The 18th April 2011 marked the 75th anniversary of the installation of the first Ordnance Survey Trig Pillar at Cold Ashby, Northamptonshire. Often made from cast concrete or sometimes stone, these monoliths are dotted throughout the UK having been used to triangulate the distance between significant and visible points on the landscape. These trig pillars enable surveying to be performed using accurate and repeatable methods which has lead to the Ordnance Survey being able to capture some of the most accurate mapping in the world.
The Ordnance Survey Blog article states = "Triangulation is basically a mathematical process that makes accurate map making possible. It works by determining the location of a point by measuring angles to it from known points at either end of a fixed baseline and in this case, those known points were the 6,500 Trig Pillars erected across the country. In practice, a theodolite would have been secured to the top mounting plate and made level. It would then be directly over the brass bolt underneath the pillar. Angles were then measured from the pillar to other surrounding points. For the highest accuracy primary points in the Retriangulation, many rounds of angles would have been measured with the observations taking several hours. But why was the Retriangulation needed?
Trig with a view.
In the early 20th century, map making was still based on the Principal Triangulation which was a piecemeal collection of observations taken between 1783 and 1853. It was starting to collapse and the lower orders were unsuitable to accurately map the rapid development going on after the Great War.
In 1935 it was decided to implement a complete new control network for the whole country and at the same time unify the mapping from local county projections onto a single national datum, projection and reference system. And thus the OSGB36 datum and The National Grid were born, both of which are still with us today."
The Exe Valley in it's geographic nature is not going to have trig points very close to the river but this area of the Devon does experience some significant terrain relief which means that we don't have to go too far to find one. If you've got an Ordnance Survey Explorer (Leisure) 1:25,000 scale map, look out for the symbol of a triangle with a dot in the middle. Most will be at the top of hills.
Here's a good site which shows a Google Map (Bing Maps of trig pillar detail) of all the trig pillars in the Ordnance Survey SS map sheet http://www.haroldstreet.org.uk/trigpoints/trig-bagging.php?area=SS the same site provides views of the trig points in SX which also covers the Exe Valley area
http://www.haroldstreet.org.uk/trigpoints/trig-bagging.php?area=SX.
This website shows the last trig point the Exe passes to be at Addelpool just east of Ebford SX989881 at 53m above sea level (the trig point that is, not the Exe) and the closest to the river's rise at Exe Head (450m) on Exmoor being Chains Barrow SS734419 at 448m above sea level, which is 2m lower that the Exe's rise.
Exe Head incidentally is on both the Tarka Trail and the Two Moors Way.
These humble trig points have even spawned their own hobby; Trig Bagging. For which walkers will endeavour to 'Bag' as many trig points as they can. If this is the 'bag' you are into why not check out
http://www.trigpointinguk.com/
The Ordnance Survey blog finishes it's story and nod to the humble trig pillar with "Trig Pillars and The Retriangulation might now be largely redundant, but we still have a responsibility to maintain and provide access to a national mapping and survey control network.
So, what do we do now to realise and maintain a national control network? The modern equivalent of the Trig Pillar and Retriangulation is the OS Net network of 110
Global Navigation Satellite System (GNSS) receivers.
Our surveyors use OS Net and GNSS technology everyday to instantly position new map detail to within a few centimetres. The system is also used to position our two aircraft as they fly the country capturing aerial imagery. What took many hours at Cold Ashby in 1936 we can now do in seconds and to a far greater degree of accuracy.
But let’s not forget the lonely, steadfast Trig Pillar, who still stands guard on our countryside and the role it played in reshaping how we view the country.
Happy birthday Trig Pillar, Ordnance Survey salutes you."
So next time you are out and about and see a pillar on the top of a hill, check it out, have a look at the brass plate in the top and try to imagine how important that little pillar once was.
Profile: Adrian Gray runs his company Viewshed Ltd from his home in Silverton, Devon. He's a trainer, software and web developer specializing in Ordnance Survey maps, Pitney Bowes MapInfo and Open Source development for the web, desktop and mobile platforms. He can be contacted at www.viewshed.co.uk
