Behind the scenes at Ordnance Survey

Daniel Neilson gets a sneak peek behind the scenes at Ordnance Survey, and examines how the appeal of maps goes beyond the practical

Illustration photo of Behind the scenes at Ordnance Survey

Before you start reading this article, find a map and pick a grid square. Perhaps the summit of the last mountain you climbed, or your home, or something at random from the nearest map you have. Ordnance Survey 1:25,000, 1:50,000, a Harvey map, it doesn’t matter. I’ve chosen the summit of Braeriach, located within grid square NN9599.

Just focusing on that grid square alone, we can see a story emerging. We can tell if there are any buildings on it, whether it’s relatively flat, in which direction the slope is formed. Are there any water features? A lake, a stream, a river? Is there a road? Is any of it forested? Are there crags or scree? Perhaps a manmade feature: a camp site, a trig point, a pub!

NN953999: summit of Braeriach
I was on the summit of Braeriach a couple of months ago at midnight practising night navigation. In the horizontal sleet, low cloud and 40mph winds, I could see virtually nothing except my map and compass lit by my headtorch. To be honest, that one grid square didn’t fill me with confidence. It was my turn to lead the group, and I was to find the summit from a few hundred metres to the west. I could see the summit spot height at 1296m, the words Coire Bhrochain, and what is a massive drop down the West Buttress into said corrie. In that square kilometre of map you can see steep crags and scree that level off just to the east of the centre of the grid square (where my tumbling body would come to a rest, I briefly thought). There’s also a footpath just at the top of the square, and a pesky boundary line.
Pull out a little and more of a picture emerges.


Blind to all but a map and a compass on that dark night, I could understand my environment a lot better by interpreting the surrounding grid squares. I was walking from the west so I knew I’d be walking uphill, I knew how many metres I’d be pacing, and I knew, largely, what to expect. Even at a scale of 1:50,000 it told me all I needed to know to reach the summit. It mapped my route, but it also aroused my curiosity. Back in my little tent (situated just above the ‘C’ in Coire Ruadh) I pondered the words, contours and signs on my map. It seemed to ask more questions than it answered. What did Braeriach mean? Why are the words Braigh Riabhach underneath it? Are OS using an anglicised name, and if so, why? Are the Wells of Dee the source of the Dee? I could tell by looking at the map that the landscape here was dramatic, but there was also so much more to discover. Before me I had a treasure map, full of intrigue and excitement. From one look at a map we can see deep into geological, natural, etymological, human and political history.

Coire Bhrochain and Coire Ruadh where I camped were gouged out in the same glacial process that formed that classic U-shaped valley – the mighty Lairig Ghru. The scree I could see marked on the map was the product of intense freeze and thaw and water weathering over millions of years. Nan Shepherd, writing in the early 1930s, describes Loch Coire an Lochain as ‘remarkable’: “It cannot be seen until one stands almost on its lip, but only height hides it… To be so open and yet so secret! Its anonymity – Loch of the Corrie of the Loch, that is all – seems to guard this surprising secrecy.” Yet, I can see it clear as day on my map. And we can discern much else besides. We know from the heights indicated against the contour lines what plants we’ll see. We know we’ll see ptarmigan rather than red grouse. Braeriach, Britain’s third highest mountain, is immovable, vast, endlessly intriguing and thrilling. And we know this by merely letting our fingers flow over a map.


Jim Perrin, in his wonderful book Snowdon: The Story of a Welsh Mountain, begins his biography of the mountain by looking at another OS map. He writes: “In the two dimensions of the 1:25,000 Ordnance Survey map it looks animated, starfish-like, pinky-beige, its radiating ridges decorative, crag-stippled and lake-enclosing, the suggestion of motion continually hovering about them as though they were limbs waving gently in the depths of the of a clear pool… But even in linear representation the symmetries of Snowdon, radiating out from the focal point of Yr Wyddfa, draw you in.”

We can learn so much from that linear representation. There’s an icon of a train on it for goodness’ sake, a disheartening sight among an otherwise thrilling-looking square kilometre. We see the march of ‘progress’ in these maps. The history of Britain can be seen in their lines. To investigate more, I travelled to GR: SU373155, the home of Britain’s national mapping authority.

