Bolt

A bolt is a common type of triangulation mark in both OSGB36 and passive stations.

There are 6032 instances in the OSGB36 list. Of these, 40 are Order 1, 705 Order 2, 3260 Order 3, 1977 Order 4, 2 Order 12 and 48 Order 13 (orders 12 and 13 denote GPS stations). Roughly one-third of OSGB36 bolts are roof stations on church towers; other common locations include mills, water tanks, colliery headgear and towers. Roof station bolts are mostly inaccessible but those which can be viewed include Haverbrack Tank, Sandscale Pillbox and Hospice of Hampsfell.

There are 94 instances of bolts that have been used as passive stations some of which may have been re-used from OSGB36.

Ground station bolts are typically installed in bedrock but there are exceptions; e.g. Ravenfield Park is installed in concrete and is visually indistinguishable from a block trig.

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  OSGB36 bolt on an outcrop at Scrafton

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  OSGB36 2 inch bolt on Grin Low tower

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  Passive station bolt on the roof of York Minster

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  Buried bolt at Ravenfield Park