Life after the trig pillar

Talking about the 79th anniversary of the trig pillar this month has sparked a flood of reactions. Lots of our lovely followers on social media sent us pictures of them (or their dogs) with our trig pillars around Britain. Others expressed delight that they now know what those odd concrete pillars were for. Some wanted to adopt a trig pillar if we no longer used them (sorry, not something we offer). Still more people were amazed that we no longer use the vast majority of trig pillars and asked us what we use instead. The answer to that is OS Net.

Some of the fab trig pillar photos sent in to us recently

Some of the fab trig pillar photos sent in to us recently

Whereas we once needed 6,500 trig pillars and it took more than 25 years to measure out the whole country for the retriangulation of Great Britain, today it’s managed with GNSS (Global Navigation Satellite System) stations and data. The well known GPS (Global Positioning System) is an example of a GNSS and can be used to pinpoint the location of a user’s receiver anywhere in the world. At OS, we use OS Net, our own network of 110 very accurately positioned GNSS receivers spread across Great Britain. What took many hours to measure at that first trig pillar in Cold Ashby in 1936 we can now do in seconds and to a far greater degree of accuracy.

The data streams from OS Net enable the correction of GNSS errors to be computed in real time and, when transmitted to our surveyors, allow them to coordinate new map features to an accuracy of just a few centimetres using a technique called RTK (Real Time Kinematic) GNSS. The system is also used to accurately position our two aircraft as they fly the country capturing aerial imagery.

Some of our OS Net stations, as shown on our website

Some of our OS Net stations, as shown on our website

Advances in technology mean that the kit our surveyors carry has changed dramatically over the last 79 years too. Whereas they would once have carried measuring chains, heavy theodolites and more, working in teams for weeks at a time at a location – now they’re largely operating solo and carrying just a few pieces of kit. They carry a tablet type computer that is coupled via Bluetooth to a survey-grade GNSS unit mounted on a pole, which they use to pinpoint the position of new features or check the position of existing data on the maps.

Such technology isn’t completely fail proof however. Some areas are so built up that our surveyors are hit by the “urban canyon” syndrome, where their GNSS kit cannot receive sufficient signals from the satellites. So our surveyors also carry back-up kit to make sure they can get the job done regardless. They have a hand-held laser measuring instrument, or failing that a good old-fashioned 20-metre tape measure!

How our surveying techniques and kit have changed over the years

How our surveying techniques and kit have changed over the years

Our surveyors then plot the new points they are measuring onto the maps on their tablet computers and begin the task of joining them up to reflect the shape of what is actually on the ground in front of them. The points joined up, they then attribute the lines and shapes within the lines to correspond to what they are, for example public road edge, public road, obstructing feature, private garden, building outline and built structure to name a few. This then allows our customers to interrogate our data for their own means.

Once the field surveyor has created the framework, joined the dots and attributed the lines to reflect what is on the ground then the ‘job’ is sent back to the servers at our Southampton head office where it is incorporated into ever-growing 450 million features forming the master map of Great Britain by the expert teams there, and a little time later becomes available to the customer.

It’s all a far cry from the surveying techniques that saw us carrying out the retriangulation of Britain back in 1936, although many of the trig pillars still remain around the country. While we no longer use them and don’t visit them regularly, they are still under our responsibility, so if you spot one in need of repair, do let us know, so that we can take a look. You can contact us on customerservices@os.uk.

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5 Responses

  1. David Findlay

    Great post Gemma. I’m one of those sad individuals who actually takes photos of trig pillars. I was able to add to this collection by snapping a New Zealand trig point recently.

  2. Michael Hastings-Hunt

    I was walking in Snowdonia at the weekend and saw that the triangulation pillar on Garnedd Ugain has suffered severe damage, with the brass plate on the floor. It was hard to tell if it has been vandalised or just suffered extreme weathering.
    Are these still owned by OS and are they ever rebuilt?

    1. Hi Michael

      Yes, they are still owned by OS, although we haven’t used them for surveying for many years now. Our team only tend to step in now if a trig has become unsafe and looks like it may topple. I’ll pass the condition along to the team regardless, but do let us know if it was unsafe.

      Many thanks

  3. Clifford

    Hello Gemma

    Although adoption is not now something OS offer it was offered to individuals with a special interest in maintaining them back in the early 1990’s

    I still check on the one I adopted and although I ran out of the postcards printed by OS to report on the visits I do send in a short report every two years based on the questions that were on the original postcard.

    Kind Regards

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