SU373155: Ordnance Survey offices
I feel the same pang of excitement pulling up into the car park of Ordnance Survey’s offices by an industrial estate just outside Southampton as I do when I open a map to plan my next mountain walk. Out of this modern building opened by the Duke of Edinburgh in October 2011, OS produce wonderful maps that are known around the world for their accuracy, legibility and integrity. OS HQ is an awe-inspiring piece of architecture. The main door opens on to the vast central atrium. Rising up towards the naturally ventilated ceiling are XXX floors, each relating to different part of Ordnance Survey’s business. There are XXX employees, many of them specialists in areas I hadn’t even considered before. OS work with local authorities, emergency services, planning authorities, energy companies, health service providers and insurance companies. In Birmingham, the organisation has been collaboratiing with the NHS and city council to combat obesity. They learned that 71 percent of all schools had a fast food outlet within 400 metres and are now controlling their numbers by using mapping. Hampshire Search and Rescue converted a Land Rover into a mobile office that receives real-time OS information on paper and to the team’s GPS devices. Surrey Heath Borough Council managed to increase its recycling rate from 30 percent to 65 percent in a year thanks to an analysis of OS data.

The message is clear – and I’m about to use the buzzword of 2014 – ‘Big Data’ is all important. And boy, do Ordnance Survey have Big Data. The MasterMap, the map from which all others are derived, contains more than 460 million man-made and natural landscape features. 230 surveyors on the street collect up to 10,000 changes every day and the organisation’s air surveying planes take around 50,000 images a year. Ordnance Survey is a vast business, of which map-making comprises only around seven percent.

TQ123743: Hounslow Heath
From what we can see around grid reference TQ123743, there is a nature reserve, a cemetery and crematorium, and a recreational route – in this case the London Loop. There’s a weir, a golf course, a school and church… with spire (I had to check, I never remember that one). There’s nothing to suggest that this was the first area in Britain to be surveyed. But in 1784, General William Roy, working for the Board of Ordnance and based in the Tower of London, was commissioned to start a baseline from where the south-east of England could be mapped. He carefully lugged Jesse Ramsden’s three-foot-high theodolite on to the boggy heath (it was later found to have sunk a little, throwing off some calculations) and triangulated the distance between the Heath and Hampton Poor House five miles away – from this line he could continue south and east to Kent.

The military had decided they needed a map of Great Britain after losing their way a little in the Highlands during the Jacobite Rebellion of 1745. However, by the time the Board of Ordnance were ready to start (the theodolite took three years to make alone), the main threat to Britain was from the south in the shape of Napoleon’s army. So the vulnerable coast of Kent and Sussex was mapped first. It was finished in 1801, 11 years after General Roy died, and the mapping continued at an incredible rate. Within 20 years, a third of England and Wales had been mapped.

Progress was interrupted for the need of a six-inch-to-a-mile survey in Ireland. Not for war this time, but taxes. More than 2,000 men were employed in the exercise, which required carrying huge theodolites and heavy chains around the country. When they finished a decade later, they had the most detailed maps of any country in the world. A six-inch map for England and Wales was then commissioned with the 1841 Ordnance Survey Act, allowing its surveyors on any land in the country (‘on His Majesty’s business’). They used a system of triangulation that would still be used until the advent of GPS.

TQ 644101: Herstmonceaux Castle
I collect maps, just anything I see from a bootsale. The oldest I have is a ‘one-inch’ map of south-east London and Dartford from 1937. It cost 1/9 then and 50p last year, and it’s a thing of beauty. The hand-rendered map and careful lettering is charming and almost as clear as what we use today. For me it’s a remarkable piece of art as well as a remarkable technological endeavour, yet it was only two years before this map was published that Ordnance Survey began their most ambitious project: the retriangulation of Great Britain. It required the building of more than 6,500 concrete pillars – triangulation points – from which on a clear day you needed to see at least two others. The theodolite was then secured on top, directly above a brass bolt.

Trig pillars (the first is at Herstmonceaux Castle, a couple of miles from my house) are now obsolete, although around 5,500 of them remain. Many hillwalkers have a peculiar relationship with these honking great lumps of concrete on the top of some of Britain’s most beautiful places. Trigs have a strangely magnetic attraction, even if the hole in the top is so often found stuffed with an empty packet of Quavers. They mark a summit, a destination, a tick. Love them or loathe them, we don’t need them anymore. The advent of GPS in the mid-1980s has rendered them useless, but the basic system of using triangulation has not.

