Trig Stations of the 19thC Principal Triangulation
The Principal Triangulation of Britain was the first high-precision trigonometric survey of the whole of Great Britain (including Ireland), carried out between 1791 and 1853 under the auspices of the Board of Ordnance. The triangulation stations used were not the permanent markers of the 20th century Retriangulation which employed the well-known Hotine trig pillar along with a host of other permanent above or below ground markers that could be quickly re-located and re-used. It's not even clear what form the old stations took, but it is known that many of the sites were re-used for the Retriangulation, therefore, by studying old maps and the sites of the current pillars an educated guess can often be made as to what might have constituted an old station. This wiki page has been created to list marks on rocks that could possibly be old stations, some will be benchmarks in their own right and can be added to the Benchmark Database, others will not be benchmarks, typically an arrow engraved into a boulder with no datum line, or with a large hole at the apex of the arrow that might have taken a sighting staff. This is by no means a definitive or exhaustive list and is largely speculative. It is hoped that keen triggers will add to the marks listed here.
| Back Tor, South Yorkshire | SK 1976 9100 | Large old arrow on flat rock approx 4.5m SW of the trig pillar |
| Brunt Knott, Cumbria | NY 4844 0062 | Old pivot benchmark on flat rock next to trig pillar |
| Castle Dow, Perth and Kinross | NN 9296 5135 | Old pivot benchmark on embedded boulder at the highest point next to a small pile of stones |
| Crutchenber Fell, Lancashire | SD 7221 6030 | Old pivot benchmark, very worn. On flat rock close to edge and a few yards from pillar |
| Deerstone Moor, Lancashire | SD 9093 3676 | Hole in boulder with cut arrow |
| Fan Llia, Powys | SN 9379 1855 | Old pivot benchmark on flat rock |
| Featherbed Moss, South Yorks | SK 1903 9386 | Large old arrow on prominent boulder, no datum |
| Floddaymore, Western Isles | NF 9142 5693 | Cassini marker - circular hole drilled into embedded boulder |
| Great Whernside, North Yorks | SE 0021 7391 | Old pivot benchmark approx 4m east of trig pillar |
| Meldon Hill, Cumbria | NY 7722 2906 | Old pivot benchmark approx 65m SE of the trig pillar |
| Newton Fell, Lancashire | SD 7147 4748 | Old pivot benchmark adjacent to pillar, listed as an Old Trig in the OSGB36 database. Rock now on side. OS list it as Destroyed 1/89 |
| Parson's Pulpit, North Yorks | SD 9184 6875 | Old pivot benchmark on a rock approx 3m west of trig surface block (converted pillar) |
| Red Screes, Cumbria | NY 3965 0875 | Old pivot benchmark on loose rock at cairn shelter entrance |
| Roan Fell, Scottish Borders | NY 4496 9207 | Old pivot benchmark on flat rock 2-3m SW trig pillar |
| Silver How, Cumbria | NY 3248 0664 | Old pivot benchmark cut into bedrock just north of the summit cairn |
| Simon's Seat, North Yorks | SE 0789 5981 | Old pivot benchmark on summit rocks approx 2m SW of trig pillar |
| Strandlud Hill, East Ayrshire | NS 5825 0606 | Old pivot mark on summit boulder |
| Thornthwaite Crag | NY 4313 1001 | Large arrow Pivot BM on rock next to large cairn |
| Ullioch Hill, Dumfries and Galloway | NX 6869 6773 | Old pivot benchmark cut into slab west of base of summit cairn |
| Walla Crag, Cumbria | NY 2765 2129 | Old pivot benchmark on flat rock close to edge. |
| Moel Wnion, Gwynedd | SH 6497 6971 | Old pivot benchmark on slab in Wind Shelter next to (damaged ) pillar S7321 |
