This week marks 80 years since the trig pillar was first used in the retriangulation of Great Britain on 18 April 1936. We’ve been celebrating by sharing the story of the humble trig pillar, and giving you the chance to join our celebrations with The Trig Pillar Trail Challenge. But if the trig pillar is now obsolete – just how do we survey Great Britain today?
Life after the trig pillar
Trig pillars might now be redundant, but at OS, we still have a responsibility to maintain and provide access to a national mapping and survey control network.
The modern equivalent to the network of trig points 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. What took many hours at a 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.
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!
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.
Hear our surveyors talking about a recent re-survey of Ben Nevis and see their equipment in action.
As well as our surveyors on the ground, we have our Flying Unit capturing aerial imagery of Britain. Find out more about the team here.
Get involved in the #TrigPillar80 celebration