Martin Hotine's Ordnance Survey (1935-1950) Map Man

This is John o'Groats, in northern Scotland.

It's the very end, or the very beginning, of mainland Britain.

Between here and the other end of Britain - Land's End, in Cornwall -

are 600 miles of mountains, forests, city, and farmland.

And the amazing thing is that every little bit of

those British landscapes is marked on one of these, an Ordnance Survey map.

Ordnance Survey maps

are incredibly accurate, incredibly detailed,

and incredibly successful.

This Landranger series sells two million copies a year.

Ordnance Survey maps are used by everyone, from hillwalkers

to fast-jet pilots. In fact, by anyone who needs to know

exactly where they are on British soil.

I'm going to take a journey across the toughest terrain in Britain,

the Highlands of northern Scotland, to find out exactly why these maps

are still regarded as the best in the world.

I'm starting my journey at Britain's busiest bomber base.

The Ordnance Survey began in 1791 as a military necessity.

You'd think that the modern RAF

would use ultramodern electronic maps for their navigation.

So they do, but just like us,

they use Ordnance Survey maps when they fly over Britain.

This room is where the pilots plan their missions.

To do so, they use a custom-made,

computerised navigation system and an OS map.

Stu Gillies is planning a navigation exercise,

using a series of prominent hilltops as his targets.

Stu will be flying at 500mph

over some key landmarks on my journey,

putting the Ordnance Survey maps to the test.

If you're going to fly an aircraft that low that fast,

you've got to be able to rely on very accurate maps.

You use an old-fashioned paper map?

The great thing about the 50,000 is that you get the big picture,

but you've got also very clear detail.

It's the same map as you've got there.

What we intend to do is come from Lossiemouth, south, then west,

-then head for Longa Island.

-There's Longa Island.

-Where you're going.

-Sithean Mor there.

-Yeah.

-Are you going to fly over them like that?

Because of the turning circle of the aircraft, I'll move that point out.

With aeroplanes, you have to keep them pointing at the right county,

or you end up in the wrong country!

And then up towards Caithness here.

Probably, to line it up,

-we'll come out to the north, then back down again.

-So a triangle.

-Yeah.

-Do you like looking at maps?

I have to admit, it's one of the pleasures of the job

because really, the map is, I think, a fusion of science and art.

You have the very accurate distances and heights and elevations marked.

For me, the great pleasure of flying is that

when you fly around this stuff,

that visualisation that you have often comes real.

Stu's list of targets will take him from Lossiemouth

to the Highlands of Wester Ross,

then up to Caithness and back. I'll make the same journey on the ground.

As in the song, he'll take the high road and I'll take the low road,

but we'll both use the same map!

The first stage in my journey takes me up to Gairloch, in Wester Ross,

in the Highlands of Scotland,

to discover how, more than 50 years ago,

small groups of men on foot managed to produce maps so accurate

that they can be used today by ground-hugging bombers.

The story of the modern Ordnance Survey map goes back to the 1930s,

when the decision was made to survey the whole country in one go.

The scale of the project was breathtaking.

It took nearly 30 years to complete.

The aim was to map the country to an accuracy of less than 20 metres.

The process really began far away in Wiltshire.

There, in 1937, on the Ridgeway Path near Swindon,

they measured by hand a distance on the ground just over 11km long.

This line, the Liddington baseline,

was to become the basis of the entire scheme.

It would form one side of a series of triangles,

which would eventually cover the whole of Britain.

Up here, in the Highlands of Scotland, the Ordnance Survey

faced their biggest challenge.

The place I'm headed for is so wild and so rugged that even today,

the Army uses it as a navigational training area

for their elite soldiers.

If things were going to go wrong, they'd have done so up here.

The master mapper of the enterprise

was Major Martin Hotine, of the Royal Engineers,

head of Triangulation and Levelling at the Ordnance Survey.

A veteran of World War I,

who had learned his trade in the hard schools of Afghanistan and Iraq,

Hotine was the brains behind the new survey.

Hotine was clearly an inspirational leader,

loyal to his troops and driven by a passion for accuracy.

He'd pick a fight with anybody who stood between him

and getting the correct result.

But first, he had to get the data,

and he did that in a characteristically Napoleonic fashion.

But how Napoleonic can map-making get?

I'm off to Longa Island to find out.

Hotine had decided that the survey required such a vast number of measurements

that the survey team's job should be made as simple as possible.

The surveyors would have to climb thousands of hilltops across Britain,

many of them extremely remote.

Each one would have to provide a footing for a theodolite, the surveyor's basic tool.

Hotine's solution was a grand concept in itself.

