Wednesday, 20 January 2010

Cut Marks, Bolts and Flush Brackets

At the end of last year we bumped into the greater world of Ordnance Survey marks; a world than includes but extends well beyond the Trig Points found on some hill tops. The best place to start is the Bench Marks database which “contains records of Ordnance Survey bench marks, with particular focus on the numbered flush brackets which appear on walls and buildings across Britain. In addition, fundamental bench marks, projecting brackets, tidal observatories, and other bench marks are also included, to help give a more complete picture of the levelling lines.”

This is a national treasure hunt or rather, like an iceberg, just the tip of a national treasure because in addition to the marks recorded in the database there are between 250 and 500,000 other OS marks. Search using your own Post Code to find those nearest to you but be careful this is highly addictive, believe me.

The table below describes the different types of bench marks -

Type

Description

Flush Bracket

These are rectangular metal plates affixed to triangulation pillars, walls, buildings etc. If they are not affixed to a triangulation pillar they are often referred to as Non-Pillar Flush Brackets (NPFB). They are about 6" x 3" in size and the location of most of them is known through Ordnance Survey (OS) records. Each Flush Bracket (FB) has a unique serial number which makes them highly 'collectable' and there are several numbering types.

One to four figure number

The earliest FBs you can find and date from 1912-1921. Numbered from 1-3000. A few can be found on triangulation pillars.

S-prefix

Introduced in the 1920s these are found on pillars and as NPFBs. There are two sub-types, S below the number and S left of the number.

G-prefix

First appeared in 1936. Unlike S-series brackets numbers below 1000 do not have leading zeroes. Used extensively in Scotland and never found on triangulation pillars.

L-prefix

Just sixteen brackets used in London in the early 1930s.

Five figure number

These are effectively S-series brackets above S9999 where they ran out of room for the S (S-below was discontinued as the S interfered with the measuring equipment).

Projecting Bracket

An early type of metal bracket used for a short time before the introduction of flush brackets. They are all the same and have no unique attributes.

OSBM Bolt

Used alongside the G-series of flush brackets these are placed where there wasn't a convenient building or wall to provide a vertical surface on which to affix a flush bracket. They are domed metal bolts about 1" (50-60mm) in diameter fixed to horizontal surfaces engraved with OSBM and the benchmark symbol.

Cut Bench Mark

By far the most common type. Used and made from the 1800s to around 20 years ago. You won't have to walk (or drive) very far in any village, town or city in Britain before you spot one of these. Chiselled into stone, brick or wood on all sorts of vertical structures. A familiar horizontal levelling line with a three line arrow pointing towards it (usually upwards). Each one is unique depending on the mason who cut it, some are plain, some decorated. Some roughly cut, some exquisitely cut with high accuracy. Some small, some huge.

Cut Bench Mark with Bolt

Old and rare these have a metal bolt screwed either alongside the horizontal cut of a cut bench mark or at the point of the cut arrowhead. Usually has what appears as a screw head horizontal in the head of the bolt. These are highly prized by benchmarkers.

Rivet

Usually found on horizontal surfaces these are cut marks with a small metal domed brass rivet at the apex of the cut arrowhead marks.

Pivot

Fairly rare these are used on horizontal surfaces such as soft sandstone, where the insertion of a rivet would break away the stone. They consist of a small hole or depression cut to take a pivot, a steel ball bearing of 5/8" diameter (16mm). In use, the pivot is placed in the depression and the levelling staff held on top of the pivot.

Fundamental Bench Mark

These are the key to the whole levelling of the UK. Granite blocks with large domed metal caps. Just like an iceberg this is just the tip of a fairly extensive underground structure. Highly accurate height stations still used today as the baseline to levelling.

Triangulation Pillar

Familiar to anyone who walks in the British countryside, these can often (but not always) be found at hilltops. Most have a flush bracket affixed to one side.

Last weekend we travelled to Glasgow to celebrate my Dad’s recent birthday. As usual I try to get some miles in even if it is very early in morning and quick look at the Bench Marks database showed a number of churches on my regular route had bench marks. Two of them are Cut Marks with Bolt dating from the first survey conducted between 1844 and 1860. I also found some other churches I knew that also had marks so parts of the weekend were given over to finding them – 8 marks in total, all found.

Glasgow Bench Marks (1 of 5)

Cut Mark with Bolt (at the top of the arrow where it meets the horizontal line) on Old Mearns Kirk

 Glasgow Bench Marks (2 of 5) Cut Mark with Bolt on Eaglesham Parish Church

The most observant will already have spotted the above were taken using flash – the first one was especially difficult as it involved skulking around the Churchyard with a torch at around 6 o’clock in the morning as the last of the Saturday night revellers made their way home. The second, at Eaglesham, is in the centre of the village where I was again happy to avoid the local constabulary – explanations could have been difficult especially had I been unable to find the bench marks.

Glasgow Bench Marks (3 of 5) Cut Mark with Bolt on the derelict Old Parish Church at Cathcart

Glasgow Bench Marks (5 of 5)close up of simple Cut Mark on Mount Florida Church

Glasgow Bench Marks (4 of 5)  simple Cut Mark on Mount Florida Church

Finding bench marks is, like any treasure hunt, great fun and I have been astonished to find how many there are in places I have walked, run, cycled and driven passed for years completely unaware of their existence. The Bench Marks database allows you to log your own visits and record details of the condition of the particular mark. If you want to log visits to Triangulation Pillars have a look at Trigpointing UK

Back to the hills next week, I hope, and perhaps a few more bench marks too.

 

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