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.
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 levelling 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. Accurate locations of both active and passive stations are published on the OS's National GPS Network website.
This website uses the word "trigpoint" quite loosely, and includes FBMs, together with all the new passive (and active) stations as well as the historical triangulation pillars with which people will be most familiar. For a list of all the different types of "trigpoint" included in our database, please see this document.
Our database currently includes a total of 25604 trigpoints. Initially, various existing lists of trigpoint locations were combined and, since then, our users have been filling in the gaps by adding trigpoints they find which are not in our database. The Ordnance Survey maintain an online database of all active and passive stations and have stated that they intend to make further details of the historical networks available in future. We will, of course, merge any newly released data into the database of this website.