As a former land surveyor I can tell you a lot about trig. points. But (thankfully you might say) I will try to summarise in a few sentences. They are the corners of a series of hundreds of triangles which cover the whole of the country, hence the name triangulation points. Surveyors measured the angles between points to a very high degree of accuracy (fractions of a second of arc) and also the length of a couple of sides at opposite ends of the country. Then by using good old-fashioned trigonometry they calculated the position of each pillar. The pillars were then used as the basis for mapping the surrounding areas.

To get good, long-range lines of sight trig. points are located on hill tops and in open country. On the top is three-legged mounting plate, known as a spider plate, that a surveyor can mount a theodilite on. On one side of the pillar is a benchmark plate for vertical control.

Of course the introduction of satellite positioning, electronic surveying instruments and aerial photography has largely rendered the old techniques obsolete, but the pillars remain as very useful to hikers, letterboxers etc.

DM

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has largely rendered the old techniques obsolete

I'm impressed DM, somebody who knows how they are used! I would have loved to see them actually being used. I understand that some of the defunct ones can be adopted now in order that they can be maintained instead of going to rack and ruin.

It is benchmarks that really puzzled me for a long time, because you see them on lots of things as you are walking, from boundary stones to moorland crosses to walls. You also see them in towns. For a long time I thought they must be an ancient symbol because they are on ancient artifacts, but now I assume that it is because the modern(ish) engravers didn't consider what they where carving on.

YT

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would have loved to see them actually being used.

As a student I spent many hours, often cold and damp, on hilltops taking rounds of angles. I don't know if if they still teach the old techniques, but if they do there is chance you might come across a group of students from Newcastle out in the wilds of Northumberland or north west Cumbria.

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I understand that some of the defunct ones can be adopted now in order that they can be maintained instead of going to rack and ruin.

Not suprisingly there is a website for trig point enthusiasts where you can check on which points are nearby and the condition they are in. http://www.trigpointinguk.com/trigs/

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benchmarks ... you see them on lots of things as you are walking, from boundary stones to moorland crosses to walls. ... I assume that it is because the modern(ish) engravers didn't consider what they where carving on.

I think they are carved on ancient(ish) buildings because they were less likely to be destroyed. BMs are often carved on struchtures such as churches, town halls and bridges because they are easy to identify and are fairly permanent. Did you know that as well as the familiar broad arrow with a cross bar there are other forms of BM? Quite a common form in the country is a round headed bolt set into the top surface of a low wall or even a rock.

DM