0:00:02 > 0:00:04This is John o'Groats, in northern Scotland.
0:00:04 > 0:00:08It's the very end, or the very beginning, of mainland Britain.
0:00:08 > 0:00:12Between here and the other end of Britain - Land's End, in Cornwall -
0:00:12 > 0:00:15are 600 miles of mountains, forests, city, and farmland.
0:00:15 > 0:00:18And the amazing thing is that every little bit of
0:00:18 > 0:00:23those British landscapes is marked on one of these, an Ordnance Survey map.
0:00:27 > 0:00:28Ordnance Survey maps
0:00:28 > 0:00:31are incredibly accurate, incredibly detailed,
0:00:31 > 0:00:33and incredibly successful.
0:00:33 > 0:00:37This Landranger series sells two million copies a year.
0:00:37 > 0:00:41Ordnance Survey maps are used by everyone, from hillwalkers
0:00:41 > 0:00:45to fast-jet pilots. In fact, by anyone who needs to know
0:00:45 > 0:00:48exactly where they are on British soil.
0:00:48 > 0:00:51I'm going to take a journey across the toughest terrain in Britain,
0:00:51 > 0:00:55the Highlands of northern Scotland, to find out exactly why these maps
0:00:55 > 0:00:58are still regarded as the best in the world.
0:01:24 > 0:01:29I'm starting my journey at Britain's busiest bomber base.
0:01:29 > 0:01:32The Ordnance Survey began in 1791 as a military necessity.
0:01:32 > 0:01:36You'd think that the modern RAF
0:01:36 > 0:01:40would use ultramodern electronic maps for their navigation.
0:01:40 > 0:01:42So they do, but just like us,
0:01:42 > 0:01:46they use Ordnance Survey maps when they fly over Britain.
0:01:59 > 0:02:01This room is where the pilots plan their missions.
0:02:01 > 0:02:04To do so, they use a custom-made,
0:02:04 > 0:02:08computerised navigation system and an OS map.
0:02:08 > 0:02:11Stu Gillies is planning a navigation exercise,
0:02:11 > 0:02:14using a series of prominent hilltops as his targets.
0:02:14 > 0:02:16Stu will be flying at 500mph
0:02:16 > 0:02:19over some key landmarks on my journey,
0:02:19 > 0:02:22putting the Ordnance Survey maps to the test.
0:02:22 > 0:02:26If you're going to fly an aircraft that low that fast,
0:02:26 > 0:02:29you've got to be able to rely on very accurate maps.
0:02:29 > 0:02:31You use an old-fashioned paper map?
0:02:31 > 0:02:35The great thing about the 50,000 is that you get the big picture,
0:02:35 > 0:02:39but you've got also very clear detail.
0:02:39 > 0:02:42It's the same map as you've got there.
0:02:42 > 0:02:47What we intend to do is come from Lossiemouth, south, then west,
0:02:47 > 0:02:52- then head for Longa Island.- There's Longa Island.- Where you're going.
0:02:52 > 0:02:56- Sithean Mor there.- Yeah.- Are you going to fly over them like that?
0:02:56 > 0:03:00Because of the turning circle of the aircraft, I'll move that point out.
0:03:00 > 0:03:05With aeroplanes, you have to keep them pointing at the right county,
0:03:05 > 0:03:07or you end up in the wrong country!
0:03:07 > 0:03:10And then up towards Caithness here.
0:03:10 > 0:03:11Probably, to line it up,
0:03:11 > 0:03:16- we'll come out to the north, then back down again.- So a triangle.
0:03:16 > 0:03:19- Yeah.- Do you like looking at maps?
0:03:19 > 0:03:23I have to admit, it's one of the pleasures of the job
0:03:23 > 0:03:28because really, the map is, I think, a fusion of science and art.
0:03:28 > 0:03:34You have the very accurate distances and heights and elevations marked.
0:03:34 > 0:03:36For me, the great pleasure of flying is that
0:03:36 > 0:03:38when you fly around this stuff,
0:03:38 > 0:03:42that visualisation that you have often comes real.
