Surveying the surveyors: the landscape legacies of the OrdnanceSurvey

Lilley, K. (2018). Surveying the surveyors: the landscape legacies of the Ordnance Survey. British Archaeology,159, 22-27.

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Surveying the surveyors: the landscape legacies of theOrdnance Survey

The modernlandscape is wellmapped, but it hasan overlookedhistoric component:monuments left bythe surveyorsthemselves. KeithLilley explains howyou can find them

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The author onCadair Idris

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by the os from its inception and in theproposal for mapping the nation atone-inch to one mile (1:63,360).Triangulation was long known as ameans for creating accurate mapsthrough indirect measurements, byobserving angles between knownpoints such as church towers andhilltops. Proceeding from the 1790sinto the 19th century, thistrigonometrical survey left its mark inthe British (and later Irish) landscape.

This tangible imprint of the surveyors’work offers some interestingarchaeological insights into how the osand its surveyors operated some 200years ago.

Occupying summits From its earliest days beginning underWilliam Roy in the 1740s and 1750s in

Scotland, the os has long played a rolein fostering field archaeology throughrecording antiquities and markingthese on its larger-scale maps. Yet the os has its own “field archaeology”in the landscape through which its past activities can be traced. Thisarchaeology of survey relates to thefieldwork of the surveyors themselves,and one of their most importantoperations: triangulation. The triangulation of Great Britain

and Ireland under the os eventuallycovered the two islands with a networkof trigonometrical “stations” that werekey to fixing the control points onwhich accurate mapping depended.Triangulation networks not onlyconnected the two islands to eachother, however. They also, early on inthe os’s work, linked Britain to similarnetworks on the near continent, inFrance and Belgium. The stationsestablished in Britain and Ireland were part of this ambitiousinternational programme of theEnlightenment in Europe, to surveyand map according to the latestscientific principles using the mostmodern survey instruments available. For the os, the earliest

trigonometrical recordings used a largeprecision instrument, the Ramsden 3-foot theodolite. Accurate observationscould be made with this over a distanceof up to 70 miles (110km). Thedisadvantage was that the instrumentwas delicate and cumbersome.Weighing 300lb (136kg), it had to belugged around the country, and fixed to

Survey is very much a part of thearchaeological toolkit. Look in almostany textbook on field or landscapearchaeology and there will be a chapterdedicated to how to use surveyingequipment – dumpy levels, totalstations, and nowadays high-techdifferential gps and gnss (globalnavigation satellite systems) – to fix the locations and coordinates ofarchaeological features, and map them. There is another link between

archaeology and survey, however,which is relatively rarely explored.Neglected and largely forgotten aresites and monuments constructed andused by surveyors in their fieldwork.Hidden in the upland landscapes ofBritain and Ireland are legacies of theearly Ordnance Survey (os).The os itself is familiar to every

archaeologist in these islands throughits large-scale maps, used widely notjust as a means of getting around but also as an important source inarchaeological fieldwork and desktopstudy. The origins of the os and itssignificance in the nation’s psyche – asmuch a part of British cultural identityas the bbcWorld Service and the wi,some would argue – is well-troddenground, covered by numerous books,tv series and radio programmes. But sooften missing from this revelry for theosmap is an appreciation of how themaking of the 19th- and 20th-centurymaps themselves also shaped theBritish and Irish landscape. The Board of Ordnance’s early work

in surveying the land was concernedwith creating a scientific basis fordetermining and calculating key points.The technique was triangulation, used

Below: Trig pillar inWiltshire, withfittings for surveyequipment; once akey part of thenation’s mappingsystem, such pillarshave been maderedundant bysatellite data and are no longermaintained ormonitored by the osfor geodetic purposes

Left: The earlyOrdnance Survey inthe field –RoyalSappers and Miners,1837

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specially chosen stations in high places.In the south and east of Britain, thesepoints were usually church towers. In the upland areas to the north andwest, however, and also in Ireland as a whole, the sites chosen for stationswere invariably elevated landformssuch as hilltops and mountain peaks.For the trigonometrical survey, theRamsden theodolite had to be placedon all of these and readings takenbetween the stations, often in harshand difficult conditions.Thus as one of the higher peaks of

Wales, Cadair Idris was selected as a“principal” trigonometrical station by the early os, forming part of thetriangulation network as a whole.Anyone who has climbed to the 2,900feet (880m) high summit will knowwhat a challenge this is, even in

relatively benign weather. Imaginedoing so carrying heavy and fragilesurvey equipment.The earliest one-inch osmap notes

the triangulation station on CadairIdris with the now familiar and fairlyuniversal sign of a triangle with a dot at its centre. It is a station that is alsonow marked on the ground by anotherfamiliar feature of the os, the “trigpillar”, those sturdy concretestructures that dot the landscape, andso often form the focus of a summitphotograph opportunity for hardy hill-walkers to record their achievement. These concrete pillars belong not to

the original os survey, however, but to

the retriangulation of Britain carriedout by the os over 20 years between the1930s and 1950s. They are no longermaintained by the os as part of theirtrigonometrical network. The trigpillar is itself now an archaeologicalfield monument, a landscape legacy of the os’s surveying work. But onCadair Idris an earlier, 19th-centurypredecessor lies beneath the later ostriangulation station.When in 1811 the third volume of

