Trig Points

Richard Ellis
 North Craven 
 Heritage Trust 

The approach to the top of Burn Moor is a fascinating experience in itself; there is no direct path. Even a flat slightly-inclined piece of dried-out Yorkshire moorland in hot sun retains a feeling of mystery and anticipation when, as the high point is reached, a view of Lancashire and Morecambe Bay is revealed. But in the foreground there is the familiar but still strange-looking object made of rough concrete and around 4 feet high. It has been visible on the near horizon for at least an hour due to the flatness of the terrain. As we get closer we can see it is truncated but with a flat top. It has four sides and on one a bench mark (an arrow under a horizontal line) has been imprinted, while on another a brass plaque shows it has an idiosyncratic number. It is built on base rock for stability and longevity.

We are, of course, looking at one of thousands of Triangulation Pillars (commonly and affectionately known as ‘Trig Points’) which form an elaborate network across the country.

The best way to Burn Moor from the north is to strike in a southerly direction from the B road at Mewith Head and after passing West Borronhead to follow Alder Gill Syke passing ‘Pile of stones’ from where the Trig Point is visible. In approaching this particular Trig Point I have an advantage by starting from Hawksheath Farm which is of a mile from the Keasden crossroads. The first part of the ascent is along Thorny Gill and then ascending Great Breast and passing the now derelict remains of what I believe to have been a shooting hut. Interestingly this obvious feature is not marked on the 1:25,000 OS map of Forest of Bowland (Edition A revised 1996), though it is retained on the 1:50,000 Landranger map of Wensleydale and Upper Wharfedale (Edition B1 revised in 2002 and 2006). In an almost direct extension of the line from Hawksheath Farm and the shooting hut the Trig Point is reached. It lies almost two miles (3 km) from Hawksheath Farm and a slightly shorter distance from Mewith Head.

Burn Moor is at a modest 402 metres above sea level. Yet it is so strategically positioned as to have been awarded a Triangulation Pillar all to itself. This pillar is designed to take on its flat top a theodolite, an optical instrument which measures angles and through which other Trig Points can be seen. Even with the aid of a cheap binocular the discerning eye can pick out nearby Trig Point locations on Caton Moor 7 miles (11 km) to the west at 361m, Whins Brow 8 miles (13 km) in the southwest at 476m, White Hill 4 miles (6 km) to the south at 544m, Fountains Fell 11 miles (18 km) to the east at 593m, and Rye Loaf Hill to the east of Settle at 547m, Pen-y-Ghent summit 10 miles (16 km) to the north east at 694m, and Ingleborough summit 6 miles (10 km) to the north at 724m.

This local array is only a small part of a national network of Trig Points, the purpose of which was the accurate re-mapping of our island leading to the production of some of the world’s finest maps - the various Ordnance Survey sheets such as the recent Outdoor Leisure Series. The network was established between 1936 and 1961. Several hundred primary sites such as Ingleborough summit were chosen together with 6000 secondary sites which include the others adjacent to Burn Moor.

The structures themselves represent an industrial archaeology which many now believe to be irrelevant due to the emergence of the Global Positioning System or GPS and consequential satellite navigation. Yet they retain an important function for the fell walker, especially on windswept and misty wild places, as a navigational guide. Why should we be at all concerned about Trig Points? They are, after all, relics of the past. It would be difficult now to find a suitably accurate theodolite. Many Trig Points have been ravaged and the three brass theodolite runners ripped off, as well as the brass number plaque on the side of the tower. Only rarely, thankfully, have some been partly or completely destroyed. Yet they are still part of a beautifully conceived mapping system based on triangulation - the use of triangles.

The use of triangles to estimate distance goes back to the ancient Greeks. Thales, six centuries BC, estimated the height of a pyramid by measuring the length of its shadow at the moment when his own shadow was equal to his height. Triangles were used to measure slope angles, and therefore gradients, and this required a sighting rod which itself naturally led to the development of the theodolite, which is really an optical sighting rod using accurate lenses, and nowadays laser beams.

The basis of land surveying using triangulation is to have a measured baseline. A point at a distance from this line can be determined by measuring the angle created between the ends of the base line and the distant point. If the distant point is a Trig Point, its position on the map can be identified - nowadays indicated on the OS map by a small blue triangle. In this way a new baseline can be established and more distant points determined, and so on.

Rough and ready triangulation was probably used by Christopher Saxton in his English Counties maps of 1570, although less charitable people have suggested he may have used estimated distances and guesswork to create his beautiful maps.

The Principal Triangulation of Great Britain was begun by the Ordnance Survey in 1783 and this continued for another 70 years. The roots of the Ordnance Survey itself go back to 1747 when Lieutenant-Colonel David Watson proposed the compilation of a map of the Scottish Highlands to facilitate the subjugation of the unruly Highland Clans following the Jacobite rebellion in 1745. The survey was commissioned by King George ll and the first map produced largely by William Roy - surprisingly a Scot. Perhaps Roy, who was born in Carluke in South Lanarkshire, had little sympathy with the highland clans and maybe he was a Protestant, or perhaps he was just ambitious. He became an FRS in 1767, a Colonel in 1777, a Major-General in 1781 and the Director of the Royal Engineers in 1783. He chose an initial baseline for the triangulation on Hounslow Heath near to the present Heathrow Airport. The cumulative error of the survey was an astounding 20 metres over the whole of the country. A modern GPS is hardly any better than this at 15 metres likewise.

But this is not the end of the story. After the establishment of the Trig Point network, the Ordnance Survey introduced, after retriangulation, the British National Grid Reference System - known as ‘The Grid’. In effect the Grid is an overlay of squares 500 km wide on the map of Great Britain and its outlying islands (though excluding all parts of Ireland which, as could be predicted, has its own grid). The large squares are subdivided into smaller squares of 100 km width and these are identified by two letters, the first for the large square and the second for the smaller. At Hawksheath Farm we are in SD territory, but as we travel east we cross over into the alien SE square just before Grassington in Wharfedale. The OS map is further overlaid by the thin blue network of 1 km squares from which Grid References can be derived. So we move from triangles to big squares then to small squares then to smaller squares then to specific reference points. Hawksheath Farm is SD 719 658 and Burn Moor Trig Point is SD 695 646.

We have inherited the ‘Trig’ network and we can decide what to do about it. The Trig Points may be largely anachronistic, yet there remains a magic in what they represent. They remain navigational beacons in all weather, fine or foul. They are all marked on our walking maps, and so can act as convenient and specific meeting places. We know when we stand close they will always provide a distant and wonderful view.

But if we feel they are old friends, they need to be respected, nurtured and protected. There is nearly always a Trig Point within eyesight but in order to pick out a 4 foot tower of concrete on a high point near us, we have to lift our eyes to the horizon - and perhaps to contemplate our future.

(Note all distances are rough estimates and all elevations are taken from OS maps. The description of the approach to the Burn Moor Trig Point may cross private land for which permission may need to be obtained).