Archaeological ProspectionArchaeol. Prospect. 10, 153–157 (2003)Published online 29 November 2002 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/arp.201

Short Report

Cawthorn Camps, North Yorkshire—aPhotogrammetric Approach†

JANE STONE*English Heritage, 37 Tanner Row, York Y01 6WP

ABSTRACT Cawthorn Camps is a Roman site of probable late first and early second century date comprisingtwo forts, one with a later annexe, and a camp. The site survives as earthworks and, within themain defences, there are many slight embanked structures. The current programme of researchat Cawthorn is a joint initiative between the North York Moors National Park and English Heritage,the aim being to produce a revised management plan for this nationally important site. It is amultidisciplinary project combining the following disciplines: excavation, geophysical prospection,ground and aerial survey. Digital photogrammetry has been used to produce a plan, scale 1 : 500,of the earthworks to a precision of š10 cm. Copyright 2002 John Wiley & Sons, Ltd.

Key words: Cawthorn Camps; Roman forts and camp; earthworks; aerial photographs; digitalphotogrammetry; hachured plan

Introduction

Cawthorn Camps lie within the area of the NorthYork Moors National Park in North Yorkshire.They are situated on the edge of an east–westescarpment commanding excellent views overmoorland to the north (Figure 1). CawthornCamps is primarily a Roman site of probable latefirst and early second century date; it comprisestwo forts, one with a later annexe, and anunusual coffin-shaped camp. The site survives asearthworks and, within the main defences, thereare many slight embanked structures. (Plate 1).

In addition to the Roman earthworks, thereis evidence of pre- and post- Roman activityincluding at least one sunken featured buildingof early medieval date (Wilson and Lee, 2000b).

* Correspondence to: Jane Stone, English Heritage, 37 TannerRow, York Y01 6WP.E-mail: Jane.stone@english-heritage.org.uk†Paper presented at the 4th International Conference onArchaeological Prospection, Vienna, September 2001.

Some of the earthworks at Cawthorn wereexcavated in the 1920s by F.G. Simpson (Simpson,1926) and Sir I.A. Richmond (Richmond, 1932).Aerial photography taken in 1925, speciallycommissioned by the excavators, shows theextensive nature of the excavations, in particularin the western fort and its annexe. Many of thetrenches and spoil heaps resulting from thoseexcavations are still visible today.

The current programme of research at Caw-thorn is a joint initiative between the North YorkMoors National Park and English Heritage, themain aims being to improve academic under-standing, information available to the public andalso to produce a revised management plan forthis nationally important site (Wilson and Lee,2000a). It is a multidisciplinary project combiningthe following disciplines: excavation, geophysicalprospection, ground and aerial survey.

The earthworks at Cawthorn were surveyed ata scale of 1 : 1250 by the Royal Commission onthe Historical Monuments of England (RCHME)

Copyright 2002 John Wiley & Sons, Ltd. Received 1 May 2002Accepted 20 September 2002

154 J. Stone

Figure 1. Location map of Cawthorn Camps. Copyright: March 2002 English Heritage. NMR.

in 1976. This plan was revised by the RCHMEin 1992 for a publication on Roman Camps(Welfare and Swan, 1995). As part of the currentresearch project, further field survey of thewestern fort and its annexe was undertaken by EdDennison Archaeological Services (EDAS) from1998 to 2001 at scale of 1 : 200 and 1 : 500 (EDAS,1998).

In August 1999, prior to commencement of theexcavations in 1999 and 2000, magnetic geophys-ical survey was carried out in two sample areasin the interior of one of the forts and the annexe(Lyall, 2000). The survey was undertaken usinga Geoscan Research fluxgate gradiometer (modelFM36), employing the traverse method of sur-vey. The sensitivity of the machine was set todetect magnetic variation in the order of 0.1 nT.The two areas surveyed proved to be of very lowmagnetic susceptibility, this was aggravated bythe uneven ground conditions and dense moor-land vegetation. Weak anomalies were detectedin both survey areas, although the majority ofthese are thought to represent glacial features. Ifstone structures were present at Cawthorn, resis-tance survey could prove productive, however,

the excavation evidence to date indicates a dis-tinct lack of stone-built structures.

The series of excavations undertaken by DrPete Wilson in 1999–2000 (Wilson and Lee, 2000a)investigated the defences and interiors of the twoforts and the annexe. The majority of the interiortrenches were positioned across features alreadyidentified by previous field survey (Welfareand Swan, 1995; EDAS, 1998). To date, oneof the excavated turf structures has producedan archaeomagnetic date of late first and earlysecond century AD (Wilson and Lee, 2002–2004).As observed above, another of the excavatedfeatures is thought to represent a sunken featuredbuilding of early medieval date (Wilson and Lee,2000b). The excavations have confirmed the slightnature of the turf-built structures.