William Mudge and Thomas Colby’sAccount of the Trigonometrical Surveywas published, included among itsappendices was “An Alphabetical Listof the Latitudes and Longitudes of the principal Stations, together withseveral Church, Steeples, Lighthouses,and other remarkable Objects.” Thislong tabulated list of stations recordsthe position of Cadair Idris withtypical military precision andgeographical exactitude: 52˚ 42’ 2’’ nand 4˚ 28’ 3’’ w. These coordinates forCadair Idris make clear too that thesummit had now been “occupied” by

the survey team, its location

Above: Principal triangulation diagram for Wales linked withIreland, adapted from Henry James, Ordnance TrigonometricalSurvey of Great Britain and Ireland (1858)

Right: Pen y Gadair,the summit ofCadair Idris, in a lithograph by T Comptonpublished in 1818

Above: YorkMinster, site of aBoard of Ordnance(later OrdnanceSurvey) early“principal station” of thetrigonometricalsurvey of GreatBritain

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providing further “control” for thenationwide survey.

Huts and campsAs part of this occupation of thesummit of Cadair Idris – from whichother distant but visible summits wereobserved, such as Snowdon andPlynlimon – the surveyors made theirmark on the ground. As you approachthe summit from the west, the laterconcrete trig pillar hoves into view. Itsits atop what appears to be a tumbledpile of stones, a summit cairn at ngr(national grid reference) sh 711130.Survey teams involved in the 1930s and50s retriangulation were instructed toput the new trig stations, as far aspossible, on the sites of the old. To do this the original trigonometricalstations were excavated, and the surveymarkers buried by the first surveyteams located and used to position thenew concrete pillars. The pile of stones

at the summit of Cadair Idris beneaththe later concrete trig pillar representsthe tangible remains of the earliertrigonometrical station. It is not theonly vestige of the surveyors’ timespent occupying the summit there in the early 1800s.

As well as the summit cairn, otherstone-built structures are visible in the immediate vicinity of thetrigonometrical station. The low-roofed “bothy” or hut frequented bywalkers today may well be the same“small hut built near the mountain top as a place of shelter to tourists”, as the os later recorded in 1858 in adescription of all the stations used inthe “principal triangulation” compiledunder the then director general of theos, Henry James. Nearby, however, isanother, smaller stone structure,circular and with low walls. Hardly distinguishable among the

rocky ground of the summit of CadairIdris, it is unrecorded and unknown onthe heritage environment record forWales. It has within it a small fireplace,with the remains of an iron hearth – inother words a site of occupation. Closeto the summit cairn of thetrigonometrical station, these stoneremains have all the characteristics of a structure built by the earlytrigonometrical surveyors. It wouldhave been a place of refuge while theirobservations were carried out, offeringsome protection for them and theirinstruments on what is an exposed and isolated site. While these unrecorded remains

on the summit of Cadair Idris are as yet not fully evaluated, their likelyconnections with the trigonometricalwork of the early os are furtherendorsed by parallels with siteselsewhere. Much better known inScotland as “Colby’s camps”, surveysites associated with the early os havebeen identified from archaeologicalfieldwork and aerial reconnaissance. Of these, the camp at Creach Bheinn in Argyllshire (ngr onm 879576) hasgained statutory protection by beinglisted by Historic Scotland (Canmorercahms (nm85ne 2).

Below and below left:Remains of structurewith stone-builtfireplace and ironhearth, Cadair Idris– former shelter foros survey team?

Left: RamsdenBoard of Ordnancetheodolite (1791) atthe Science Museum,London

Left: Detail showingtrigonometricalstation at summit ofCadair Idris onOrdnance SurveyOld Series one inchmap of England &Wales, Sheet 59neMachynlleth,surveyed 1816–34,revised 1834

Left: Summit cairnwith trig pillar onCadair Idris

Here, the summit site has aparticularly impressive set of stone-built structures, including substantialwindbreaks, the remains of the summitcairn itself, and a range of circular-formed stone walls built to protect thesurveyors’ tents. The interpretation of these physical remains at CreachBheinn is assisted by a contemporaryillustration of the camp published in1862 in an Aide-Mémoire to theMilitary Sciences. Equally, otherwritten contemporary accounts by thesurveyors about their time on summit-top survey stations in Scotland, andindeed elsewhere in both Britain andIreland, reveal the substantial nature of these sites. The survey work waslengthy and arduous, requiringtemporary camps as well as the morepermanent trigonometrical stations. The surviving structures of the

camp at Creach Bheinn are paralleledelsewhere too in Scotland, for exampleat Beinn an Oir, Jura (ngr nr 495749),and on Ben Alder (nn 496718), but their identification in other parts ofBritain and in Ireland has been perhapsrather overlooked. Yet, as the examplefrom Cadair Idris suggests, there isother survey archaeology still to bediscovered in the field, for those willingto go and seek it out.