0:03:42 > 0:03:46Stu's list of targets will take him from Lossiemouth
0:03:46 > 0:03:48to the Highlands of Wester Ross,
0:03:48 > 0:03:53then up to Caithness and back. I'll make the same journey on the ground.
0:03:53 > 0:03:57As in the song, he'll take the high road and I'll take the low road,
0:03:57 > 0:03:59but we'll both use the same map!
0:03:59 > 0:04:03The first stage in my journey takes me up to Gairloch, in Wester Ross,
0:04:03 > 0:04:05in the Highlands of Scotland,
0:04:05 > 0:04:09to discover how, more than 50 years ago,
0:04:09 > 0:04:13small groups of men on foot managed to produce maps so accurate
0:04:13 > 0:04:17that they can be used today by ground-hugging bombers.
0:04:19 > 0:04:23The story of the modern Ordnance Survey map goes back to the 1930s,
0:04:23 > 0:04:27when the decision was made to survey the whole country in one go.
0:04:27 > 0:04:31The scale of the project was breathtaking.
0:04:31 > 0:04:34It took nearly 30 years to complete.
0:04:34 > 0:04:39The aim was to map the country to an accuracy of less than 20 metres.
0:04:39 > 0:04:43The process really began far away in Wiltshire.
0:04:43 > 0:04:47There, in 1937, on the Ridgeway Path near Swindon,
0:04:47 > 0:04:52they measured by hand a distance on the ground just over 11km long.
0:04:52 > 0:04:54This line, the Liddington baseline,
0:04:54 > 0:04:57was to become the basis of the entire scheme.
0:04:57 > 0:05:00It would form one side of a series of triangles,
0:05:00 > 0:05:03which would eventually cover the whole of Britain.
0:05:15 > 0:05:18Up here, in the Highlands of Scotland, the Ordnance Survey
0:05:18 > 0:05:20faced their biggest challenge.
0:05:20 > 0:05:25The place I'm headed for is so wild and so rugged that even today,
0:05:25 > 0:05:27the Army uses it as a navigational training area
0:05:27 > 0:05:31for their elite soldiers.
0:05:31 > 0:05:34If things were going to go wrong, they'd have done so up here.
0:05:42 > 0:05:44The master mapper of the enterprise
0:05:44 > 0:05:48was Major Martin Hotine, of the Royal Engineers,
0:05:48 > 0:05:52head of Triangulation and Levelling at the Ordnance Survey.
0:05:52 > 0:05:54A veteran of World War I,
0:05:54 > 0:05:58who had learned his trade in the hard schools of Afghanistan and Iraq,
0:05:58 > 0:06:02Hotine was the brains behind the new survey.
0:06:02 > 0:06:06Hotine was clearly an inspirational leader,
0:06:06 > 0:06:10loyal to his troops and driven by a passion for accuracy.
0:06:10 > 0:06:14He'd pick a fight with anybody who stood between him
0:06:14 > 0:06:16and getting the correct result.
0:06:16 > 0:06:18But first, he had to get the data,
0:06:18 > 0:06:23and he did that in a characteristically Napoleonic fashion.
0:06:30 > 0:06:34But how Napoleonic can map-making get?
0:06:34 > 0:06:37I'm off to Longa Island to find out.
0:06:37 > 0:06:42Hotine had decided that the survey required such a vast number of measurements
0:06:42 > 0:06:46that the survey team's job should be made as simple as possible.
0:06:48 > 0:06:52The surveyors would have to climb thousands of hilltops across Britain,
0:06:52 > 0:06:54many of them extremely remote.
0:06:54 > 0:07:00Each one would have to provide a footing for a theodolite, the surveyor's basic tool.
0:07:00 > 0:07:04Hotine's solution was a grand concept in itself.
0:07:04 > 0:07:08He invented a completely new feature for the British landscape,
0:07:08 > 0:07:13one that's become familiar to millions, the triangulation pillar.
