The National Archives
In 2012, the Ordnance Survey transferred the paper documents for many triangulation stations to The National Archives (TNA) at Kew .
Each triangulation station has an Office File, as described in The History of the Retriangulation of Great Britain . This contains a varying number of loose-leaf pages from 2-3 to 20 or more, typically including reconaissance and maintenance reports, lists of rays and bearings to other nearby stations, maps and location sketches. The Ordnance Survey Act of 1841 gave the OS a legal right to enter into and upon any land for survey purposes. Correspondence with the Landowner is therefore also held in the files.
For tracking down elusive trig stations (especially blocks) the sketches are particularly useful.
Catalogue entries were allocated to each document by TNA, each comprising a prefix OS69, followed by a catalogue number and station name, for example: OS69/4873 Millstone Rigg. OS69 is the Series identifier for "Ordnance Survey: Trigonometrical and Levelling Divisions: Triangulation Stations, files". Some documents in related series are listed at deformedweb.
The archive may be searched via http://discovery.nationalarchives.gov.uk/SearchUI/ using 'OS69' and the station name as keywords.
Few, if any, of the documents have been digitised and so the expected search response is therefore "This record has not been digitised and cannot be downloaded".
GPS stations were not part of the initial transfer and so, for these, the search response was "This record is closed and retained by a government department". For example, a search for OS69 Rombalds Moor produced this response.
It was subsequently reported that all trig files were being transferred to TNA, via temporary warehouse storage and that "the only paper files retained by the OS relate to FBMs as these still currently realise the National height datum" . However, as of July 2022, the access issue with GPS station records still applies.
A few Office Files appear to have been lost. For example, a search for OS69/8495 The Poplars responds "This record is not available to order. Missing at transfer."
Although it does not occur with the OS69 document series, another possible search outcome is "This record can only be seen under supervision at The National Archives". This applies, for example, to early theodolite photos
Individuals may register with TNA then pre-order access to specific paper documents. These will then be available for inspection on a subsequent visit to Kew, but note that the OS Office Files are held off-site and a 4 day lead time is required. This also impacts on a personal visit to the Archives as no further documents can be ordered for viewing during the day of the visit. It should also be noted that the Office Files are held in boxes of between 10 and 20+ files. Ordering one file will result in the complete box being available during a visit. Details will be found on the TNA web site This service is free, notwithstanding the visitor's travel costs.
It is possible to request a digital copy of a document from TNA. These are high-quality scans. The current scale of charges is provided on the web site.
Others organisations, such as https://www.arcre.com/ and http://www.searcher-na.co.uk/ can visit TNA on your behalf and obtain photos (not scans or photocopies) of specified documents. The image quality is satisfactory for interpreting the documents. The images are made available for download as a zip archive. The fee for this service depends on the number of pages required but is typically lower than the charge levied by TNA.