Photogrammetry

Digital photogrammetry was used at Cawthornto produce a plan (scale 1 : 500) of the earthworksto a precision of š10 cm (Stone, 2002). WithinEnglish Heritage, photogrammetric plotting ofarchaeological earthworks has rarely before been

Copyright 2002 John Wiley & Sons, Ltd. Archaeol. Prospect. 10, 153–157 (2003)

Photogrammetry of Cawthorn Camps 155

undertaken at such large scale and to such a highdegree of accuracy. The Cawthorn project hashad an experimental approach, one of the mainaims being to investigate the potential of detailedmapping of earthworks from specially commis-sioned, large-scale, colour vertical photography,using digital photogrammetric technology. Thephotogrammetric plotting has also been informedby the use of further oblique and vertical pho-tographs ranging in date from 1925 to 2001.Each set of photographs has contributed differentinformation resulting in a greater understand-ing of the earthworks that exist on site today. Theaerial photograph plotting has attempted not onlyto identify and interpret the multiperiod archae-ology at Cawthorn, but also to locate as many ofthe 1920s excavation remains as possible.

The colour vertical aerial photography, usedfor the photogrammetric plotting, was commis-sioned in May 2000 at a scale of 1 : 1600. Thequality and scale of this stereo photography issuch that individual rabbit holes and tree stumps

are visible. The control used to geo-referencethe stereographic models was established via aglobal positioning system (GPS) to an accuracy ofš12 mm. Using a Leica Helava Systems DigitalPhotogrammetric Workstation running SOCETSet and Windows NT 4, the three-dimensionalstereographic models from which plotting wasundertaken were viewed on the workstationmonitor using polarizing glasses. Additionally,the workstation is connected to Microsoft Micro-Station to allow the investigator to produce vec-torized line detail of the archaeology. Figure 2is the plotted drawing for the site, with the linedetail depicting the tops and bottoms of features.The final product will be a hachured plan at ascale of 1 : 500.

One of the benefits of working in a digital envi-ronment is the ability to manipulate the imageryto create further products. Plate 2 is an orthopho-tograph of Cawthorn generated through thephotogrammetric software; it is a rectified, geo-referenced image produced from several vertical

0 m 50 m 100 m 150 m

Figure 2. Interpretative plan of the site produced through the photogrammetric workstation. Copyright: March 2002 EnglishHeritage. NMR.

Copyright 2002 John Wiley & Sons, Ltd. Archaeol. Prospect. 10, 153–157 (2003)

156 J. Stone

aerial photographs. To produce an orthophoto-graph a digital elevation model (DEM) of thearea is required. This can be created automati-cally by the photogrammetric software and takesone of two forms. The first form of DEM is agrid of three-dimensional points, the density ofwhich can be dictated by the operator. The sec-ond form is a ‘triangular irregular network’ (TIN)consisting, as the name suggests, of a network oftriangles. When creating a TIN DEM, the softwarerecords less points in flat areas and more in areasof undulation. The benefit of this is that it canhelp reduce the number of points used, reducingthe size of the DEM file.

The DEM used to produce the Cawthornorthophotograph was a grid DEM generated at1-m intervals. In addition, a trial was undertaken

to see whether a more dense DEM wouldpick up the subtle embanked features withinthe main defences. Plate 3 is an extract of theorthophotograph overlaid with a grid DEM,taken at half-metre intervals, and illustratedas contours. Low turf banks representing theremains of structures of possible Roman dateare clearly being picked up; some of these banksare only 20–30 cm high.

The aerial imagery and DEM can be usedtogether to create three-dimensional perspectiveviews of the site. The images are ‘draped’ overthe DEM by the software and can be viewed fromvarying elevations and aspects as dictated by theoperator, as in Plate 4. Figure 3 shows the manyslight embanked features within the area of thefort and its annexe. This is another perspective

Figure 3. This perspective view has been changed to greyscale and the shadows increased to show the subtle embankedearthworks within the main defences to best advantage. Copyright: March 2002 English Heritage. NMR.

Copyright 2002 John Wiley & Sons, Ltd. Archaeol. Prospect. 10, 153–157 (2003)

Plate 1. Cawthorn Camps from the east comprising, from top to bottom, a fort, a camp and a second fort with an annex.Subtle earthwork features within the main defences are highlighted by the low oblique sunlight. Copyright: SE7890/21 NMR12381/26 07 May 1993 English Heritage. NMR.

