Keeping our fundamental benchmarks in order
Guest blog by Colin Fane, Geodesy and Positioning Consultant.
Most people are familiar with trig pillars. There are over 7,000 of them scattered around Great Britain. However, there is another, considerably more elusive, type of pillar to be seen across the land – the fundamental benchmark (FBM). FBMs are the physical realisation of our national height datum ‘Ordnance Datum Newlyn’ (ODN – mean sea level at Newlyn, Cornwall, 1915-1921) and are still crucial in defining this reference system today.
There are nearly 200 FBMs around Great Britain, mostly constructed in the first half of the twentieth century at sites carefully selected to provide an anchor to bedrock. 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 approximately three-quarters of a million bench marks, using less precise levelling. These lower-order bench marks are often seen cut into stone at the base of a building, church or bridge and about half a million of them are still in existence today.
Approximately half the FBMs are surrounded by railings, particularly in urban areas. To the casual observer the railings could be mistaken for a receptacle for other people’s rubbish (or at least this seems to be what some people use them for). However, the railings do provide protection and some of those FBMs without them can get damaged.
Lampeter FBM (SN5752) is one such example. It was brought to our attention that the pillar had been broken in two and the visible section of the pillar, above ground, was now laying on the ground. As these monuments are still important to our work, it was essential to rescue the broken pillar (before someone liberated it as a garden ornament!) and to repair it.
The visible section of an FBM pillar is usually about nine by 11 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’.
The broken part of the Lampeter pillar was repaired using a high-tech resin (in a delicate shade of bubble-gum pink) which was allowed to harden for 24 hours. We then re-heighted the top bolt on the pillar to determine its new altitude relative to ODN. This was done with a modern digital level – capable of measuring a difference in height to a precision of 0.00001 metres.
While we were working on the site, locals informed us that a Spitfire had crashed there during World War II, but we didn’t find any sign of it. Lampeter FBM is the first FBM we’ve had to re-height in over 20 years – so who knows when we’ll be working on one again?