0:07:13 > 0:07:16There's one of them on top of that island.
0:07:20 > 0:07:24It is easy enough for the RAF to find its target on Longa Island,
0:07:24 > 0:07:26but for me, it is a lot harder,
0:07:26 > 0:07:29and for the pillar builders, it was even worse.
0:07:29 > 0:07:33The pillars were made of concrete, which meant lugging bags of gravel,
0:07:33 > 0:07:37sand and cement to what were often very inaccessible hilltops,
0:07:37 > 0:07:39like one I am hoping to get to now.
0:07:49 > 0:07:52Where they could, they transported all the raw materials
0:07:52 > 0:07:56by lorry or, failing that, horse and cart.
0:07:56 > 0:08:01but some spots were so remote that nothing short of human brawn would do.
0:08:02 > 0:08:08I've lugged my 25 kilograms of cement ashore and this burden is just a small proportion
0:08:08 > 0:08:13of the amount that the construction teams had to carry.
0:08:13 > 0:08:16They would have been carrying more cement, sacks of sand,
0:08:16 > 0:08:21sacks of gravel, picks, shovels, and their food and shelter.
0:08:21 > 0:08:24And they were working in some of the most dramatic
0:08:24 > 0:08:26and adventurous landscapes in the land.,
0:08:26 > 0:08:29but they were doing it for a very good reason.
0:08:29 > 0:08:33It was a characteristically military approach - prepare the ground first
0:08:33 > 0:08:35and then commence operations.
0:08:35 > 0:08:37In true military style,
0:08:37 > 0:08:41the manual laid down exactly what they had to carry.
0:08:41 > 0:08:45A pillar complete with standard-sized base and block
0:08:45 > 0:08:48requires 32 cubic feet of concrete.
0:08:48 > 0:08:55At the required proportions, this represents cement - 4 cwt, sand - 11 cwt, and chippings - 16 cwt.
0:08:55 > 0:08:59Hotine's reasons for building the pillars were entirely operational -
0:08:59 > 0:09:01to ensure absolute accuracy.
0:09:01 > 0:09:08These concrete tripods meant that each time a team returned to a particular place,
0:09:08 > 0:09:11as they'd often have to do during the survey, they'd know
0:09:11 > 0:09:13they were taking measurements
0:09:13 > 0:09:15from the same place.
0:09:15 > 0:09:19There'd be no chance of putting down a tripod in slightly the wrong place.
0:09:19 > 0:09:23Hotine's measurements were set in stone, literally.
0:09:23 > 0:09:25ROAR OF JET ENGINE
0:09:30 > 0:09:32Once the pillars were up,
0:09:32 > 0:09:38it was time for the surveyors to start laying down Hotine's triangles.
0:09:38 > 0:09:40Here we are!
0:09:40 > 0:09:44'Ian McManus, once a surveyor for the Ordnance Survey himself,
0:09:44 > 0:09:48'has joined me at Sithean Mor near Gairloch to show me how it was done.'
0:09:48 > 0:09:52- This is the daddy of them all. - That's the big theodolite?
0:09:52 > 0:09:59'The theodolite was the key to the process because it measured not distances, but angles.'
0:09:59 > 0:10:06OK, before we go up the hill, shall we just establish the principles of a triangulated survey?
0:10:06 > 0:10:11We measure a baseline along one side of a triangle, between A and B.
0:10:11 > 0:10:15We do that very accurately with a tape measure.
0:10:15 > 0:10:19Then, again very accurately, we measure the angles of the triangle.
0:10:19 > 0:10:24And then a very simple mathematical equation gives us the length
0:10:24 > 0:10:26of the other two lines, doesn't it?
0:10:26 > 0:10:29We measured that baseline and the three angles.
0:10:29 > 0:10:32Mathematics does the rest.
0:10:32 > 0:10:36At each corner of these triangles, you have a trig pillar,
0:10:36 > 0:10:40then you just add triangles until the entire country has been surveyed.
0:10:40 > 0:10:43- Simple.- That's the way to do it.
