Measuring up to put county on the map

The trig point (triangulation station) on Shillhope Law. Picture by Ian Hall.
The trig point (triangulation station) on Shillhope Law. Picture by Ian Hall.

In this age of satellite navigation systems and Google maps, there is still a quiet pleasure to be had in opening out an Ordnance Survey map and seeing the landscape spread out in fine detail.

The very paper these maps are made from inspires confidence in the way that they withstand frequent folding, and even the odd drenching.

A three-legged spider used to position the theodolite. Picture by Ian Hall.

A three-legged spider used to position the theodolite. Picture by Ian Hall.

The Ordnance Survey has been creating and updating the maps of Britain for more than 200 years.

Today, most of this survey work is done from the air – either by aeroplane or satellite. However, it was not always so.

In the past there were teams of surveyors walking the landscape, measuring and recording as they went along.

And the tell-tale signs of their work are still there to be seen, if we take the time to look.

Your phone may be good at telling you where you are, but it is not so good at telling you what is just over that hill, or round the next corner.

In making maps, the surveyors generally are doing two separate things – they look to fix the position of places in relation to each other in a horizontal sense, and to measure the height of these same places.

The location of a place is done by a process called triangulation, where the landscape is divided up into a series of triangles. One side of the first of them has its length measured very accurately. From then onwards it is a matter of trigonometry to determine all the other distances.

The most obvious signs of these measurements are the trig points (or triangulation stations) that can be found on many of our hilltops and other high points.

Each pillar has a three-legged metal ‘spider’ at the top, which was used to position the theodolite. The surveyor would then measure the various angles between adjacent pillars.

Belford fundamental bench mark. Picture by Ian Hall.

Belford fundamental bench mark. Picture by Ian Hall.

These pillars are no longer in use, but their construction was so good that they are likely to remain in place for many years to come.

Perhaps surprisingly, these columns are only the top of what is a much more massive structure.

The fundamental triangulation marker is set below ground level, where it is built onto bedrock, and the pillar that we see is built on top.

In this way, the measurement point will survive the complete destruction of the column.

A metal bench mark. Picture by Ian Hall.

A metal bench mark. Picture by Ian Hall.

The measurement of heights is done by a process called levelling.

Starting out at the chosen ‘zero’ point, which for Great Britain is set at the mid-tide level at Newlyn in Cornwall, surveyors created a network of about 200 ‘fundamental benchmarks’, which each had their height measured to a very high degree of accuracy.

In Northumberland there are three of these primary benchmarks – at Hexham, Longhorsley and Belford.

They are still maintained as part of the national levelling system and can all be visited.

The one at Belford is on the side road to Swinhoe Farm, just to the north of Belford, sitting inside a set of railings.

Its height is marked as 236.88 ft above Newlyn.

A carved bench mark. Picture by Ian Hall.

A carved bench mark. Picture by Ian Hall.

As with the trig points, what we can see above the surface is only a small part – there is a bigger structure below ground, which goes down to the underlying rock.

From this primary network, a series of secondary benchmarks were then created.

These were cast from metal and set into the faces of walls, with each one being uniquely identified.

Many of these are still in existence and we walk past them every day without even noticing them.

Other benchmarks are carved into walls of buildings, some of which date from much earlier map making.

There are many websites which have lists of the different surveying marks, such as www.bench-marks.org.uk

These sites will show you the various surveying marks that can be found close to your home.

So have a look and see how many of these lost marks you can find.

Each one of them reveals part of a fascinating history of mapping in Britain.

And to get you started, here are a few of the metal benchmarks you can see in this area of North Northumberland:

Alnwick – Next to the door of Chisolms in the Market Place.

Belford – On the angle of the wall opposite St Mary’s Church.

Berwick – On the South East face of the Town Hall.

Thropton – On the North East corner of the Three Wheatheads Inn.

Wooler – On the corner of Dene House in Church Street.

We often hear how, with the rise in the use of satellite navigation and other electronic devices, we might be seeing the end of paper maps.

But we need to be careful – there is something important in a paper map, and not just because it doesn’t need batteries.

Your phone may be good at telling you where you are, but it is not so good at telling you what is just over that hill, or round the next corner.

Unfolding an Ordnance Survey map will reveal the whole countryside and show the landscape in a totally new way.