Bolt

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OSGB36 bolt on an outcrop at Scrafton
OSGB36 2 inch bolt on Grin Low tower
Passive station bolt on the roof of York Minster

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 on church towers; other common locations include mills, water tanks, colliery headgear and towers.

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