He invented a completely new feature for the British landscape,

one that's become familiar to millions, the triangulation pillar.

There's one of them on top of that island.

It is easy enough for the RAF to find its target on Longa Island,

but for me, it is a lot harder,

and for the pillar builders, it was even worse.

The pillars were made of concrete, which meant lugging bags of gravel,

sand and cement to what were often very inaccessible hilltops,

like one I am hoping to get to now.

Where they could, they transported all the raw materials

by lorry or, failing that, horse and cart.

but some spots were so remote that nothing short of human brawn would do.

I've lugged my 25 kilograms of cement ashore and this burden is just a small proportion

of the amount that the construction teams had to carry.

They would have been carrying more cement, sacks of sand,

sacks of gravel, picks, shovels, and their food and shelter.

And they were working in some of the most dramatic

and adventurous landscapes in the land.,

but they were doing it for a very good reason.

It was a characteristically military approach - prepare the ground first

and then commence operations.

In true military style,

the manual laid down exactly what they had to carry.

A pillar complete with standard-sized base and block

requires 32 cubic feet of concrete.

At the required proportions, this represents cement - 4 cwt, sand - 11 cwt, and chippings - 16 cwt.

Hotine's reasons for building the pillars were entirely operational -

to ensure absolute accuracy.

These concrete tripods meant that each time a team returned to a particular place,

as they'd often have to do during the survey, they'd know

they were taking measurements

from the same place.

There'd be no chance of putting down a tripod in slightly the wrong place.

Hotine's measurements were set in stone, literally.

ROAR OF JET ENGINE

Once the pillars were up,

it was time for the surveyors to start laying down Hotine's triangles.

Here we are!

'Ian McManus, once a surveyor for the Ordnance Survey himself,

'has joined me at Sithean Mor near Gairloch to show me how it was done.'

-This is the daddy of them all.

-That's the big theodolite?

'The theodolite was the key to the process because it measured not distances, but angles.'

OK, before we go up the hill, shall we just establish the principles of a triangulated survey?

We measure a baseline along one side of a triangle, between A and B.

We do that very accurately with a tape measure.

Then, again very accurately, we measure the angles of the triangle.

And then a very simple mathematical equation gives us the length

of the other two lines, doesn't it?

We measured that baseline and the three angles.

Mathematics does the rest.

At each corner of these triangles, you have a trig pillar,

then you just add triangles until the entire country has been surveyed.

-Simple.

-That's the way to do it.

'Those who took part in Hotine's triangulation

'never forgot the experience.'

We were all single, we climbed these mountains

and got paid for it! It was fantastic.

Getting heavy, unwieldy equipment up one mountain is a very serious task.

Taking it up hundreds of mountains is an even more incredible thought.

Surveyors were issued with detailed instructions about the method to use.

The Triangulation Handbook left them in no doubt

how records were to be kept.

-That was the bible.

-What was?

Each trig point had its own file with all the relevant information.

There's no way that I can undertake

a series of triangulation observations on my own,

so I've roped in the Dundonnell Mountain Rescue Team

who have put 15 men on 6 different peaks in a circle around this mountain behind me.

They're going to lead us up the mountain to the observation point by the trig pillar.

-Which way do we go?

-The top's up there.

-Oh, no!

In Gaelic, this mountain's called the hill of spirits.

I think I can see why.

But there's something else, too.

Because the heat haze distorts the accuracy of the readings in daytime,

all the surveying was done at night.

On all the peaks around us, our colleagues will shine a light towards our position at the trig pillar.

When they do so, we'll measure

the bearing with a theodolite.

This is a five-inch theodolite, self-centring.

It's a magnificent instrument. It sits in these slots?

It's a massive piece of precision engineering.

'The exact process of setting up the theodolite was laid down precisely

'in the triangulation handbook.'

-Did you know the handbook by heart?

-Yes, oh, yes.

What do you remember from it particularly?

Thou shalt not fiddle the results!

The handbook also said that each bearing must be measured 32 times.

The results would then be collated back at base.

We need the Longa Island light on.

-Do you want the light on?

-Yeah, light on!

Can we have the light on, please?

Just waiting for Longa Island light to come on.

I've got it! I've got it!

That's incredibly clear.

Just going to fine-set the light with the cross.

It's got to be right in the middle of the light.

Got it. OK, I'll read it off.

-Three, three, zero...

-Three, three, zero...

-Zero, six, two, nine.

-Zero, six, two, nine. Three, three, zero, zero, six, two, nine.

-Correct.

OK, so the next one should be An Groban.

They haven't put the light on yet. We'll have to wait a sec.

