I have been meaning to place a cache here for a long while, but never got around to it. I always knew this concrete pillar was here, but I had no idea what it was until I did a bit of research after coming across some more when I started caching. I love maps, and anything to do with maps, so always get excited to come across a 'trig'. For those of you unaware of what it is, see below for more info. The triangle shown above in blue with a dot in the centre, is the symbol used to mark them on maps.
Trigpoints or 'trigs' are the common name for "triangulation pillars". These are concrete pillars, about 4' tall, which were used by the Ordnance Survey in order to determine the exact shape of the country. They are generally located on the highest bit of ground in the area, so that there is a direct line of sight from one to the next. By sitting a theodolite (an accurate protractor built into a telescope) on the top of the pillar, accurate angles between pairs of nearby trigpoints could be measured. This process is called "triangulation".
A major project to map out the shape of Great Britain began in 1936. The network of triangulation pillars, with accurately known positions, led to the excellent OS maps which we enjoy today. The coordinate system used on these maps is known as the "National Grid".
The triangulation pillars determined the exact shape of the country, but what about distances? Triangulation itself only shows you the shape of the land, not the scale. The scale of the mapping was determined in 1784 by laying a series of glass rods along Hounslow Heath. Using this single measurement, plus the network of triangulation pillars, the size and shape of the country was determined to within about 20m!
In order to determine heights, a different type of surveying was required. Around 200 "fundamental bench marks" (or FBMs) were located across the country. These consist of an underground chamber topped by a small pillar. Between these 200 FBMs, around 750,000 "lower order benchmarks" were scratched into walls etc. Precise leveling built up a picture of the profile of the land, giving rise to the spot heights and contour lines you see on today's maps.
Just as the triangulation pillars determined the shape of the land, but not its size, the FBMs determine the profile, but not the absolute height. In order to define a zero height, the sea level, measured by a tide gauge in Newlyn, Cornwall was averaged over a period of 6 years from 1915 to 1921. This gave "Mean Sea Level" (MSL) and all heights on OS maps are quoted as height above mean sea level.
Nowadays, most of the monuments described above have fallen into disuse. However a number of the old triangulation pillars and FBMs, along with various newly installed bolts and rivets, now form the "Passive Station" network. This is a set of around 1000 locations which have been accurately measured using GPS (Global Positioning System) technology. There are also about 30 "active stations" which transmit positional information, in much the same way as the GPS satellites, which can be received by survey-quality GPS receivers.