Aerial survey for archaeology

AERIAL SURVEY FOR ARCHAEOLOGY

Robert H. Bewley ([email protected])

English Heritage

(Based on papers read at the Thompson Symposium of the Remote Sensing andPhotogrammetry Society at Loughborough University on 6th April 2002 and at a jointTechnical Meeting of the Remote Sensing and Photogrammetry Society and the Royal

Institution of Chartered Surveyors on 7th November 2002)

Abstract

Aerial photography for archaeology has been developing its approaches

and techniques over the past 100 years so that it now integrates the results ofreconnaissance with extensive interpretative and analytical surveys. Thispaper introduces the philosophy and approach of the English Heritage (EH)Aerial Survey team, covering aerial reconnaissance and the National

Mapping Programme (NMP), as well as the potential developments andopportunities in Europe. In the 1980s there was a debate over the nature ofthe evidence derived from aerial photographs, especially how to describe

archaeological features. As part of NMP a classification and recordingsystem has been devised which meets most of the users’ needs, be theynational organisations, county archaeologists, commercial contractors or

university-based researchers. The maps and records produced by NMP areused to further our understanding of the past human settlement in England,not only at the individual site level, but also in regional or landscapecontexts. This paper provides an overview of the current progress of NMP

and acts as an entree for explaining the current research and recording ofarchaeological landscapes throughout Europe. Recent developments inBritain and Europe have provided the opportunity for a greater priority to be

given to aerial survey and accelerating programmes of mapping. In Europethe ending of the cold war has allowed greater access to aerial photographsand the possibility of beginning new reconnaissance, as well as introducing

new forms of remote sensing. All these developments have led to atransformation of our understanding of prehistoric, Roman and medievalarchaeology.

Keywords: aerial photography, archaeology, interpretation, mapping,National Mapping Programme (NMP), photogrammetry, reconnaissance

Introduction

This paper is based on two presentations during 2002, one to the Remote Sensing andPhotogrammetry Society and the other to a joint meeting of the same society and the

Photogrammetric Record, 18(104): 273–292 (December 2003)

Photogrammetric Record, 18(104), 2003 273

Royal Institution of Chartered Surveyors. Rather than focus on the technical aspects ofarchaeological aerial survey, the paper focuses on the contribution to knowledgethrough interpretation and synthesis which archaeologists are now providing throughaerial survey. The reaction to the presentation at the Thompson Symposium confirmedthat conveying the value of the work undertaken is as important as detailing the meansby which the information is obtained. The technical specification can be obtainedthrough other sources, in particular a revised manual for the National MappingProgramme (NMP) that is being revised in 2002/3.

At the turn of the millennium there was much speculation that the 21st centurywould be a new dawn for technological advancement and that old methods of flyingaround in aircraft taking photographs using traditional films would be superseded by anew digital age using satellite and airborne sensors which can record everything thatmight possibly be wanted. There was a danger that the baby would be thrown out withthe bathwater, and from a great height, and that traditional aerial survey had had itsday. Archaeologists have always been at the forefront of new developments in thequest for more information about their sites or landscapes. With regard to aerial survey,however, the wealth of information collected over the past 100 years has yet to beexamined, analysed and understood, so there is a balance between developing ourknowledge, based on the existing information and embracing new technologies.

Background

An aerial photograph of Stonehenge, taken from a balloon in 1906, is oftenconsidered the first archaeological aerial photograph in England (Fig. 1). Develop-ments in photography, cameras and aircraft in the First World War accelerated the use

Fig. 1. Stonehenge from an army balloon, taken in 1906. Probably the earliest aerial photograph of anarchaeological site in the National Monuments Record for England. English Heritage. NMR.

Bewley. Aerial survey for archaeology

274 Photogrammetric Record, 18(104), 2003

of aerial photography and there was one man in particular, O. G. S. Crawford, whopioneered the technique for archaeology (Crawford, 1928, 1929; Bewley, 1997;Bowden, 2001).

The first major landmark in the brief history of aerial survey was the publicationin 1928 of Wessex from the Air (Crawford and Keiller, 1928) which highlighted thepotential of aerial reconnaissance and mapping for archaeology. The second landmarkwas the publication in 1960 of A Matter of Time (RCHME, 1960). This seminal workprovided important clear messages that aerial reconnaissance was an essentialtechnique in archaeological investigation; that river valleys and especially the gravelterraces contained dense concentrations of archaeological sites (visible as cropmarksand only understandable from the air, or through large-scale excavation); and that theseareas were under enormous threat from mineral extraction. The sands and gravels werein huge demand for the re-building programmes and the creation of new towns afterthe Second World War. However, the significance of these discoveries could not berealised unless the information from the aerial photographs could be interpreted,classified and mapped in a usable form (see RCHME, 1960). Before this could happenthe material had to be made accessible and it was a far-sighted move in 1965 to create alibrary of aerial photographs as part of the National Monuments Record in England(Hampton, 1989).

