The Angry Corrie 32: Jun-Aug 1997


Lost: 4500 bench marks

In a rare and refreshing outbreak of girls-and-numbers, Barbara Jones becomes the latest TAC reader to admit to a fascination with the monolithic pillars dotted around Britain's high (and low) points ...

I have recorded over 700 trig point numbers: enough to start drawing conclusions and asking questions. Trig point numbers? Yes, fixed to the side of every trig - well, almost every trig - is a brass plate or "flush bracket" with a unique number. Some mis-recording of numbers has led to duplication in my records - usually occuring after a visit to a remote trig and then not discovered until back home. Would anyone like to check Peter Hill, OS44, NO578886, and Craig of Dalfro, OS45, NO673898, both down as S5858 in my list? Craig of Dalfro is shown as thickly forested on the 1989 Landranger, but when I was there in 1992 the trees had been felled.

My records run from S1501 (Butser Hill in Hants) to 11900 (Peniston Hill in West Yorks). All four-figure numbers, bar one (2951, Dunstable Downs in Bedfordshire), have an S either in front of the figures or, in the case of many numbers in the 3000s, above them. The numbering suggests that there are, or were, at least 11900 - 1501 = 10899 trigs. But various sources state there are "over 6500" (1), "some 6500" (2), and "6154" (3). So where are the phantom 4500 or so?

When the OS first surveyed Britain in the 1840s and 1850s, and again before and after WW2, they began by establishing a network of primary trig stations at approximately 30-40 mile intervals on prominent hilltops. (Or not so prominent: 47, Selsey in West Sussex, is probably the lowest at 7m.) The OS then filled in the network with secondary and tertiary trigs, from which they surveyed the fine detail.

So not all trigs are equal and some are not even trigs in the conventional sense. The OS made many observations from church towers and the like, and among primary trig stations are York Minster, Lincoln, Ely and Peterborough Cathedrals, water-towers, a gasometer, and two castles (Belvoir in Leics, Orford in Suffolk), and six assorted towers. I've scrutinised Leith Hill Tower in Surrey and Orford Castle (a small, simple keep), and haven't found any flush brackets at either ground- or rooftop-level. However, on the rooftop at Leith Hill Tower are sundry brass knobs which probably have something to do with the OS (top brass knobs? - Ed.), and at ground-level a couple of bench marks are cut into the stonework. I could find nothing to suggest that the OS had been at Orford Castle.

The old-style Pathfinder maps - covers showing the land white with orange roads, and phased out in the mid-80s - show some churches with little white holes in their black squares/circles. In the legend these are "triangulation stations on churches". Also shown are "triangulation stations on chapels, lighthouses, beacons, buildings and chimneys". Has anyone come across examples of these latter five?

On my four local old-style Pathfinders I've found various trig-churches: TQ04/14 Dorking, dated 1985: three; TQ05/15 Woking and Leatherhead, 1981: four; SU84/94 Farnham and Godalming, 1980: two; SU85/95 Farnborough and Aldershot, 1981: six. Some of these have been visited, but I've not found any flush brackets. I suspect these may be some of the missing trig numbers. Here in the southeast I've visited most conventional trig pillars. Their numbers often run sequentially, three, four, five or six in a row (like Rangers and the Scottish league title - Ed.), and then comes a gap of one or two (unlike Rangers - Ed.). Perhaps these gaps are trig stations on churches, numbered in the OS scheme of things but not in the field.

So, S1501, Butser Hill, is the lowest trig in my records. Furthermore, Butser is one of the 340 primary trigs (4) spread across Britain. All trigs noted with numbers in the 1500s are primary, and Butser is number nine in the OS listing of these. I've not visited any of the eight primary trigs listed above it, but it rather looks as if numbers before 1500 may not be trigs at all.

So where are these lower numbers? A special bracket can be clipped on the flush bracket to provide a horizontal ledge. This is the height level, equivalent to the horizontal groove of a bench mark. Several lines of primary levelling snake across Britain, with flush brackets fixed to permanent structures and bridges along these lines. The first phase of primary levelling was completed in the years before 1921, but the first trig pillar was not built until 1936. I've come across eight of these non-trig flush brackets, including 0596, 0597 and 0601 on the Isle of Wight. Maybe the pre-1500 "missing" trig numbers are on these levelling lines.

The building next to the post office in Killin bears flush bracket G1225, whilst G1692 is on the Greenwich Observatory, S1692 is on the trig on Quarley Hill, Hants, OS185, SU263424 - the only repeat number thus far encountered between trig and non-trig flush brackets; but one begins to see the importance of the S and the G.

Flush brackets G1768 and G1769 turn up on stone bridges along the old road from the Cluanie Inn towards Loch Loyne, while G10009 is on the church at Mickleham, near Dorking, Surrey. This isn't one of the trig stations on churches but does happen to be where I was married - and I didn't notice the flush bracket until some thirty years later! (Having doubtless been too flushed at getting bracketed with Mr Jones at the time - Ed.)

I also record whether the centre bit of the tribach/spider (the bronze thing they attach the theodolite to) is there, is missing, or has its hole plugged with a cement-like substance. The centre screws in and is theoretically prevented from being undone by a couple of grub screws. Sometimes these are bronze, sometimes stainless steel. I've not recorded this, but lie awake at night wondering if I should have done or should start doing do!

(1) Ordnance Survey - Map Makers to Britain since 1791, Tim Owen / Elaine Pilbeam, HMSO, 1992.

(2) A History of the Ordnance Survey, WA Seymour (Ed.), Dawson, 1980.

(3) Article in TGO, Roger Smith, 1994.

(4) History of the re-triangulation of Great Britain 1935-1962, HMSO, 1967.