As we’ve said above, most of these means of measuring height are now redundant for national mapping. The modern equivalent to the network of trig points is the OS Net network of 110 Global Navigation Satellite System (GNSS) receivers. Our surveyors use OS Net and GNSS technology everyday to instantly position new map detail to within a few centimetres.