SH664593: Tryfan
In the atrium of Ordnance Survey in Southampton, Mark Greaves introduces himself as a Geodetic Analyst. Responding to my blank look, Mark explains: “Geodesy the science of measuring and mapping of the Earth’s surface and its gravity field”. It is, very simply, the science making our hilly country look flat on a piece of paper or computer screen. “Today we run a permanent GPS network called OS Net,” he continues. “The original trig network had thousands of points, took decades to build and several years to compute. It had an accuracy of 20 metres. The first GPS network in the mid-1990s had 30 points across the country, took two years to build and is computed constantly. The accuracy is 0.01 metres.” The system continues to develop. Since 2004, OS Net (Real-time Kinematic System for those who really want to know) has 110 stations roughly 70km apart. Mark is the guy who is called when, as happened a couple of years ago, amateur surveyors, such as G&J Surveys, reassessed Tryfan to 917.51m (3,010ft) – 2.43 metres higher that Ordnance Survey’s assessment. It was Mark’s job to verify the result.

“Tryfan is like a boulder field at the top, and we’d have measured it to the shape of the land. It’s hard to find the base.” What Mark is basically saying is that G&J Surveys measured the height from the top of Adam (or Eve), while the OS system would have naturally measured the base. “I was happy to verify it,” Mark adds. “Whether it’s on a list or not is important. We understand that.”

There’s no trig pillar on Tryfan, and it’s unlikely that, until Mark climbed it, anyone from Ordnance Survey had stood on it, in an official capacity at least. The 230 surveyors spend most of their time in urban areas measuring new roads, new buildings and developments such as Crossrail or the Olympic Park. The rural areas of Britain, some 90 percent of the landmass, are surveyed by two planes flying out of East Midlands Airport. They cover the entire country every three to five years. From 8,000 feet, the planes follow a strict grid, taking thousands of high resolution photos. The photos then go to the photogrammetry department. Here, lots of people in dark glasses look at special screens to get topographic information from aerial photos and, increasingly, satellite imagery. The latest software can convert that tree in your back yard to three dimensions from a digital photo taken 8,000 feet in the air. It’s also in photogrammetry where the next great step in cartography is being developed: 3-D mapping.

SE 056 515
My final call on my visit to the Ordnance Survey offices is to the cartography department. This is where the Landranger and Explorer maps are drawn. I’m introduced to Rob Dodd, who is working on a 1:50,000 map of a location near Leeds. In a coincidence that seems designed for The Great Outdoors, he is currently dropping little wind turbine icons onto the map. It is through Rob and the others in his team that all the vast amounts of information – from the surveyors, the aeroplanes, local authorities and agencies such as Sustrans – are interpreted and drawn on the map. While he is adding icons for the new wind turbines, he also points at places where they are being removed. Compare the two images above. There are now only two turbines on Chelker Reservoir, the other two removed for scrap. It takes time for the information to filter through of course, but on average, the digital mapping of any area is updated every quarter. It’s a painstaking task.

“Twenty years ago, we were still drawing on the maps, but now we update the tiles by computer,” Rob explains. “Although we’re using a very different system, it takes the same dedication to accuracy and eye for detail. The art of what we’re doing remains the same, and that is to simplify. Our job is to turn the mess into clarity.” The mantra in cartography is ESSO: Exaggerate, Select, Simplify, Omit.

TQ 577022
I tend go for lunchtime runs on the South Downs near my house at a place called Butts Brow. I know it very well. I have a map of the area of course, but I’ve never looked at the grid square I’m most familiar with. The tumuli burial mounds I know and recognise. But what is a ‘Causewayed Enclosure’ and why is it in an Old English font? I had to look on the key to find out what the little stars were (visible earthwork). At the 1:25,000 scale it gets even more intriguing. It turns out the ‘Causewayed Enclosure’ is a ‘Neolithic Camp’. There are names I’ve never heard before, despite walking up here on and off for 20 years. It turns out I live below Babylon Down, which is quite cool. In this one square kilometre 6,000 years of human history are marked, and aeons of geological history. But looking at it, I also remember family walks and bike rides with mates, nights in the chalk circle – Pit (dis) – listening to REM’s Automatic for the People and drinking beer when I was 17. I remember the rope swing in the woods from when I was a kid, and picking marjoram with my daughter last summer.

And that gets to the heart of mapping. It’s about planning and navigation, but it’s also about memories. As people who look at maps almost daily, we read them like we flick through a photo album: to remember.