Trigonometrical point V Triangulation station

[Rocky074]

I hope that there is an expert out there who can answer this question.
I have been looking at an old 1 inch to the mile OS map dating back to 1948. The map features small triangles with a dot and next to each is a height in feet. These are not always at the highest point of a hill. There are called Trigonometrical Points on the map key.Now I realise that at this time the process of constructing triangulation stations was still ongoing.
The question is, what relationship does the trigonometrical points have to the triangulation stations as not all points became stations?
Thanks
Rocky

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And why are they still shown on the OS maps as a triangle with a dot where the pillar was not erected when this symbol generally now explains where a pillar is, on the key- it says 'pillar'. Stanage Pole is an example of this. There has never been a triangulation point there but is still on the map as the dotted triangle. Presumably where they were going to erect one but never did so why is it on the map...... this has drove me mad for years!

[agentmancuso]

Only about one third of triangulation stations are pillars. Previously, pillar stations were shown on OS maps in blue ink and non-pillar stations in black ink. At some stage, the decision was taken to not show non-pillar stations on maps any more, and the vast majority were removed. But a few non-pillar stations were accidentally missed during the editing process, and ended up appearing in blue ink, giving the impression that pillar stations were to be found in places they had never been, hence 'phantom trigs' as these were often known (by us).

[Rocky074]

Thanks for the replies.
Interesting comment there agentmancuso regarding the pillars on the maps.
My question though is about the difference between trigonometrical point and the triangulation stations. I have been looking at old maps (mid 1800's) and see that these points mark a height and are shown on maps a the triangle with a dot. Am I right in assuming that well before the OS started the triangulation process many places were logged by height using theodolites? If so, any idea when this was done?

[agentmancuso]

Triangulation Stations listed on T:UK are those used in the Retriangulation of Great Britain which took place from 1935-62, often known as OSGB36. Approximately 1/3 of these are pillars, shown on OS 1:50k & 1:25k maps by the familiar blue dotted triangle. The original triangulation of Britain started in 1783 and was completed by 1858; it is known as the Principal Triangulation. These are the trig stations marked on earlier maps. Some locations from the Principal Triangulation were reused for OSGB36, and appear as 'Old Trig' in the records; physical remains of these are almost never to be found. I suspect that in a number of cases no physical mark was ever made, a bit like temporary versions of OSGB36 Intersected Stations, but I don't know for sure. I suppose the definitive answer would be found in the "Account of the Observations and Calculations of the Principal Triangulation and of the Figure, Dimensions and Mean Specific Gravity of the Earth as derived therefrom" published by A.R.Clarke in 1858, but I've never seen it.

[MAC.HAWK]

You could try visiting https://www.ordnancesurvey.co.uk/about/ ... story.html and look in some of the free pdf documents. In 'A history of OS' there's a section that covers the document agentmancuso refers to, whether it helps you or not I couldn't say.

[Rocky074]

Thank you for your help, that has been most useful. Not sure if it has satisfied my curiosity or encouraged me to delve even deeper