Lasting testimony The challenge then is to begin to seekand record these landscape legacies ofthe os, to identify sites associated with the early os, and to survey theirremains. This is not as difficult orarduous as may first appear. The 21st-century field archaeologist interested

in “surveying the surveyors” has at theirdisposal the means by which to identifythe trigonometrical stations and locatethem in the field. Over the past few years, historic os

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Below: “Camp of theparty employed onthe OrdnanceSurvey” at CreachBheinn, in Aide-Mémoire to theMilitary Sciences byJames Weale (1862)

Right: Sub-circularstone structures onlower slopes belowsummit cairn andtrig station atCreach Bheinnsurvey camp

Above: A substantialstone wall crosses theshallow valley atCreach Bheinn,Argyllshire,probably built as a windbreak

maps have become more accessible tousers through online resources andplatforms. While not all are free-to-view, many are, including the NationalLibrary of Scotland (nls) historic map viewer (http://maps.nls.uk/geo/explore/#). Such online resources for locating the landscape legacies ofthe os provide archaeologists withviewable digital copies of early osmapping. They also, by usinggeographical information systems(gis), enable the historic osmaps to becompared with modern aerial imagery.Features shown by the historic map canbe juxtaposed against the landscape asit is today.Moreover, the nls “Explore

georeferenced maps” viewer containsos one-inch to one mile maps whichhave been “georectified”. These yieldmodern coordinate information for allthe features they show, including thesites of early trigonometrical stationsmarked by the triangle with dot symbol

triangulation, just as in Great Britain.The contemporary written account

of principal stations in Britain andIreland, compiled in 1858 under James,refers often to particularly impressivestructures marking the trigonometricalstations, in some cases cairns clearlyhaving been specially constructed forthe purpose. Similarly, accounts of lifein the field by surveyors such as JosephPortlock, who worked in Ireland andScotland under Thomas Colby, refer to these survey camps, as well as theinfrastructure associated with them. This monumentalising of the os in

the field through its early 19th-centurysurvey work is a lasting testimony tothose involved in mapping the nationsome 200 years ago. Today, these sites and monuments of the early oshave become almost forgotten. Theydeserve greater archaeologicalrecognition, repaying closer study inthe field. In so doing, the archaeologistwilling to “survey the surveyors” willuncover an undervalued aspect ofBritish and Irish field archaeology,while at the same time enduring thesame outdoor challenges and enjoyingthe same landscape vistas as the ossurveyors did themselves.

Keith Lilley, recently elected as a cbatrustee, is professor of historical geographyat Queen’s University Belfast

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Above: nlsExploregeoreferenced mapsviewer for theCheviot os trigstation of os six-inchmap (1888–1913)overlaid on modernaerial imagery;mapped circularstructures west of theword “Cairn” showclearly

Below: proniHistorical maps viewer showing early os summit stationon Slieve Donard, Co Down. The principal triangulation station, laterthe site of a concrete trig pillar (green peg), was at the centre of a largecairn (large red circle); small circle highlights sub-circular stonestructure perhaps associated with Colby’s 1820s survey

even in the 19th century. The principalstations of the os listed and describedin 1858 are all therefore easily identifiedon the maps that were producedthrough the survey work. Equally asimportant, they are now identifiable onthe ground and in the local landscapeby reading their modern coordinatesoff from the nlsmap viewer. Even before setting a foot outdoors,

the nlsExplore georeferenced mapsviewer with its aerial imagery enablessome useful site reconnaissance oflikely survey camps: the imagery itselfis often sufficiently high-resolution topick out structures in the landscape.This is the case not just for GreatBritain but for Ireland too. This isimportant, as the trigonometricalsurvey of Ireland undertaken by the osin the 1820s and 1830s formed part ofthe overall work of the os, at that timeunder the auspices of Colonel Colby. For Ireland, there is similar survey

archaeology still to be explored in thefield – and similar online resources tothe nlsmap viewer are available, suchas the Ordnance Survey of Ireland (osi)Geohive and the Public Record Officeof Northern Ireland (proni) Historicalmaps viewer. Both of these platformscontain early osmaps (particularly the first edition six-inch to one milemapping of the 1830s–40s) and high-resolution aerial imagery. Laying thehistoric map layer over the modernimagery reveals otherwise unrecordedarchaeological structures on summittops in Ireland used for the principal