Plate 2. Orthophotograph of Cawthorn Camps. The red rectangle shows the location of the extract illustrated in Plate 3.Copyright: March 2002 English Heritage. NMR.

Copyright 2002 John Wiley & Sons, Ltd. Archaeol. Prospect. 10, (2003)

Plate 3. An extract of the orthophotograph overlaid with the DEM seen as contours. Slight turf banks highlighted by the DEMrepresent the remains of structures of possible Roman date. The light feature to the left is one of the excavation trenchesopened in 1999 as part of the current programme of research. Copyright: March 2002 English Heritage. NMR.

Plate 4. This three-dimensional perspective view clearly illustrates the location of the Roman earthworks on the edge ofan escarpment commanding excellent views across the North York Moors to the north. Copyright: March 2002 EnglishHeritage. NMR.

Copyright 2002 John Wiley & Sons, Ltd. Archaeol. Prospect. 10, (2003)

Photogrammetry of Cawthorn Camps 157

view, which has been changed to greyscaleand the shadows increased, highlighting theearthworks to best advantage; the softwareenables us to choose where to put the sun inrelation to the earthworks, in this case low in thenorth-east.

Conclusion

The work at Cawthorn has gone some way todemonstrating the variety of products that can begenerated through digital photogrammetric soft-ware. One of the main uses for orthophotographsand perspective views is in the presentation ofthe site to others, making the archaeological andtopographical information more understandable.In addition, there is the potential to use DEMs,perspective views and the lighting tools in thesoftware to aid the interpretation process.

At the beginning of the project, it was uncer-tain how much earthwork detail would be visibleon the photogrammetric stereographic models,especially given the nature of the rough moor-land vegetation at Cawthorn. The results have farexceeded initial expectations; not only have themain defensive earthworks of the forts and campbeen mapped, but also many slight internal fea-tures, some only approximately 10 cm in height. Itis also worth observing that other recent obliqueand vertical photography, taken of the site inparticularly favourable conditions (low obliquesunlight, low vegetation), has proved invaluablefor the plotting process in terms of identifica-tion and interpretation of features. This clearlydemonstrates the continuing potential of aerialphotography, taken in the right conditions, forenhancing our understanding of earthwork sites.

The aerial photographic interpretation hasrevealed potential new features and further detailfor known features; some of these would bene-fit from clarification through future field sur-vey. Also, many extant earthworks have now

been positively identified as 1920s’ excavationremains. Here the use of rectified photographstaken in 1925 of the excavations, also Rich-mond’s published excavation plan (1932, plateXX), were an invaluable aid to interpretation. Theresults of the aerial photographic interpretation,including the plan, are fully documented else-where (Stone, 2002).

Finally, the research work at Cawthorn hasdemonstrated the benefits of using complemen-tary survey techniques, each of which can con-tribute its own set of data. Combining the variousstrands of data allows us to achieve a more com-prehensive understanding of the site. The projectoverview and conclusions will be collated in thefinal project report.

References

EDAS. 1998. Annexe B Cawthorn Camps, Pickering,North Yorkshire. Earthwork Survey. Ed DennisonArchaeological Services, Report commissioned bythe North York Moors National Park, Helmsley.

Lyall J. 2000. Fluxgate Gradiometer Survey carried outat Cawthorn Camps, North Yorkshire. In CawthornCamps, North Yorkshire. Assessment and Revised ProjectDesign. Wilson P, Lee G (eds). English Heritage: FortCumberland.

Richmond IA. 1932. The four Roman Camps atCawthorn, in the North Riding of Yorkshire. TheArchaeological Journal 89: 17–78.

Simpson FG. 1926. The Roman Camps at Cawthorn,near Pickering. Preliminary Report, 1923. YorkshireArchaeological Journal 28: 25–33.

Stone JL. 2002. Cawthorn camps, North Yorkshire: AirPhotograph Evaluation Phase II. English Heritage:York.

Welfare H, Swan V. 1995. Roman Camps in England.HMSO: London.

Wilson P, Lee G. 2000a. Cawthorn Camps, NorthYorkshire. Assessment and Revised Project Design.English Heritage: Fort Cumberland.

Wilson P, Lee G. 2000b. Cawthorn Camps. TrialExcavations 1999: Interim Report. Roman AntiquitiesSection Bulletin 17: 17–24.

Wilson P, Lee G. 2002–2004. Cawthorn Camps2000—Interim Report. Ryedale Historian 21: 30–33.

Copyright 2002 John Wiley & Sons, Ltd. Archaeol. Prospect. 10, 153–157 (2003)