OK, the light's on.

'They may look close in the darkness,

'but those lights are actually miles away, lighting up the Highlands from hilltops up to 3,000 feet high.'

-1812.

-Correct. Next one.

Meall an Dubh.

'No-one has done anything like this for nearly 40 years.'

-God, this is difficult.

-Imagine doing this 32 times!

There it is. It's on.

'Doing the initial observations was only the first step.

'The primary triangles were huge, with sides up to 30 miles long.

'The Ordnance Survey still had to put in the detail.

'That meant two further levels

'of triangulation, with the triangles getting smaller each time.

'When you're down to the size of a field, you get the tape measure out.

'This is the secret of the Ordnance Survey's reputation.

'Triangulation, as a means of avoiding errors on the ground

'and a foolproof method of pinpointing errors later,

'all because of the simple

'mathematical properties of triangles.'

Then and now, it was a hard struggle, but it was worth it.

There are many different series of Ordnance Survey maps,

but the two that I am using on this journey

are this one here - the 1:50,000 scale Landranger map -

which is as familiar to lovers of the Great Outdoors as

the British landscape itself,

and this one here - the 1:25,000 scale Explorer map -

which is particularly suitable for mountain navigation.

Now, the detail in the Explorer map is quite extraordinary.

It is absolutely crammed with geographical information,

so let's have a closer look.

If I flip it over to this area here,

we have got all sorts of diverse information,

all organised systematically,

because that's the key to a great map.

So we can start by looking at the contour lines, these brown lines

that indicate altitude above sea level,

and they are arranged at ten-metre intervals.

The closer together the contour lines, the steeper the gradient.

If they become vertical, you have a symbol for a crag or a cliff.

And then we have vegetation cover.

There are two different symbols for trees.

A coniferous forest is marked by little fir trees,

and then a deciduous forest is marked by something looking like

an oak tree. There is another symbol to show open grassland

and bracken, so you know when you are about to trip over tussocks of grass.

the built landscape is represented by all sorts of symbols.

So in a built-up area like Gairloch,

there is a police station marked, a school, several houses marked.

But the most amazing aspect of any Ordnance Survey map

is the unique system of grid referencing.

Ordnance Survey maps are covered by little squares.

On this map, they are quite large,

because it is a large-scale map.

Each of these squares is a one-kilometre square,

and it allows you to give any feature on the map

a unique six-figure number, a grid reference.

You read those off by starting along the bottom.

The first three are read off the bottom,

and the second three numbers are read off the side.

To demonstrate the kind of accuracy Hotine's survey was aiming for,

I've been set a little challenge.

The BBC's production team has dumped me by the roadside

and given me a grid reference to find.

The number they've given me is 844737. That's the grid reference.

Let's see where that is. 844, along the bottom.

737, I read that off down the side of the map.

Follow your finger along, 844737.

That grid reference is a tiny loch.

Actually, a lochan, it's so small, with an island in the middle of it,

or at one end of it. It's set behind some very rugged-looking mountains

on the far side of a wilderness of heather and bog.

So they've picked a knotty problem.

I'm going to use a technique called dead-reckoning navigation

and I'll start by going to a corner in the road.

There are two phases to a dead-reckoning exercise.

The first is to fix your precise location on the map.

I know exactly where I am, because there's a stream

running under the road and I'm on the apex of the bend.

Now I set my compass on a bearing,

which will be the first in a series of doglegs to my destination,

this remote lochan behind all the mountains. So I set my compass.

It tells me that I should go precisely up there.

In country like this, it's impossible to keep a straight path,

but if the map's correct, I need to be at the top of this stream.

What I need to do is keep going up.

Well, the rain's beginning to ease. Thank goodness.

This is one of the most useful bits of mountain kit I've come across.

Walking in the mountain ranges of Europe, you find that all the shepherds use umbrellas,

not just for the rain and the snow,

but it stops you getting sunburn in the intense sunlight.

You can also fend off rabid shepherd dogs and the odd wolf or bear.

I'm just going to check my compass bearing,

to make certain I'm not straying off course.

My next spot to aim for is that rocky hillock.

Well, here's the rocky knoll, but I can't see my destination.

It's vital that I don't get lost.

One way of making sure is to time myself to the way point I've chosen.

That's this crag, overlooking a stream.

According to the map, it's roughly 500 metres away.

In this country, that should take me about 15 minutes.

On days like this, I love my job!

Where HAVE they sent me?

Fantastic!

Here's the little cliff, and it's right above the elbow in the river,

and it's taken me exactly 15 minutes.

So far, so good.

