'Concrete Ring' is a trig bagger's term denoting a distinctive type of Buried Block seen in Lakes/Northern Pennines and elsewhere. Typically, the block comprises a central bolt of 2" diameter. This is surrounded by a concrete ring of approx 15" internal and 21" external diameter. The ring may bear the inscription "Ordnance Survey Trigonometrical Station" although this may be eroded.
The OS records do not distinguish Concrete Rings from other types of Block 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. SU and SS.
A concrete ring as described above is distinct from an 'emplacement ring'. The latter is a term used by the OS to describe a metal ring (possibly with a concrete or similar casing) placed loosely on a buried triangulation mark. 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 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.