The response to the recommendations (RCHME, 1960) was to appoint a few morestaff to the Ministry of Works Inspectorate with the aim of excavating more sites inadvance of destruction. With hindsight it is easy to see that the response to this workshould have been to follow the recommendation that ‘‘the requirement for continuingair-survey and air-photographs at increased tempo is self-evident from the foregoingstudy.’’ and not just to excavate (although excavation was also a strong recommen-dation). This was a time when excavation was synonymous with archaeology and itshould be no surprise that it took another 15 years before the next report on cropmarksites in a river valley was prepared (Benson and Miles, 1974), followed rapidly byLeech (1977), and other attempts to examine these types of sites in a landscape context(Riley, 1980).

The next major milestone in developing our understanding of the aerial evidencewas the volume on the environs of the hillfort at Danebury, Hampshire (Palmer, 1984),some 24 years after A Matter of Time. The classification, analysis and approach in thiswork provided a good model for future surveys that aimed to understand landscapes,and the sites within them, as multiperiod and dynamic (in that the landscapes havechanged, from the Neolithic period to the present day).

Since 1984 then there has been a growing understanding that interpretation,synthesis and analysis of aerial photographic evidence must have a higher priority thanhitherto. This has partly been as a result of EH’s Monuments Protection Programme(MPP) (Fairclough, 1996), but also because of a growing realisation within theprofession that understanding the nature, extent and significance of the aerial evidenceis a fundamental requirement for the conservation and management of archaeologicalsites (Bewley, 1993, 1998a, b).

Aerial Reconnaissance for Archaeology

Archaeological aerial reconnaissance is a topic which has been covered manytimes elsewhere, both in terms of results and techniques (Crawford, 1954; St Joseph,



1966, 1981; Whimster, 1983; Griffith, 1990; Featherstone et al., 1999). There havebeen so many changes to the landscape in terms of crop conditions (as a result of theEuropean Common Agricultural Policy), climate change, aeronautical restrictionsbeing lifted (or changed) that a 50 year campaign is often required to really begin tounderstand an area. There is one example worth highlighting to show the potential andthe dynamic nature of aerial survey.

The example is the Cherwell valley in North Oxfordshire (Featherstone andBewley, 2000). Here a combination of factors led to a series of remarkable discoveriesbeing made in the 1990s. The first change was the transfer of EH aircraft operationsfrom Biggin Hill to Oxford (as a result of relocation of staff out of London) in 1990.The second major change was on an international scale—the ending of the cold waralso in 1990—which led to the closure of the Upper Heyford (USAF) airbase and thusallowed access from the air to a previously inaccessible landscape. In themselves thesefactors would not have been enough for the new discoveries to be made. The thirdstroke of luck was the series of hot dry summers in the early and mid-1990s. The hotdry summer of 1989 was an annus mirabilis for archaeologists (Griffith, 1990), as thedrought conditions started early and lasted so long that many new sites werediscovered. The warm dry summers continued with 1995 and 1996 in particular beingvery hot and dry.

Aerial reconnaissance in the Cherwell valley not only dramatically increased thenumber of known sites but also transformed our understanding of the Iron Agesettlement in southern England. Hitherto the distribution of Iron Age settlements ofone particular type, known as banjo enclosures (because the circular part looks like thesoundboard and the trackway the fretboard of a banjo) (Fig. 2), did not extend into thispart of England. Fig. 3 graphically illustrates the quantity and distribution ofarchaeological sites in this small area.

There have been many other examples of important discoveries from the air overthe past 70 years, but discovery is only the beginning. Another very important aspectof aerial reconnaissance is monitoring the condition of archaeological sites from the

Fig. 2. An Iron Age settlement in North Oxfordshire of a type often referred to as a banjo enclosure. Thiswas one of many enclosures found in the hot dry summers of 1995 and 1996, and is unusual as the outer

ditches encircle the inner settlement. SP 3431/12. NMR 15350/33. NMR � Crown Copyright.



air. The Royal Commission for the Ancient and Historical Monuments in Wales(RCAHMW) pioneered the process of monitoring scheduled (that is, legally protected)ancient monuments from the air. EH now devotes part of its annual programme ofreconnaissance to this monitoring role but often the changes to a site are subtle andonly noticeable over a number of years. Monitoring from the air is a small part ofassessing the condition of sites, as the major resource is devoted to field visits bytrained staff known as Field Monument Wardens. Less frequently the aerial photographshows a dramatic and devastating change to a site, as in the example of the CistercianNunnery at Legbourne, Lincolnshire shown in Figs. 4 and 5. In this instance the earlierphotograph (Fig. 4) had been taken as part of routine reconnaissance but wheninformed of the damage further photographs were taken in October 1988 (Fig. 5),which led to a prosecution and a substantial fine for damaging an ancient monument.This site has been monitored regularly since.