0:10:43 > 0:10:46'Those who took part in Hotine's triangulation
0:10:46 > 0:10:49'never forgot the experience.'
0:10:49 > 0:10:52We were all single, we climbed these mountains
0:10:52 > 0:10:55and got paid for it! It was fantastic.
0:10:58 > 0:11:02Getting heavy, unwieldy equipment up one mountain is a very serious task.
0:11:02 > 0:11:07Taking it up hundreds of mountains is an even more incredible thought.
0:11:07 > 0:11:12Surveyors were issued with detailed instructions about the method to use.
0:11:12 > 0:11:16The Triangulation Handbook left them in no doubt
0:11:16 > 0:11:18how records were to be kept.
0:11:18 > 0:11:20- That was the bible.- What was?
0:11:20 > 0:11:25Each trig point had its own file with all the relevant information.
0:11:31 > 0:11:34There's no way that I can undertake
0:11:34 > 0:11:37a series of triangulation observations on my own,
0:11:37 > 0:11:41so I've roped in the Dundonnell Mountain Rescue Team
0:11:41 > 0:11:47who have put 15 men on 6 different peaks in a circle around this mountain behind me.
0:11:47 > 0:11:53They're going to lead us up the mountain to the observation point by the trig pillar.
0:11:53 > 0:11:57- Which way do we go? - The top's up there.- Oh, no!
0:12:19 > 0:12:22In Gaelic, this mountain's called the hill of spirits.
0:12:22 > 0:12:25I think I can see why.
0:12:25 > 0:12:27But there's something else, too.
0:12:27 > 0:12:32Because the heat haze distorts the accuracy of the readings in daytime,
0:12:32 > 0:12:35all the surveying was done at night.
0:12:35 > 0:12:41On all the peaks around us, our colleagues will shine a light towards our position at the trig pillar.
0:12:41 > 0:12:44When they do so, we'll measure
0:12:44 > 0:12:46the bearing with a theodolite.
0:12:46 > 0:12:49This is a five-inch theodolite, self-centring.
0:12:49 > 0:12:53It's a magnificent instrument. It sits in these slots?
0:12:53 > 0:12:57It's a massive piece of precision engineering.
0:12:57 > 0:13:01'The exact process of setting up the theodolite was laid down precisely
0:13:01 > 0:13:04'in the triangulation handbook.'
0:13:04 > 0:13:07- Did you know the handbook by heart? - Yes, oh, yes.
0:13:07 > 0:13:11What do you remember from it particularly?
0:13:11 > 0:13:13Thou shalt not fiddle the results!
0:13:15 > 0:13:20The handbook also said that each bearing must be measured 32 times.
0:13:20 > 0:13:24The results would then be collated back at base.
0:13:25 > 0:13:27We need the Longa Island light on.
0:13:27 > 0:13:31- Do you want the light on? - Yeah, light on!
0:13:31 > 0:13:34Can we have the light on, please?
0:13:34 > 0:13:37Just waiting for Longa Island light to come on.
0:13:37 > 0:13:40I've got it! I've got it!
0:13:40 > 0:13:42That's incredibly clear.
0:13:44 > 0:13:46Just going to fine-set the light with the cross.
0:13:46 > 0:13:50It's got to be right in the middle of the light.
0:13:50 > 0:13:52Got it. OK, I'll read it off.
0:13:52 > 0:13:58- Three, three, zero...- Three, three, zero...- Zero, six, two, nine.
0:13:58 > 0:14:02- Zero, six, two, nine. Three, three, zero, zero, six, two, nine.- Correct.
0:14:02 > 0:14:06OK, so the next one should be An Groban.
0:14:06 > 0:14:09They haven't put the light on yet. We'll have to wait a sec.
0:14:09 > 0:14:11OK, the light's on.
0:14:11 > 0:14:14'They may look close in the darkness,
0:14:14 > 0:14:21'but those lights are actually miles away, lighting up the Highlands from hilltops up to 3,000 feet high.'
