Pre-1936 Trig Points

[Alduma]

Newly registered user here. I’m a big cartography fan, and have lately been consuming anything I can find on early OS maps.

I have a trig point near me that is marked on 19th century maps but not late 20th century ones. I would guess it was an intermediate trig point used as a reference point when mapping between primary survey trig points in the late 1800s.

There’s nothing visible at the location other than a lot of moss and scrub trees now. I’ve looked at every significant lump and bump.

My question is - would there have been a physical marker there if it warranted marking on an OS map? And if so what did pre 1936 trig points in Argyll look like?

Any information gratefully received!

[Alduma]

PS I should add the track shown on the map is long gone!

[agentmancuso]

...would there have been a physical marker there if it warranted marking on an OS map?

Sometimes! The 19th Century Principal Triangulation made use of intersected stations in much the same way as the OSGB36 Retriangulation, generally church spires or similar.

.. And if so what did pre 1936 trig points in Argyll look like?

Physical remains from the Principal Triangulation are pretty scarce, though this might reflect the fact that many appear to have consisted of nothing more than a carved arrow on embedded stone or bedrock and hence were indistinguishable from benchmarks. Some appear to have been dressed stones buried under the surface, a precursor of later Blocks without the brasswork.

If you do find anything, please supply a picture & GR.

[Alduma]

Thank you - very interesting! If I find anything at all I’ll certainly post it!

[Stephen Roddick]

Pre-1936 Trig points may have taken the form of "blocks" or "buried blocks".
There may still be a small stone structure buried below the ground surface at the specific point.

[wheresthatpenguin]

Archeologists actually discovered a trig point from the Principal Triangulation carried out between 1783 and 1853 back in October 2020. It was right next to the Beacon Ring pillar (TP1112). An excellent article on the discovery can be found here - https://archaeology.co.uk/articles/news ... n-ring.htm

The main section from the article reads: "The excavations at Beacon Ring revealed the remains of an earlier trig point and benchmark, which had been laid out as part of the ‘Principal Triangulation’ in the 19th century. The older benchmark was in two parts. The lower part was a stone slab, about 900mm square. This had been set into the underlying subsoil, and would have been at or just below the contemporary ground surface. A Board of Ordnance ‘broad arrow’ symbol was carved into the top surface, with a circular hole at the apex marking the precise triangulation point. The second part was a later concrete block, cast on top of the ‘broad arrow’ benchmark. Its form suggested that it had been created in situ by pouring concrete into a hole cut into the ground – so it seems likely to be a later addition. It is tempting to associate the stone-cut benchmark with the Ordnance Survey’s use of the sea-level at Liverpool as a national datum (from 1844 to 1915), and the later concrete block with the change to the Newlyn Datum, which took place between 1912 and 1921."

A photograph of what the archeologists found is below

[agentmancuso]

Thanks for that link, very interesting; information about the Principal Triangulation is hard to come by! ( I've crossposted to the FB group too)