It has always been the case that the flying and the photography have taken centrestage, whilst the next stages in the process, the interpretation and mapping, althoughrequiring the majority of the resources (in terms of manpower and equipment) have,until recently, been less prominent. In EH, of the resources available for aerialsurvey, about 20% is spent on aerial reconnaissance, and 80% on mapping andinterpretation.

It is this interpretation, mapping and synthesis which will form the major effortover the coming years.

Fig. 3. Distribution of archaeological sites discovered up to 1996 in North Oxfordshire. � EnglishHeritage.



English Heritage’s National Mapping Programme (NMP)

The aim of NMP is to enhance our understanding of past human settlement, byproviding primary information and syntheses for all archaeological sites andlandscapes (visible on aerial photographs) from the Neolithic period to the 20th century(Bewley, 2001).

The programme has its origins in the projects already mentioned (especiallyDanebury) but also in the Yorkshire Wolds survey (Stoertz, 1997) and the pilot projectsfunded by EH for the MPP. In 1985 the earthwork and stonework sites on Dartmoorwere mapped from aerial photographs. In 1988 the former Royal Commission on the

Fig. 4. The remains of the Cistercian Nunnery, founded in ad 1150, at Legbourne, Lincolnshire. Thisphotograph was taken before the medieval fishpond and other earthwork features were damaged by theconstruction of an ornamental pond. TF 3684/23 NMR 4189/04. 6th June 1988. NMR � Crown

Copyright.

Fig. 5. Even fewer remains of the Cistercian Nunnery, Legbourne, Lincolnshire. The (non-archaeolo-gical) excavation for an ornamental pond took place some time between June 1988 and September 1988and was recorded soon after. TF 3684/8. NMR 4261/21. 10th October 1988. NMR � Crown Copyright.



Historical Monuments of England (RCHME) and EH ran cropmark classificationprojects in three areas: the Thames Valley (Fenner and Dyer, 1994), Kent (Edis andMacLeod, 1989) and Hertfordshire (Fenner, 1992). In 1989 they expanded the scope toinclude earthwork and stonework sites in the Yorkshire Dales (Horne and MacLeod,1995).

These projects led to the creation of NMP in 1992 and current progress, with over22 projects either completed or ongoing, means that around 30% of England has beenmapped to an NMP standard or equivalent (Fig. 6). The standard mapping scale is1:10 000 (Fig. 7) and there are standard recording systems so that the site descriptionsand interpretations are systematically and consistently recorded. Throughout NMP’sdevelopment there has been the need to classify the sites being interpreted and recordedin a repeatable and systematic way, so that subsequent analysis of the records can beaimed at improving our understanding of past human behaviour. Classification allowsgroups of sites to be identified and studied in a landscape context rather than asindividual sites. The basic recording system and the assumptions behind it have beenpublished (Edis et al., 1989) but a brief description of the approach to recording isprovided here.

The basic means of classification used in NMP are not only monument type, dateand form but also location, shape and size. The underlying assumption is that sites of asimilar shape and size (and in similar locations) might have had a similar date and/orfunction. This belief is derived from existing archaeological practice in categorising

Fig. 6. Progress of English Heritage’s National Mapping Programme (NMP) to 30th September 2003.� English Heritage.



sites into classes, such as henges, barrows, Roman camps, and Roman forts, but wasadopted as a means to an end and not an end in itself. The aim of morphologicalclassification is to attempt to understand classes or groups of sites, in a landscapecontext (to explore their distribution and associations), where the evidence is oftensolely from aerial photographs. It is recognised that information about archaeologicalsites and landscapes derived from aerial photographs has its limitations: the majority ofsites are undated, except by association, some are undatable without furtherinvestigation, and the sites seen are only a small percentage of what may haveexisted or of what is visible using this technique (Carter, 1998).

Therefore, the approach that has been developed is to record and classify sites in asystematic way (defining circular or rectilinear features and attributes about size andlocation) and then to decide on an interpretation. This method allows for subsequentanalysis to create groups of sites, based on location, size and shape (which may changethe interpretation for any individual site, subsequently) and provide for anunderstanding of sites within a landscape context. The hypotheses thus generatedcan be tested by other research projects. The information has been used by MPP todetermine which sites and types of sites are worthy of protection through scheduling orother means.

Fig. 7. An example of NMP transcription based on interpretations of aerial photographsfor the Stonehenge area. � English Heritage.



