Detector Plate

Detector plate at Redscar

Exposed detector plate at Wansfell Pike

Detector plate at Fochno

Detector plate at Therfield Heath

Chamber Bolt at Tory Island photo © Rossographer

Lake District

A very small number of bolts have been found covered with a metal plate approx 12" square with the warning sign 'ORDNANCE / SURVEY / DO NOT / DISTURB' embossed on a raised circular disc at the centre of the plate. At installation, the covers were buried but in some cases were later exposed by ground erosion. To date (March 2026) all of these have been located within the Lake District National Park.

'Covered bolt' is a trig baggers term; the corresponding OS term for the cover is 'Detector Plate'. This is mentioned, for example, in the Office File sketches for Ponsonby Fell and Kentmere Hall. In T:UK and OSGB36 the trig mark itself is listed as either a Bolt or Buried Block. As with Detector Rings, the purpose of the cover is to aid in the detection of the trig when using a metal detector.

Trig Stations with a Detector Plate

T:UK	Station name	Order	Computed	Block	Date found	Finder
TP16495	Blake Fell	3	1952	NY11	5 Nov 2023	CJO
TP16484	Clints	3	1952	NY11	6 Aug 2023	CJO
TP17120	Gilpin Park	3	1950	SD47	26 Feb 2022	peregrinus
TP17121	Kentmere Hall	3	1950	SD47	11 Feb 2023	peregrinus
TP16490	Lorton	3	1952	NY11	13 Aug 2023	CJO
TP8079	Lowick	2	1950	SD47	14 Sep 2013	peregrinus
TP16492	Ponsonby Fell	2	1952	NY11	16 Oct 2019	Dusty
TP8081	Redscar	3	1950	SD47	18 Apr 2014	painterman61
TP13003	Top o' Selside	2	1950	SD47	10 Oct 2019	Dusty
TP7820	Wansfell Pike	2	1950	SD47	22 Apr 2020	projectionist
TP16497	Watch Hill	3	1952	NY11	6 Aug 2023	CJO

Trig Stations where a Detector Plate is suspected but not confirmed

T:UK	Station name	Order	Computed	Block	Notes

Others

A single instance of a cover with a different style has been found at Fochno in Dyfed. This in turn resembles the passive station sketch for Pegler's Tump.

A small number of OSI bolts, covered by a hinged lid marked "OS Station", have been found. These have been designated as 'Chamber Bolts'.

Trig Stations with a Chamber Bolt

T:UK	Station name	Date found	Finder
TP26008	Tory Island (Toraigh)	11 Jul 2025	Rossographer
TP7360	Dunglow	5 May 2005	peregrinus
TP25924	Knockomagh	23 May 2024	peregrinus
TP7346	Croaghgorm	5 May 2005	peregrinus
TP26506	Gregory Hill	31 May 2026	peregrinus

An unmarked iron plate covers the bolt at Therfield Heath.

At Stocks Lane Bridge correspondence in the OS Office File indicates that, in 1966, a 6"x6" water hydrant box was used to cover the bolt and protect it from the impending footpath construction. However, a subsequent report in 1967 states that the box and bolt were removed by GPO engineers while cable laying operations were in progress .

At Riber the block location corresponds to a water meter cover. In this instance however, it appears that the cover was used to replace the trig rather than to preserve it.

At Stornoway a bin lid was used to cover a passive station bolt.