taken 17 years ago, 3 km from Fort Augustus, Highland, Scotland
The Pillar Bolt
The anatomy of a triangulation pillar. There is one of these upper centre marks inside every triangulation pillar. A plumb bob would be accurately lowered through the vertical centre hole from pillar top. If the point of the bolt and the point of the plumb bob did not meet when viewed through the bottom sighting holes on both horizontal dimensions, then the pillar would be deemed 'off plumb' and could not be used. This bolt is inside cylindrical trig pillar 11881, Great Glen Project Station M.
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