Examples of this approach, the analysis involved as well as the results, haverecently been published (for example, Stoertz, 1997; Jones, 1998; Winton, 1998;Crutchley, 2001) and an early example can be found from research in the Solway Plain(Bewley, 1994). Here the small square enclosures (of around 1000 m2) were identifiedas a group, and one of the sites had been excavated and dated to the 3rd–4th centuryad and was thought to be a Romano-British farmstead. By association it was possibleto extrapolate that the others in the group had a high probability of having the samedate and function.

Another example, from the Thames Valley NMP project, is that of the HighworthCircles. These are circular (earthwork and cropmark) sites which derive their namefrom the early interpretation that they were concentrated within the medieval hundredof Highworth, a small market town in north Wiltshire. The analysis (Fenner and Dyer,1994) shows that these sites form a coherent group of 51 sites which each have adiameter of between 40 and 90 m and have no discernible entrances. They appear to bemorphologically distinct from henge monuments and their distribution now extendswell beyond the Highworth hundred. At present they have not been securely dated andfurther research has increased the number of sites to 68 (Wilson, 1999) but they remainan enigma in terms of date and function. Their form suggests a prehistoric origin butthe most likely explanation is that they are some form of medieval stock enclosure.

An article using information from the Yorkshire Dales project (Horne andMacLeod, 2001) examines the results relating to coaxial field systems andmorphological recording that highlighted 12 sites of rectangular ground plan ofunknown function and date. The sites are remarkable for the consistency of theirground plan with internal subdivisions, which suggests they are of similar date andfunction probably dating to the Roman or (more likely) medieval periods. There are, asyet, no examples of this type of site known beyond the Yorkshire Dales project area,which helps to emphasise the regional nature of much of English archaeology. WhenNMP has covered a greater proportion of the country this aspect of regional variationsthrough time and space can be explored.

In examining one landscape, that of the Salisbury Plain (military) Training Area incentral southern England, the combination of aerial survey and fieldwork developed anunderstanding of the ancient landscapes on the Plain. The area is unique in southernEngland in that it is chalk downland, unploughed since the Army bought the land in1897. Thus the approach to investigating and understanding its past required theintegration of many techniques. The results have now been published (Crutchley, 2000,2001; McOmish et al., 2002). One important contribution from aerial survey was toidentify a series of curvilinear enclosures, in three size groups (McOmish et al., 2002).The interpretation and analysis revealed two important aspects of this series; one wastheir location on promontories overlooking the vale below. The other was the date, inthe Iron Age (from 1000 bc up to the arrival of the Romans). By grouping these sitestogether on the basis of their morphology it was possible to suggest that they werecontemporary. Although this is not proven, some sites had been excavated(Widdington Farm, Chisenbury Field Barn, Warren Hill and Everleigh) and shownto be from the Middle to Late Iron Age, so there is a strong suggestion that the othersare also Iron Age in date.

Other examples of this approach to analysis and understanding can be found inPalmer (1984), Whimster (1989), Fenner (1992), Fenner and Dyer (1994), Stoertz(1997), Boutwood (1998), Jones (1998), Winton (1998) and Deegan (1999).



The surveys and research undertaken so far for NMP have also shown us thatnot all the features which make up the historic environment are easily classifiablebut that they need to be recorded in a more general way to assist future moredetailed survey and investigation. This is especially true of landscape features (suchas field systems and linear boundaries), industrial complexes (lead mines, tin mines,copper mines and salt production (Grady, 1998)) and military sites (airfields, ranges,training areas, defence structures) where the significance is the extent and layout ofthese large archaeological sites. Boutwood’s (1998) paper on linear boundaries is agood example of using NMP data to re-evaluate the existing evidence for multiple-ditched boundaries (now mainly visible only as cropmark sites) and providing a newhypothesis for their function. The ability to use the NMP maps and records to examinelandscape zones means that these linear features are no longer regarded as largeterritorial markers interconnecting large tracts of lowland England on a tribal orregional scale, but much more as local land divisions for smaller groupings of peopleand their livestock.

The maps, records and reports from NMP can be used in a variety of ways andmost commonly they form part of the relevant Sites and Monuments Records. Themaps are now available digitally and can therefore be used for GIS applications.Manipulation of the data allows the creation of maps for all the sites in an area, whichis very useful for examining distribution patterns and relationships with soils andtopography. Alternatively selections can be made for particular periods for closerexamination and study of particular aspects of landscape development.

Over the past 3 years EH has allocated an increased resource (in part through itsArchaeology Commissions Programme) to NMP. As Fig. 6 shows, 22 projects havebeen completed and new ones begun (especially a large one in Norfolk) so thatapproximately 30% of England has now been covered.