0:14:21 > 0:14:24- 1812.- Correct. Next one.
0:14:24 > 0:14:26Meall an Dubh.
0:14:26 > 0:14:30'No-one has done anything like this for nearly 40 years.'
0:14:30 > 0:14:34- God, this is difficult. - Imagine doing this 32 times!
0:14:34 > 0:14:36There it is. It's on.
0:14:38 > 0:14:42'Doing the initial observations was only the first step.
0:14:42 > 0:14:47'The primary triangles were huge, with sides up to 30 miles long.
0:14:48 > 0:14:52'The Ordnance Survey still had to put in the detail.
0:14:52 > 0:14:54'That meant two further levels
0:14:54 > 0:14:58'of triangulation, with the triangles getting smaller each time.
0:14:58 > 0:15:03'When you're down to the size of a field, you get the tape measure out.
0:15:03 > 0:15:07'This is the secret of the Ordnance Survey's reputation.
0:15:07 > 0:15:11'Triangulation, as a means of avoiding errors on the ground
0:15:11 > 0:15:16'and a foolproof method of pinpointing errors later,
0:15:16 > 0:15:18'all because of the simple
0:15:18 > 0:15:21'mathematical properties of triangles.'
0:15:23 > 0:15:27Then and now, it was a hard struggle, but it was worth it.
0:15:27 > 0:15:30There are many different series of Ordnance Survey maps,
0:15:30 > 0:15:32but the two that I am using on this journey
0:15:32 > 0:15:36are this one here - the 1:50,000 scale Landranger map -
0:15:36 > 0:15:39which is as familiar to lovers of the Great Outdoors as
0:15:39 > 0:15:41the British landscape itself,
0:15:41 > 0:15:45and this one here - the 1:25,000 scale Explorer map -
0:15:45 > 0:15:49which is particularly suitable for mountain navigation.
0:15:49 > 0:15:53Now, the detail in the Explorer map is quite extraordinary.
0:15:53 > 0:15:56It is absolutely crammed with geographical information,
0:15:56 > 0:15:58so let's have a closer look.
0:15:58 > 0:16:00If I flip it over to this area here,
0:16:00 > 0:16:03we have got all sorts of diverse information,
0:16:03 > 0:16:05all organised systematically,
0:16:05 > 0:16:07because that's the key to a great map.
0:16:07 > 0:16:11So we can start by looking at the contour lines, these brown lines
0:16:11 > 0:16:14that indicate altitude above sea level,
0:16:14 > 0:16:17and they are arranged at ten-metre intervals.
0:16:17 > 0:16:20The closer together the contour lines, the steeper the gradient.
0:16:20 > 0:16:25If they become vertical, you have a symbol for a crag or a cliff.
0:16:25 > 0:16:27And then we have vegetation cover.
0:16:27 > 0:16:30There are two different symbols for trees.
0:16:30 > 0:16:33A coniferous forest is marked by little fir trees,
0:16:33 > 0:16:36and then a deciduous forest is marked by something looking like
0:16:36 > 0:16:41an oak tree. There is another symbol to show open grassland
0:16:41 > 0:16:45and bracken, so you know when you are about to trip over tussocks of grass.
0:16:45 > 0:16:48the built landscape is represented by all sorts of symbols.
0:16:48 > 0:16:50So in a built-up area like Gairloch,
0:16:50 > 0:16:55there is a police station marked, a school, several houses marked.
0:16:55 > 0:17:00But the most amazing aspect of any Ordnance Survey map
0:17:00 > 0:17:02is the unique system of grid referencing.
0:17:02 > 0:17:06Ordnance Survey maps are covered by little squares.
0:17:06 > 0:17:08On this map, they are quite large,
0:17:08 > 0:17:10because it is a large-scale map.
0:17:10 > 0:17:12Each of these squares is a one-kilometre square,
0:17:12 > 0:17:15and it allows you to give any feature on the map
0:17:15 > 0:17:19a unique six-figure number, a grid reference.