Now I've got to reset the compass on the next landmark I'm going to use

on my dead-reckoning exercise to take me to this lake.

I'm going to choose a little rock outcrop marked on the map,

about 400 metres from here.

And...

it's on a bearing of eight degrees.

I'm jolly glad that Hotine's people have been here before me.

The weather up here can be extremely treacherous.

You have to be on the lookout all of the time. If the clouds roll in,

visibility can change very quickly.

Now I'm on this exposed ridge, the wind's incredibly strong and gusty,

which makes it even more dangerous than it was before.

These mountains are very difficult to find a route through.

I'll have to be careful because that mountain's covered in sheer crags.

It's going to throw me off course.

This is getting tense.

I'm now on my fourth dogleg bearing

and I'm completely dependent on the dead reckoning.

If I've made any mistakes, each one will have put me further out.

But if I haven't, and the map is as accurate as Hotine hoped,

the lochan should be just over this crag.

There it is!

Fantastic!

What a beautiful sight! There's the little loch, with its tiny island

sitting in the middle of nowhere.

That map has brought me the whole way from the road that I was dropped on

across several miles of the wildest and most rugged terrain in the UK

and has delivered me with pinpoint precision on a tiny puddle

in the middle of nowhere.

The accuracy of Hotine's triangulation is scarcely believable.

But there's more to it than that.

I've passed my test using a map based on Hotine's work.

But Hotine himself had to face a far stiffer test.

He had to produce a survey that was accurate across the whole country,

not just a few miles.

And on that scale, the danger was that errors would accumulate

and the results would be useless.

The Ordnance Survey weren't having that.

They wanted a single, consistent scheme of mapping across the whole country,

which raised the stakes further.

Would mathematical triangulation

be accurate across hundreds of miles of country? There's only one way to find out.

ROAR OF AIRCRAFT ENGINE

Back in 1937, they had measured the original baseline

as accurately as possible.

And now they faced their big test, here in Caithness.

After the first baseline, none of the triangles in Hotine's scheme

had actually been measured.

They'd all been calculated,

so to check the scheme's accuracy, they went back to measuring.

They chose a triangle up here, with sides 25 kilometres long,

a triangle as far away from Wiltshire as it was possible to get.

Then, in 1951,

they measured one of its sides with a 30-metre steel tape.

According to the maths, the distance between this trig pillar

and that one over there in the murk,

in the far corner of the survey triangle,

should have been 24,828.423 metres.

The question facing the Ordnance Survey was that

if they ran a tape measure between the two trig pillars,

whether that measured distance

would tally up with the computed distance.

If it did, it would be a miracle of modern science and of surveying.

But if it didn't, all Hotine's work would have been in vain.

'I've got with me some experts from the Ordnance Survey to show me what it was like.'

So you pull that, is that the idea? To get the tension right?

'Back then, the line they measured was laid out exactly,

'using a theodolite. That was just the start.'

I've no idea how you do this with frozen fingers.

'A 30-metre metal tape was made of a special alloy,

'which resisted the effect of temperature.

'To ensure its accuracy, the tape was tensioned to a force of 20 pounds.'

Point-nine-seven-one.

Each reading was taken four times.

That was done for nearly 25,000 metres

between the two trig points,

more than 800 separate measurements.

THEY SHOUT NUMBERS

29.818.

-What is the temperature?

-Six degrees.

-Above freezing?

-Yes.

Right, that's two done. Now we just have to get the average distance between the two measurements, yes?

-Are we allowed to do that down in the pub?

-Yep.

'It took them 54 days

'and only nine were lost to wind and rain.

'Today, that distance takes a Tornado just over two minutes.'

This is the other end of the baseline.

I'm nearly 25 kilometres from the John O'Groats pillar.

When the surveyors reached this point,

they sent their measurements back to Southampton

and waited for the results.

They were absolutely astonishing.

After measuring the entire Caithness baseline, a distance of around 25,000 metres,

they found that the difference between the amount that they expected from triangulation

and the distance that they got through measuring it with a tape measure was only 42 centimetres,

a scarcely believable level of accuracy.

And that was after measuring through triangulation

the 550 miles from the Liddington baseline to the Caithness baseline.

So Hotine's painstaking approach to surveying had been vindicated.

42 centimetres!

Less than 17 inches... after 550 miles.

Astonishing!

It's that amazing accuracy and meticulous detail

which enables the RAF pilots to hit their targets with pinpoint accuracy

and frees the British people to roam right across their own country.

And all because of the monumental efforts

made more than half a century ago.

Hotine and his Ordnance surveyors

set the British map record straight for good.