A selection of some of the trig pillars I've visited and a brief outline of their construction.
A brief explanation of trigpoints
Triangulation Pillars - measuring shape
Trigpoints 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", and it is essential that you are familiar with this system if you are to get the most of OS maps, or this website. The OS provide an introduction to the National Grid on their website.
Baseline measurement - measuring size
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!
Ordnance Survey excellent reading:
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An excellent book downloadable here:
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