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Triangulation – trig pillar art

There were 6,500 across Great Britain. Around 5,500 are still standing. Cold Ashby marks the spot as the first one used for the retriangulation of Great Britain starting on 18 April 1936. What are we talking about? The humble trig pillar of course.

The iconic pillar, much-recognised by hikers as they reach the heights of their walk, was originally created as a state-of-the-art network to re-map the country. Triangulation is 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.

The modern equivalent of the trig pillar and retriangulation is the OS Net network of 110 Global Navigation Satellite System (GNSS) receivers, leaving trig pillars largely redundant, although still much-loved.

That love of trig pillars is being celebrated through a project called ‘Triangulation’ by photographers Stephen McCoy and Stephanie Wynne. They’re visiting the 314 primary triangulation points built by us at Ordnance Survey between 1936 and 1962 and creating a visual survey of Great Britain.

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Welsh and English trig pillars completion

Today is the 83rd anniversary of the first use of an Ordnance Survey trig pillar, so the perfect time to catch up with Britain’s top trig-bagger, Rob Woodall, on his latest achievement. 

I bagged my final Welsh trig pillar in 2008 – sort of. At that time, I counted 660 trig pillars still surviving in Wales, and Red Hill, S6561, east of Builth Wells was my last, on a blustery August day. We celebrated with a Balvenie single malt (somehow not a Penderyn).

Red Hill (Photo: Rob Woodall)

But had I really finished? The OS originally built 684 pillars in Wales – what about the others? At that time, I was focused on extant pillars, trying to get around as many trigs as I could before they were lost to housing developments, road construction, farming operations and the like.  I’d visited all the remaining English, Isle of Man and Scottish pillars by 2016, so it was time to think about visiting the remaining vacant trig sites. Some were simply in-situ replacements, the pillars being rebuilt on the same site, with the same flush bracket or occasionally a new one.  Sites that used to have a trig pillar, aren’t inherently as interesting to the bagger as those where there’s something to look for, but the scenery is still there (if it hasn’t been built on), and in some cases, the pillars weren’t quite as dead as we thought: Read More


An OS history of the theodolite

If you’re an avid reader of our blog or have some surveying knowledge, you may well have heard of a theodolite before. A key instrument in the mapping of Britain (and pretty much everywhere else!), a theodolite is a simple concept built into a precision instrument – a telescope with graduated horizontal and vertical circles attached to enable angles to be measured. Intrigued by this implement, we decided to find out more… Read More


Remembering Brigadier Martin Hotine

While 2018 marks the 227th anniversary of Ordnance Survey, lest we forget it is also the 82nd anniversary of the first observations from our ‘hotine’ trig pillar!

If you want to find out who created our well-known trig pillars and the memorial we have for him at our head office, you’re in the right place.

Brigadier Martin Hotine CMG CBE RE was born on 17 June 1898, in Wandsworth, London. He read mathematics at Magdalene College and was then commissioned into the Royal Engineers (RE). Hotine was in the RE for many years and served in both the First and Second World Wars.

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Map history at OS with Antiques Road Trip

If you were watching Antiques Road Trip yesterday afternoon, you’d have seen antiques expert Paul Laidlaw visiting our Southampton head office. The modern building we’ve been in since 2009, is a far cry from our first home at the Tower of London, and even the military barracks which became our first Southampton head office. But, despite being a digital data company in a state of the art building, there are still many nods to our mappy heritage to be found.

Our CEO Nigel Clifford showed Paul our first map, and an early theodolite while filming for the programme. Here’s a bit more about them:

Paul Laidlaw (left) and Nigel Clifford (right) in from of an 1801 map of Kent and the Ramsden 18″ theodolite

Ramsden 18” theodolite Read More


Benchmark or trig pillar: what’s in a name?

We’ve had a few questions recently about benchmarks and trig pillars and what they are and how they differ, so we thought we’d clear it up.

The benchmark

Most weeks we’ll see a Twitter conversation where someone is asking what this mark is:

Many think it is War Office-related, but it is in fact an OS benchmark (BM) and a means of marking a height above sea level. Surveyors in our history made these marks to record height above Ordnance Datum Newlyn (ODN – mean sea level determined at Newlyn in Cornwall). If the exact height of one BM was known, the exact height of the next could be found by measuring the difference in heights, through a process of spirit levelling. They can be found cut into houses, churches, bridges and many other structures. There are hundreds of thousands of them dotted across Great Britain, although we no longer use them today. Read More


How to map a new country?

Update: DVD released on 24 July 2017. We have one copy of the DVD to give away. Just retweet this message by 12 noon on Friday 28 July:

March 24 sees the UK film release of Lost City of Z. It chronicles the South American adventures of British explorer, cartographer and archaeologist Lt Colonel Percy Fawcett. I joined a panel discussion in London last week, along with historian Dan Snow and Lost City of Z author David Grann, discussing how Percy would have explored and mapped a new land. Catch up on the podcast here.

Dan, David and Mark

A member of the Royal Geographical Society (RGS), Percy Fawcett first arrived in South American in 1906 to survey and map an area of jungle lying on the Brazil and Bolivian border. The border between the two countries was not fully mapped and it was agreed that an RGS survey and map would be accepted as an impartial representation of the border. Today we would complete this activity using satellite systems and sophisticated surveying technology, which obviously wasn’t available back then. So, how would Percy and his team have gone about making maps? Read More


Our top ten mappy stories in 2016

As we get closer to the end of 2016, we decided to take a look back at the blogs you liked the most over the year. We were pleased to see that the popular topics covered off our 225th anniversary and the 80th anniversary of the trig pillar as well as many other topics in between. Read your top ten:

  1. OS Maps go off the planet

mars-map-extractThe planet Mars became the latest subject in our long line of iconic OS paper maps. The one-off OS Mars map, created using NASA open data and made to a 1:4,000,000 scale, was made to see if our style of mapping has potential for future Mars missions. You can also order a copy for your wall in the OS shop.

  1. Britain’s most popular grid squares

You may have heard us saying that there are over 500,000 routes in our OS Maps service…well, we  analysed all of that data to look at which areas you most like to #GetOutside and explore. We compiled a list of the 20 most popular grid squares in Britain, using 10 years of public routing data created in OS Maps and its predecessors.

All of the public OS Maps routes recorded in the Lake District National Park

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OSTN15 – the new geoid for Britain

Seeing as an improvement in our model that transforms height from GPS to one above mean sea level has caused a hill to “grow” into a mountain – we thought it would be a good idea to explain how positions and heights surveyed by our surveyors with GPS make it onto our maps.

OS surveyor

OS surveyor

All positioning and surveying, not just that from GPS, has to take place on a mathematically simplified model of the surface of the Earth. The surface that the model attempts to emulate is called the geoid.  The geoid is a complex concept, but can be imagined as a hypothetical surface that would be formed if the water in the oceans, close to mean sea level, continued under the land and was only influenced by Earth’s gravity field. This surface is one we already refer to without perhaps thinking about it – we say oceans have “depth” (below the arbitrary zero height surface) and mountains have “height” (above the surface). The geoid is a complex shape since it is influenced by varying Earth gravity. It is too complex to act as our surface for the calculations involved in positioning and surveying, so we need to fit a simple model shape to it. Read More