Trigonometrical or triangulation points as they were also known as were used by the Ordnance Survey to map out the shape of the UK resulting in the excellent OS maps that we are all familiar with today.
There were over 6,500 trig points throughout the country on hills and mountains at one time but many have been lost over the decades since they became obsolete.
They are concrete pillars, typically 4 feet tall, with a substantial portion of the structure below ground.
The brass fitting often seen on the top of trig points is known as a ‘spider’.
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