Finally there is huge potential in aerial survey for very detailed interpretations ofsites which can be mapped and recorded with great accuracy. There have been manyexamples of these over the years (for example, Soffe, 1988; Bewley and Fulford, 1996)but the project on Cawthorn Roman Camps, which was completed in 2002, hasproduced some very good results. The work has been undertaken by Jane Stone andMick Clowes of EH’s Aerial Survey and Metric Survey teams in the York Office. Afull report detailing the results (Stone, 2002) provides an overview of the project,summarised below.

Photogrammetric Survey at Cawthorn Camps, North Yorkshire

The research at Cawthorn is a joint initiative between the North York MoorsNational Park and EH, to produce a revised management plan for this nationallyimportant site. It is a multi-disciplinary project combining excavation, geophysicalprospection, ground and aerial survey (Wilson and Lee, 2002). Digital photogram-metry is being used to produce a plan of the earthworks at a scale of 1:500 with aprecision of 10 cm or better.

Cawthorn Camps are a series of Roman earthwork enclosures, probably datingfrom the late 1st and early 2nd centuries ad comprising two forts, one with a laterannexe, and a coffin-shaped camp. Although the majority of the earthworks are Romanin date, there is evidence of pre- and post-Roman activity including at least oneprobable sunken featured building of 9th–11th century date (Wilson and Lee, 2000).



Some of the earthworks at Cawthorn were excavated in the 1920s by F. G. Simpson(Simpson, 1926) and Sir I. A. Richmond (Richmond, 1932).

Digital photogrammetry was used to produce a plan of the earthworks usingcolour vertical aerial photography specially commissioned in May 2000 at a scale of1:1600. The quality and scale of the photography are such that even individual rabbitholes and tree stumps are visible. The 3D stereomodels from which plotting isundertaken are viewed on a Leica Helava Systems Digital PhotogrammetricWorkstation using polarising glasses.

One of the additional benefits of working in a digital environment is the facility tomanipulate the imagery to create further products, in particular an orthophotograph.The orthophotographic image of the site (Fig. 8) is a rectified, georeferenced versionand was produced from portions of 15 vertical aerial photographs.

To produce an orthophoto a digital elevation model (DEM) of the area is firstrequired. This can be created automatically by the photogrammetric software andconsists of a grid of 3D points, the density of which can be dictated by the operator. TheDEM used to produce the Cawthorn orthophoto was generated at a 2 m grid interval. Inaddition, a trial was undertaken to see whether a denser DEM would pick up the subtleembanked features within the main defences. Fig. 9 is an extract of the orthophotooverlaid with a DEM taken at a 0Æ5 m grid interval from which contours have beeninterpolated. Low turf banks representing the remains of structures of possible Romandate are clearly being picked up and some of these are only 20–30 cm high.

The aerial imagery and DEM can also be used together to create 3D perspectiveviews of the site. The imagery is ‘‘draped’’ over the DEM by the software and can beviewed from varying elevations and aspects as dictated by the operator.

Fig. 8. A digital orthophoto of the Roman Camps at Cawthorn, North York Moors. � English Heritage.



The photogrammetric plotting of archaeological earthworks at such large scaleand to such a high degree of accuracy has rarely been undertaken before. At the start ofthe project it was uncertain how much earthwork detail would be visible on thestereomodels, especially given the nature of the rough moorland vegetation atCawthorn, but the results have far exceeded expectations. Not only have the maindefensive earthworks been successfully mapped, but also many other earthworkfeatures including slight banks of about 10 cm in height.

The work at Cawthorn illustrates the levels of detail and accuracy that are possiblefrom using a combination of low-level vertical stereoscopic aerial photographs, digitalphotogrammetry and skilled air photo interpreters. When the resulting plan is analysedtogether with all the existing information about the site (from previous fieldwork andexcavation) it will be an enormous benefit to those responsible for the futuremanagement and presentation of these important earthworks.

Future Potential

Building on the foundations laid in the 20th century, aerial archaeology is nowbeginning to explore other forms of airborne remote sensing. With the support ofgrants from NATO and the EU’s Culture 2000 programme, a workshop was held inLeszno, Poland in November 2000 (Bewley and Raczkowksi, 2002) which discussed

Fig. 9. Digital elevation model of part of Cawthorn Camps. � English Heritage.



