Triangulation pillars, or trig points to give them their colloqial name are a network of concrete pillars around the UK constructed as part of the great Ordnance Survey of Great Britain carried out in 1936.
Trig points are placed on locally high points, and are within sight of other pillars. The pillar, although mostly concrete, has a metal identification plate on its side and a grooved metal circle on top for placing a tripod. By placing a theodolite on top and sighting other trig points, the whole country can be mapped.
The typical shape of a pillar is a truncated square-section pyramid - around 4 feet high, with a 1-foot square top. Many points have been adopted by local walking groups, who maintain them, painting them white regularly. Others have fallen into disrepair, and may be in any condition, from good through crumbling to completely vanished.
While the pillars are the most common shape, other trig point types include:
- Fundamental Benchmark - an underground chamber, with a short concrete pillar on top. This is used for accurate elevation measurement.
- Rivet or Bolt - as it seems, a metal rivet or bolt stuck into the ground.
- Flush Bracket - the metal plate as found on a pillar, but attached to a wall or building.
On ordnance survey maps, a trig point is marked as an equilateral triangle with a dot in its center.
Other nations use similar methods, such as:
If trainspotting doesn't involve enough excercise for you, but you have the right mental state, you might want to try trigpointing. There are many points in all areas, in various conditions, and various interesting locations - public areas, private land. Collecting the plate numbers is a hobby for some - added to the excitement of arranging access when yet another point is locked into a local water authority compound - yes, reservoir tanks are often on the tops of the same hills.
Thanks to toalight for information on Norway