A 'Block' is a concrete block approx 1 cu ft in dimension emplaced such that the top surface is at ground level. A bolt, with head diameters measured to date between 3/4" and 2 3/8", is fixed at the centre of the upper face of the block. Over time surface soil/vegetation etc has built up rendering the blocks often now several inches below ground level.
For a general description of a 'Buried Block' see 'Block' above. 'Buried Blocks' were designed to be buried below ground level (0.6m is an indication of the depth), however, they can be found much closer to the surface. A tool known as a searcher was used by the OS to help relocate the trig mark. At some point the OS experimented with introducing a ferrous substance (iron filings?) into the concrete mix of some buried blocks in order to use a metal detector to aid in locating these trigs. Apparently it wasn't very successful. Another relocation technique was to cover the block with a metal sheet, for example Long Hill. See also 'Concrete Ring' below, for a variation on the standard concrete cube.
'Concrete Ring' is a trig bagger's term denoting a feature of some buried blocks seen in Lakes/Northern Pennines and elsewhere. Typically, the ring is approx 15" internal and 21" external diameter. It usually bears the inscription "Ordnance Survey Trigonometrical Station" although this may be eroded or absent. The ring is placed loosely above the block. The block itself typically has a central, slightly conical bolt of approx 3" diameter.
Initially, whilst very few examples of concrete rings had been rediscovered, they were considered by some baggers to be a decorative feature, lacking a functional use. Subsequently, it was understood that they were identical to the OS term 'Emplacement Ring' as described in Section 7.032 and Fig 7.7 of .
Some, but not all, concrete rings have an embedded metal ring (originally phosphor-bronze, subsequently iron) . With the use of a metal detector the emplacement ring can assist in locating a trig mark, but is not in itself a mark.
The OS records do not identify which blocks have emplacement rings, hence it is not known how many exist. Until 2014, 10 Concrete Rings had been located, all in Northern England (squares NY and SD). However, they have subsequently been located more widely, e.g. NZ, SO, SS, ST, SU, SX and SW with 26 having been identified by late 2019, 43 by 2022.
The photograph to the right is the Saddleback buried block located at the summit of Blencathra, Cumbria. It was reported missing in 2018 and was replaced in early 2019.
The term 'Surface Block' is derived from the OS Passive Stations Database. A number of OSGB36 Blocks were reused as Passive Stations, and some new Blocks were constructed. In practice, Surface Blocks are physically indistinguishable from Blocks.
- History of the Retriangulation of Great Britain
- email from Ian Wilson to the trigonomy mailing list, 15 July 2009