future practice for aerial survey in Europe. The topics covered the geographicalexpansion of the technique into former eastern bloc countries (Gojda, 2002; Korobov,2002; Kuzma, 2002; Urt�aans, 2002) and the developments of new techniques. Onepromising new sensor is lidar (light direction and ranging) (see Holden et al., 2002)which uses laser pulses to record the landscape below, measuring very small variationsin the shape of the earth’s surface and thus revealing archaeological sites which maynot be visible to the naked eye. Research is now being undertaken to compare thecontribution of this technique on well-surveyed landscapes (for example, Stonehengeand Avebury) with the results of aerial survey and field investigation. Preliminaryexamination shows that archaeological features are visible using lidar on particulartopographies where aerial reconnaissance is limited (Fig. 10). Lidar will also have animportant role to play in monitoring archaeological landscapes, especially if erosion ofthe sites is suspected. An example already published, for Newton Kyme Roman fort inNorth Yorkshire, shows that there is vestigial earthwork survival, on a site which hasonly previously been visible as a cropmark site (Holden et al., 2002). Other sensors,for example, the compact airborne spectographic imager (CASI) and thermal imaging(TI) sensors, are also being tested (Shell, 2002) and they too are proving very useful inexpanding information about archaeological sites and landscapes.

Aerial survey for archaeology does have worldwide potential: the pressures on thelandscape in western Europe or the Middle East are very damaging to archaeological

Fig. 10. An example of the output of a lidar survey in the Stonehenge area. It is hoped that this type ofimagery will be of benefit for detecting vestigial features and thus important for the management of

sensitive archaeological landscapes. � English Heritage.



sites. A programme of aerial survey for archaeology has been running in Jordan for thepast 5 years where, even in remote parts of the desert, large landscapes are under threatfrom agricultural expansion and experimental farming. Fig. 11 shows one of manynew sites discovered during recent surveys, adding to the already large numbers ofsites in Jordan which require protection before they are destroyed.

Conclusion

At the turn of the millennium some considered aerial survey for archaelogy to beout dated. They thought that the future lay in new technologies such as satellite imageryand other airborne remote sensing. The British Academy sponsored a small workingparty to assess the current state of aerial survey in Britain which concluded that thefuture lay in combining the old and the new and that the opportunities and challenges forthe future lay in the better organisation of skills and resources (British Academy, 2001).A key element to this is better integration of all the various techniques of archaeologicalinvestigation and research, with a common aim of improving our understanding of thehistoric environment.

Similarly the key to developing our understanding is our ability to interpret theinformation we have collected over many years. All forms of remotely sensed datarequire interpretation, and it is extracting as much archaeological information from asmany sources as possible that is the challenge for the coming century.

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Fig. 11. In 1999 reconnaissance over northern Jordan led to the discovery of this Roman fort (probablydating from the 2nd century ad) at Khirbet Ain. � Robert Bewley. 15th June 1999. RHB/SL 99/4/10.



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Resume

L’emploi des photographies aeriennes pour l’archeologie s’est deve-loppe au cours des cent dernieres annees au point d’arriver maintenant aintegrer techniquement les resultats de la reconnaissance classique avec les

releves analytiques et interpretatifs concernant de grandes etendues. Onaborde dans cet article la methode et la philosophie de l’equipe de leveaerien du service du Patrimoine anglais, en y incluant la reconnaissance

aerienne et le programme national de cartographie (NMP), ainsi que lespossibilites et les occasions de developpement en Europe. Il y a eu dansles annees 1980 un debat sur la nature des temoignages apportes par les

photographies aeriennes, en particulier sur la facon de decrire les elementsarcheologiques. On a pu concevoir, dans le cadre du NMP, un systemed’enregistrement et de classification qui satisfait la plupart des besoins desusagers, qu’il s’agisse d’organismes nationaux, d’archeologues dans les

comtes, de contractants commerciaux ou de chercheurs universitaires.L’emploi des enregistrements et des cartes etablies par le NMP facilite notrecomprehension des implantations humaines du passe en Angleterre, non

seulement au niveau d’un site individuel, mais aussi dans leur contextepaysager ou regional. On fournit dans cet article un apercu des progresactuels du NMP et une cle permettant d’expliquer les recherches en cours en

Europe et les saisies sur les paysages archeologiques. Les developpementsrecents effectues en Angleterre comme en Europe ont permis d’accorder unepriorite plus grande aux leves aeriens et ont accelere les programmes decartographie. La fin de la guerre froide a ouvert en Europe un acces bien

plus grand aux photographies aeriennes et permet d’entreprendre denouvelles reconnaissances et de recourir egalement a de nouvelles formesde teledetection. Tous ces developpements nous ont amene a modifier notre

comprehension de l’archeologie pre-historique, romaine et medievale.

