Lamberts Castle Trig Point 256m

Thick mist at Lamberts AGAIN this morning but as I walked past this Trig point I wondered if the people who passed it every day still knew what they were? They have fascinated me ever since I started to learn how to read a map and it was a great challenge to navigate to those in far remote places.

In the United Kingdom, trig points are typically concrete pillars, and were erected by the Ordnance Survey. The process of placing trig points on top of prominent hills and mountains began in 1935 to assist in the accurate retriangulation of Great Britain. In low-lying or flat areas some trig points are only a few metres above sea level and one is even at -1m (in Norfolk, TL6189). When all the trig points were in place, it was possible, in clear weather, to see at least two other trig points from any one trig point but subsequent vegetation growth means that this is not necessarily still the case. Careful measurements of the angles between the lines-of-sight of the other trig points then allowed the construction of a system of triangles which could then be referenced back to a single baseline to construct a highly accurate measurement system that covered the entire country.

In most of the United Kingdom, trig points are truncated square concrete (occasionally stone) pyramids or obelisks tapering towards the top. On the top a brass plate with three arms and a central depression is fixed and this is used to mount and centre a theodolite used to take angular measurements to neighbouring trig points. A benchmark is usually set on the side, marked with the letters "O S B M" (Ordnance Survey Bench Mark) and the reference number of the trig point. Within and below the visible trig point, there are concealed reference marks whose National Grid References are precisely known. The standard trig point design is credited to Brigadier Martin Hotine CMG CBE (1898-1968), the then head of the Trigonometrical and Levelling Division of the Ordnance Survey.[2] Many of them are now disappearing from the countryside as their function has largely been superseded by aerial photography and digital mapping using lasers and GPS measurements. To quote from a page at the OS site: "Like an iceberg, there is more of trig pillar below the surface than above it."[3] From the same source: "Today the receivers that make up the OS Net network are coordinated to an accuracy of just 3 mm over {the entire length of Great Britain}".