Happy birthday to the Trig Pillar – 75 years young today

On a clear, crisp spring morning in 1936, a group of men gathered around a strange, pale obelisk in the middle of an unremarkable field in Cold Ashby, Northamptonshire. Those men were there to begin the greatest undertaking Ordnance Survey had attempted since the early 19th century.

That shining white monolith would now be instantly recognised by any walker, hiker or geography pupil. It was of course a Trig Pillar, and today, 18 April, marks 75 years since the day when they were first used in anger at the beginning of the Retriangulation of Great Britain.

Where it all began - Cold Ashby

Where it all began - Cold Ashby

Trig Pillars now evoke the kind of sentimentalism of something quintessentially British (although there are equivalents in other countries), but at the time they were part of a state-of-the art network built to literally re-write the map of Britain. The Retriangulation was an enormous task and lasted up until 1962 (with a break for World War II), the impact of which we still live with today.

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.

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.

Today, ask someone the first thing that comes to mind when they think of Ordnance Survey and more often than not they’ll still say a Trig Pillar. But time and technologies have moved on enormously to the point where the traditional Trig Pillar is now obsolete in its original guise. They still act as a beacon for weary ramblers but no longer do they help shape our maps.

Silent vigil.

Lonely vigil.

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.

Top Trig Trivia

1. The survey control network of Trig Pillars was accurate to 20 metres over the entire length of Great Britain. Today the receivers that make up the OS Net network are coordinated to an accuracy of just 3mm over the same area.

2. 6,500 Trig Pillars were built for the Retriangulation of which around 5,500 are still standing. In total the Retriangulation had in excess of 30,000 coordinated points. The modern OS Net network performs the same function with just 110 points.

3. Measuring angles by eye from a Trig Pillar meant the Retriangulation was reliant on good weather – perhaps part of the reason it took until 1962 to complete! Modern GNSS surveying works in all weathers and is available 24 hours a day.

4. Trig Pillars are mostly made of cast concrete but a few are built from local stone cemented together.

5. Like an iceberg, there is more of Trig Pillar below the surface than above it.

[Cold Ashby photo by Bridgeman via Trigpointing UK, a great site for all things trig]

[Trig with a view photo by James B Brown via Flickr]

[Lonley vigil photo by Jared Earle via Flickr]

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