0:17:19 > 0:17:22You read those off by starting along the bottom.
0:17:22 > 0:17:24The first three are read off the bottom,
0:17:24 > 0:17:27and the second three numbers are read off the side.
0:17:32 > 0:17:36To demonstrate the kind of accuracy Hotine's survey was aiming for,
0:17:36 > 0:17:38I've been set a little challenge.
0:17:38 > 0:17:42The BBC's production team has dumped me by the roadside
0:17:42 > 0:17:45and given me a grid reference to find.
0:17:45 > 0:17:50The number they've given me is 844737. That's the grid reference.
0:17:50 > 0:17:54Let's see where that is. 844, along the bottom.
0:17:54 > 0:17:58737, I read that off down the side of the map.
0:17:58 > 0:18:01Follow your finger along, 844737.
0:18:01 > 0:18:03That grid reference is a tiny loch.
0:18:03 > 0:18:07Actually, a lochan, it's so small, with an island in the middle of it,
0:18:07 > 0:18:12or at one end of it. It's set behind some very rugged-looking mountains
0:18:12 > 0:18:15on the far side of a wilderness of heather and bog.
0:18:15 > 0:18:17So they've picked a knotty problem.
0:18:17 > 0:18:21I'm going to use a technique called dead-reckoning navigation
0:18:21 > 0:18:25and I'll start by going to a corner in the road.
0:18:28 > 0:18:30There are two phases to a dead-reckoning exercise.
0:18:30 > 0:18:34The first is to fix your precise location on the map.
0:18:34 > 0:18:37I know exactly where I am, because there's a stream
0:18:37 > 0:18:42running under the road and I'm on the apex of the bend.
0:18:42 > 0:18:44Now I set my compass on a bearing,
0:18:44 > 0:18:48which will be the first in a series of doglegs to my destination,
0:18:48 > 0:18:53this remote lochan behind all the mountains. So I set my compass.
0:18:53 > 0:18:57It tells me that I should go precisely up there.
0:19:01 > 0:19:05In country like this, it's impossible to keep a straight path,
0:19:05 > 0:19:10but if the map's correct, I need to be at the top of this stream.
0:19:10 > 0:19:13What I need to do is keep going up.
0:19:20 > 0:19:24Well, the rain's beginning to ease. Thank goodness.
0:19:24 > 0:19:28This is one of the most useful bits of mountain kit I've come across.
0:19:28 > 0:19:34Walking in the mountain ranges of Europe, you find that all the shepherds use umbrellas,
0:19:34 > 0:19:36not just for the rain and the snow,
0:19:36 > 0:19:40but it stops you getting sunburn in the intense sunlight.
0:19:40 > 0:19:45You can also fend off rabid shepherd dogs and the odd wolf or bear.
0:19:45 > 0:19:48I'm just going to check my compass bearing,
0:19:48 > 0:19:51to make certain I'm not straying off course.
0:19:51 > 0:19:55My next spot to aim for is that rocky hillock.
0:20:03 > 0:20:08Well, here's the rocky knoll, but I can't see my destination.
0:20:08 > 0:20:11It's vital that I don't get lost.
0:20:11 > 0:20:16One way of making sure is to time myself to the way point I've chosen.
0:20:16 > 0:20:19That's this crag, overlooking a stream.
0:20:19 > 0:20:23According to the map, it's roughly 500 metres away.
0:20:23 > 0:20:27In this country, that should take me about 15 minutes.
0:20:32 > 0:20:34On days like this, I love my job!
0:20:34 > 0:20:37Where HAVE they sent me?
0:20:52 > 0:20:53Fantastic!
0:20:53 > 0:20:57Here's the little cliff, and it's right above the elbow in the river,
0:20:57 > 0:21:00and it's taken me exactly 15 minutes.
0:21:00 > 0:21:02So far, so good.
0:21:02 > 0:21:06Now I've got to reset the compass on the next landmark I'm going to use
0:21:06 > 0:21:10on my dead-reckoning exercise to take me to this lake.