Zusammenfassung

Die Luftbildphotographie fur die Archaologie hat in den letzteneinhundert Jahren ihre Ansatze und Techniken so weit fortentwickelt, dasssie heutzutage die Erkundungsergebnisse mit ausfuhrlichen Interpretationen

und analytischen Untersuchungen erganzt. Dieser Beitrag fuhrt in diePhilosophie und die Vorgehensweise der Arbeitsgruppe fur Luftbildvermes-sung des staatlichen Denkmalschutzes ein. Es werden die Erkundung aus der

Luft und das Nationale Kartierprogramm (NMP), sowie weitere potentielle



Entwicklungen und Moglichkeiten in Europa vorgestellt. In den 80er Jahrendes 20. Jahrhunderts gab es eine Debatte uber die Art der Nachweise, die aus

Luftbildern gewonnen werden konnen, insbesondere daruber, wie archaolo-gische Merkmale beschrieben werden konnen. Als Teil des nationalenKartierprogramms wurde ein Klassifizierungs- und Erfassungssystem aus-gedacht, das die meisten Nutzeranforderungen erfullt, seien es nationale

Organisationen, Archaologen auf Kreisebene, kommerzielle Anbieter oderuniversitare Forscher. Die Karten und Dokumente, die durch NMP produziertwerden, werden zur Vertiefung unseres Wissens uber fruhere Besiedelung in

England, nicht nur an individuellen Orten, sondern auch im regionalen oderLandschaftskontext verwendet. Dieser Beitrag gibt einen Uberblick uber denaktuellen Fortschritt von NMP und fuhrt erklarend in die aktuelle Forschung

und Erfassung archaologischer Landschaften in Europa ein. JungsteEntwicklungen in Großbritannien und Europa haben die Moglichkeiteroffnet, dass die Luftbildvermessung mit hoherer Prioritat zur Beschleuni-gung von kartographischen Programmen eingesetzt wird. Durch das Ende

des Kalten Krieges in Europa ist sowohl der Zugang zu Luftbildern erweitertworden, als auch die Moglichkeiten geschaffen worden, neue Erkundungendurchzufuhren und neuen Formen der Fernerkundung anzuwenden. All diese

Entwicklungen haben dazu beigetragen, unser Verstandnis der prahistori-schen, romischen und mittelalterlichen Geschichte zu verandern.

DISCUSSION

(The following discussion concluded the joint Technical Meeting of the RemoteSensing and Photogrammetry Society and the Royal Institution of Chartered Surveyorson 7th November 2002)

Richard Hartley: Can you use aerial photography to extract features from shallowwaters or mud flats? I’m thinking about Woodhenge; I’m thinking about the ScottishBoat.

Bob Bewley: Yes, indeed. The lecture that I have just given, is a cut-down versionof the 4 h one, which would have included a section on the inter tidal zone. We havedone a lot of work on tidal archaeology and yes there are features that can be seen.Throughout Europe now, they are recording, particularly in the Baltic, down to 5 mbeneath the sea, a number of features, mainly shipwrecks. But in Switzerland there arelakes where you can see prehistoric dwellings or settlements, so yes there is the chanceyou can see under the water, but it depends on where you are, not just the localconditions.

Stephen Booth (Geomatics World): The last 3 or 4 years have seen, I think, fourdifferent campaigns to capture aerial imagery of some sort or other, Ordnance Survey,Getmapping, UKP and we’ve recently had Synthetic Aperture Radar. Is that data ofsome use to you?

Bob Bewley: Yes it is. The problem we have with the National MappingProgramme is that just as we get to the stage where we have reached some kind ofstability, in terms of recording, methodology, or the scope of it, somebody says ‘‘Ohhere’s another layer, or piece of information’’, and of course the staff want to have alook at it. So that any new photography that comes through that is successful, they dowant to have a look at it, almost to eliminate it from their enquiries, if you see what I



mean, because you’ve got to look at it to examine what’s there, if there is nothing therethen that’s great. If there is something there, then that’s even better, so yes that isimportant.

Richard Wylde (ExxonMobil): Have you made any use of lidar?Bob Bewley: I meant, in the ‘‘future’’ bit of my presentation, to mention lidar

because I have PowerPoint slides of lidar, but not old technology slides of lidar. Yes wehave made use of it. We have published a short article on it, in the proceedings of aNATO conference, but we commissioned a lidar survey in the Stonehenge area, partlyto test it, if you like, to destruction, because if we targeted Stonehenge’s landscape, oneof the most surveyed landscapes anywhere in Europe, and we found new informationfrom lidar, then it shows the technique has some use. We are just proposing to publishthat in Antiquity early in 2003.

Richard Wylde (ExxonMobil): I’m thinking, particularly in the inter-coastal area.Bob Bewley: Yes, we haven’t yet analysed that data, but we know it exists with the

Environment Agency staff who are working quite closely with lidar. We could almostgive up what we’re doing and just focus on lidar, it is such a powerful new technique.