0:21:10 > 0:21:14I'm going to choose a little rock outcrop marked on the map,
0:21:14 > 0:21:17about 400 metres from here.
0:21:17 > 0:21:18And...
0:21:18 > 0:21:21it's on a bearing of eight degrees.
0:21:25 > 0:21:29I'm jolly glad that Hotine's people have been here before me.
0:21:29 > 0:21:32The weather up here can be extremely treacherous.
0:21:32 > 0:21:36You have to be on the lookout all of the time. If the clouds roll in,
0:21:36 > 0:21:40visibility can change very quickly.
0:21:40 > 0:21:44Now I'm on this exposed ridge, the wind's incredibly strong and gusty,
0:21:44 > 0:21:48which makes it even more dangerous than it was before.
0:21:48 > 0:21:52These mountains are very difficult to find a route through.
0:21:52 > 0:21:57I'll have to be careful because that mountain's covered in sheer crags.
0:21:57 > 0:22:00It's going to throw me off course.
0:22:09 > 0:22:11This is getting tense.
0:22:11 > 0:22:13I'm now on my fourth dogleg bearing
0:22:13 > 0:22:17and I'm completely dependent on the dead reckoning.
0:22:17 > 0:22:21If I've made any mistakes, each one will have put me further out.
0:22:21 > 0:22:25But if I haven't, and the map is as accurate as Hotine hoped,
0:22:25 > 0:22:28the lochan should be just over this crag.
0:22:33 > 0:22:36There it is!
0:22:36 > 0:22:37Fantastic!
0:22:37 > 0:22:41What a beautiful sight! There's the little loch, with its tiny island
0:22:41 > 0:22:43sitting in the middle of nowhere.
0:22:43 > 0:22:48That map has brought me the whole way from the road that I was dropped on
0:22:48 > 0:22:52across several miles of the wildest and most rugged terrain in the UK
0:22:52 > 0:22:56and has delivered me with pinpoint precision on a tiny puddle
0:22:56 > 0:22:58in the middle of nowhere.
0:22:58 > 0:23:03The accuracy of Hotine's triangulation is scarcely believable.
0:23:03 > 0:23:06But there's more to it than that.
0:23:06 > 0:23:10I've passed my test using a map based on Hotine's work.
0:23:10 > 0:23:14But Hotine himself had to face a far stiffer test.
0:23:14 > 0:23:18He had to produce a survey that was accurate across the whole country,
0:23:18 > 0:23:20not just a few miles.
0:23:20 > 0:23:24And on that scale, the danger was that errors would accumulate
0:23:24 > 0:23:26and the results would be useless.
0:23:29 > 0:23:32The Ordnance Survey weren't having that.
0:23:32 > 0:23:38They wanted a single, consistent scheme of mapping across the whole country,
0:23:38 > 0:23:40which raised the stakes further.
0:23:40 > 0:23:43Would mathematical triangulation
0:23:43 > 0:23:47be accurate across hundreds of miles of country? There's only one way to find out.
0:23:47 > 0:23:51ROAR OF AIRCRAFT ENGINE
0:24:04 > 0:24:07Back in 1937, they had measured the original baseline
0:24:07 > 0:24:09as accurately as possible.
0:24:09 > 0:24:13And now they faced their big test, here in Caithness.
0:24:13 > 0:24:17After the first baseline, none of the triangles in Hotine's scheme
0:24:17 > 0:24:19had actually been measured.
0:24:19 > 0:24:21They'd all been calculated,
0:24:21 > 0:24:25so to check the scheme's accuracy, they went back to measuring.
0:24:25 > 0:24:30They chose a triangle up here, with sides 25 kilometres long,
0:24:30 > 0:24:34a triangle as far away from Wiltshire as it was possible to get.
0:24:34 > 0:24:35Then, in 1951,
0:24:35 > 0:24:40they measured one of its sides with a 30-metre steel tape.
0:24:40 > 0:24:45According to the maths, the distance between this trig pillar
0:24:45 > 0:24:47and that one over there in the murk,
0:24:47 > 0:24:51in the far corner of the survey triangle,
0:24:51 > 0:24:56should have been 24,828.423 metres.
0:24:56 > 0:24:59The question facing the Ordnance Survey was that
0:24:59 > 0:25:02if they ran a tape measure between the two trig pillars,
0:25:02 > 0:25:06whether that measured distance
0:25:06 > 0:25:09would tally up with the computed distance.
0:25:09 > 0:25:13If it did, it would be a miracle of modern science and of surveying.
0:25:13 > 0:25:18But if it didn't, all Hotine's work would have been in vain.
0:25:19 > 0:25:25'I've got with me some experts from the Ordnance Survey to show me what it was like.'
0:25:25 > 0:25:30So you pull that, is that the idea? To get the tension right?
0:25:30 > 0:25:33'Back then, the line they measured was laid out exactly,
0:25:33 > 0:25:36'using a theodolite. That was just the start.'
0:25:36 > 0:25:42I've no idea how you do this with frozen fingers.
0:25:42 > 0:25:44'A 30-metre metal tape was made of a special alloy,
0:25:44 > 0:25:47'which resisted the effect of temperature.
0:25:47 > 0:25:51'To ensure its accuracy, the tape was tensioned to a force of 20 pounds.'
0:25:53 > 0:25:56Point-nine-seven-one.
0:25:56 > 0:25:58Each reading was taken four times.
0:25:58 > 0:26:01That was done for nearly 25,000 metres
0:26:01 > 0:26:03between the two trig points,
0:26:03 > 0:26:05more than 800 separate measurements.
0:26:05 > 0:26:10THEY SHOUT NUMBERS
0:26:13 > 0:26:1829.818.
0:26:18 > 0:26:23- What is the temperature? - Six degrees.
0:26:23 > 0:26:25- Above freezing?- Yes.
0:26:27 > 0:26:32Right, that's two done. Now we just have to get the average distance between the two measurements, yes?
0:26:32 > 0:26:37- Are we allowed to do that down in the pub?- Yep.
0:26:38 > 0:26:41'It took them 54 days
0:26:41 > 0:26:43'and only nine were lost to wind and rain.
0:26:43 > 0:26:47'Today, that distance takes a Tornado just over two minutes.'
0:26:58 > 0:27:01This is the other end of the baseline.
0:27:01 > 0:27:06I'm nearly 25 kilometres from the John O'Groats pillar.
0:27:06 > 0:27:09When the surveyors reached this point,
0:27:09 > 0:27:12they sent their measurements back to Southampton
0:27:12 > 0:27:15and waited for the results.
0:27:15 > 0:27:17They were absolutely astonishing.
0:27:17 > 0:27:23After measuring the entire Caithness baseline, a distance of around 25,000 metres,
0:27:23 > 0:27:29they found that the difference between the amount that they expected from triangulation
0:27:29 > 0:27:36and the distance that they got through measuring it with a tape measure was only 42 centimetres,
0:27:36 > 0:27:39a scarcely believable level of accuracy.
0:27:39 > 0:27:43And that was after measuring through triangulation
0:27:43 > 0:27:47the 550 miles from the Liddington baseline to the Caithness baseline.
0:27:47 > 0:27:52So Hotine's painstaking approach to surveying had been vindicated.
0:27:56 > 0:27:5942 centimetres!
0:27:59 > 0:28:03Less than 17 inches... after 550 miles.
0:28:03 > 0:28:05Astonishing!
0:28:07 > 0:28:11It's that amazing accuracy and meticulous detail
0:28:11 > 0:28:15which enables the RAF pilots to hit their targets with pinpoint accuracy
0:28:15 > 0:28:20and frees the British people to roam right across their own country.
0:28:20 > 0:28:23And all because of the monumental efforts
0:28:23 > 0:28:26made more than half a century ago.
0:28:26 > 0:28:29Hotine and his Ordnance surveyors
0:28:29 > 0:28:33set the British map record straight for good.