Paul Bryan (English Heritage): Bob, could you just outline how you produce theline-based transcriptions, that form the backbone of the National Mapping Pro-gramme?

Bob Bewley: That’s a technical question, I said I wasn’t going to answer technicalquestions (laughter). Yes, they use Autocad 2002 and the computer program they useto do the transformation is called Aerial, now in its fifth version, which was developedby John Haigh of Bradford University, specifically for archaeologists; to create atransformation we need a minimum of four control points, and it’s better to put five, asyou all know. It’s very simple to use, you just put it on a laptop or a PC. There isanother programme developed by Irwin Scollar, called ‘‘Airphoto’’, which some of ushave used and is equally good. So it’s ‘‘horses for courses’’.

Paul Bryan: … and following on from that, can you outline your application ofphotogrammetry?

Bob Bewley: By our application of photogrammetry, do you mean in terms ofCawthorn Camps? Which I didn’t mention, or the relation we have with your unit inparticular (laughter)? Yes, before we were merged (EH and RCHME) we did have aphotogrammetric capability, and when anybody needed a very detailed map of, forexample, the Honister Slate Quarries in Cumbria, we were able to use photographs toproduce a very, very detailed drawing, and that’s what we are now doing with Paul’ssection, at Cawthorn Camps in particular, and reference is made to this work in thearticle (above). Also on, as it were, dangerous landscapes, there’s an Alum mine on thecoast of North Yorkshire, where the field surveyors don’t really want to go, in case theyfall into the sea, so the more we can map from the air the better. Obviouslyphotogrammetry plays an important part in getting a very detailed map and theaccuracies are down to a few centimetres, which for archaeological purposes is morethan adequate. Normally, in terms of the National Mapping Programme, we are happyif we are mapping something that’s within 5 m, because that is determined by thequality of the map, sometimes it’s 10 m. What we find in fact is, usually, it’s within2 m. So we can use that information to either locate a trench, or tie in to measureddistances. Does that answer your question?

Mike Curtis: When I was working for the Ordnance Survey, I was somewhatinvolved with what was then the RCHME, and acquiring selective archaeology and



publishing that on OS maps, but that seemed to fall off over the years. Could you bringus up to date on how the archaeological information is going to be made available tothe public, in a digital form. Is that going to go down an OS route or separate route, ora layer in Mastermap, or what?

Bob Bewley: Yes, I was just talking to somebody today. It could well be a layer inMastermap, but there are discussions going on at the moment. When the ArchaeologyDivision of the Ordnance Survey came over to the Royal Commission, it was alwaysagreed that there would be the flowline of information back to the Ordnance Survey,for them to do with it whatever they wanted (put it on a map or not). The biggestproject seemed to be to create the Roman Map of Britain, which has recently been re-published, so that was a big project between the Ordnance Survey and the RoyalCommission. As far as I’m aware, we have at least two members of staff whose job itis to pass information for all the sites that we have discovered in England through tothe Ordnance Survey. But there is also now a move, for a lot of the information wehave, to be available on the web for people to download it, as and when they need it. Ipersonally think it would be a retrograde step, if that archaeological information wasn’ton the maps, but I have a problem with the new information we find. How do youdecide what to put on and what not? It may be as well to say, let’s put betterinformation about the sites that are open to the public on it, but highlight the website,for people to get more information about that area, so there is a challenge.

Jim Chandler (Loughborough University): As a supplementary to Paul Bryan’squestion, and Paul might wish to contribute, I would like to enquire how much you’reusing hand-held oblique photography for spatial measurement, or are you just usingvertical imagery exclusively?

Bob Bewley: It depends on who took the oblique. We try to make sure when wetake oblique photographs, that they are high enough to have control points on, but lowenough to see the detail you need. Now sometimes that is physically not possible. Theyalso take stereo obliques, in order to get an idea of what’s going on. But it’s acombination of both. So if there are obliques available, we use them. Similarly, if thereare verticals, we use them. For any project it varies and the balance will vary. In theYorkshire Dales there are very few oblique photos, but something like 10 000verticals. So it was something like a 90/10 split. In other areas it’s 50/50.

Jim Chandler:… and also, to what extent are you using oblique archival materialfor measurement?

Bob Bewley: We would use it. When you say archival, do you mean the RAFhistorical? Yes, obliques as well. Except that we can only use such historical obliquematerial as has been catalogued. There is a programme in the National MonumentsRecord of cataloguing all the military oblique photography that was given as adonation by the Ministry of Defence, but that’s a huge task and that’s taken three orfour, maybe even five, years to get through. Where that is available, especially aroundthe coast, it’s very useful to us.

Thanks to Bob Bewley for his presentation were provided by James Kavanagh onbehalf of the RICS and RSPSoc.



Documents

Aerial survey for archaeology