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Airborne investigation

'What's that? The field of ripe barley at two o'clock, two miles, near the L-shaped wood? Those patterns can't be natural ... too regular. Let's have a closer look ... There! A series of rectangles one with a circle inside. We'll have that. ... Opening window! Wing down a bit! ... more... more... that's great!'

Grab the 35mm camera - photograph the target - reach behind you for the 70mm camera - wait - it's clearer from this angle, photograph it again. Log your position on the GPS, mark up the report form on the knee pad and go back to scanning, as you search for a new target.

All this is what aerial archaeology, or at least the photographic reconnaissance part of it, feels like. Many people are now familiar with the concept of remote sensing - epitomised for most by the geophysical survey. Well, this is remote sensing too and, lets face it, you can't get much more remote than 2,000 feet above ground level, travelling at 90 miles an hour, constantly flicking your eyes between the map in your hand and the landscape below.

Aerial archaeology sounds - and is - exciting, but there isn't much glamour about it. The inside of a light aircraft is not roomy, especially a two-seater. After three and a half hours you are aching, tired, and in winter pretty cold, but the payback for this discomfort makes it all worthwhile. Most archaeologists do what they do because they relish the thrill of discovery, and that is one thing that aerial archaeologists get more of than most.

To see below you the unmistakable shape of an unknown Roman camp, emerging in outline under rows of sugar beet, or to pick out subtle earthworks highlighted by the play of light and shadow of the low winter sun, can be an unforgettable experience. For now, though, let's get back down to earth, and consider what it is the airborne archaeologist is looking for.

Buried remains

Cropmarks of an Iron Age settlement in Barley  © The way we see things depends on whether or not the archaeological remains survive above ground or have been levelled. Standing archaeological remains are commonly in the form of earth banks, ditches or low walls, often collectively referred to as earthworks. Earthwork photography is best carried out when the sun is low in the sky, either in the winter or late evening in the summer so that the archaeology is picked out in contrasting and complimentary areas of highlight and shadow. Even substantial earthworks can be almost invisible if photographed in flat light, but in the right conditions very slight height differences can be startlingly clear from the air.

When it comes to buried or levelled remains there are many more factors - such as the time of day, the recent weather patterns, soil type, underlying geology, the agricultural regime and more - that determine whether or not the archaeology can be seen and recorded, usually as a cropmark or soilmark. Cropmarks are essentially patterns of differential growth in vegetation that correspond to and are caused by variations in the subsoil. So, for instance, fissures in the underlying bedrock or a man-made trench or pit will often fill with soils and matter that have greater moisture retention and more nutrients than the surrounding, undisturbed subsoil.

Looking for clues in the Wolds  © In drought conditions these moisture-retentive soils hold a reserve of available water that allows the plants growing above to thrive, growing fuller and taller for longer. The rest of the crop suffers stress due to lack of moisture and plants can be weaker, shorter and tend to ripen quicker. Other archaeological deposits, such as compacted surfaces or stone wall foundations, have the reverse effect, causing poor, stunted growth as the plants above struggle to survive.

Many crops will develop good marks when conditions are right, but some of the best - from an archaeologist's point of view - are cereals, sugar beet and peas. Cropmarks can be found at all stages of the growth cycle, from germination to harvest; they are sometimes seen as height differences (rather like seeing an earthwork), sometimes as colour differences - either subtle or vivid. Soilmarks are a little more straightforward. With soilmarks the observer is looking directly at the archaeological deposits brought to the surface by the plough, where they show as colour differences against the non-archaeological plough soil.

Photographic interpretation

An aerial photograph of the Yorkshire Wolds showing a funnel arrangment  © But aerial archaeology is about much more than just flying about taking photographs - in some ways that is the easy part. There would be no point taking pictures if they were just catalogued, filed and forgotten. To really understand what they contain, the photographs have to be studied and interpreted to extract as much archaeological information as possible. This is where experience, archaeological knowledge and often common sense come into play. Like field archaeologists, aerial photograph interpreters need a broad general knowledge of the processes, natural and human, that shape the landscape.

Of course it doesn't end there. The real advantage of aerial archaeology is in the overview that can rapidly be achieved by mapping the archaeology interpreted from the photographs. Mile after mile of previously hidden landscapes can be revealed by careful interpretation and mapping. A terrific example of this is the work done on the Yorkshire Wolds by the former Royal Commission on the Historical Monuments of England. Today this rolling chalk upland is covered in arable fields, and apart from a few medieval village remains and a scattering of prehistoric earthworks, it is hard to see anything much of archaeological interest.

Excavating in the Yorkshire Wolds  © Only when it was mapped from tens of thousands of aerial photographs in the 1980s could we see the incredible extent of the late prehistoric and Roman remains that lay buried beneath all those cereal crops. Thousands of Iron Age square barrows (burial sites), hundreds of farmsteads and settlements, droveways, tracks and field systems were among the sites mapped. The worth of this and subsequent mapping projects has been shown many times over, and first-level mapping is now being carried out for all of England, county by county, as part of the National Mapping Programme of English Heritage.

Picture taking

The ridge of Offa's Dyke  © Before going further, it is important to understand that there are two basic types of aerial photograph used by aerial archaeologists; verticals and obliques. A vertical aerial photograph offers a plan view and is captured by a specially calibrated camera pointing straight down at the ground. Vertical sorties are flown at predetermined altitude and speed in order to achieve a specific nominal scale (eg 1:10,000); the photographs are taken at set intervals to ensure that each frame overlaps the next by 60 per cent. The overlap ensures that each part of the ground appears on three frames (more about this later).

Verticals are rarely taken specifically for archaeological reasons, but are nonetheless a rich source of information. Most parts of the country have been photographed several times since the 1940s, so verticals offer chronological depth, often capturing changes in the landscape that are not reflected in contemporary mapping.

A Roman villa: the light lines reveal buildings and walls  © Oblique photographs are quite simply all those that are not verticals. To take obliques does not require any special equipment; all you need is a good camera, with shutter-speed priority to counter the vibration of the aircraft, and high quality lenses. A head for heights and a strong stomach are also handy. Digital cameras are not yet standard kit, but with rapidly improving technology and lowering prices it will not be long before they are. Oblique aerial photographs usually offer a more familiar view than the straight down perspective of a vertical, but the scale changes continuously from foreground to background, making measurement from the image difficult.

Image mapping

An image of Cawthorn Camps is compiled by mapping  © How then, is the archaeology mapped from all this photography? All photographs, obliques and verticals, are digitally rectified to remove inherent distortions, due mainly to height and camera tilt, and transformed into corrected plan views.

The accuracy of the resultant photo-map is largely dependent on the accuracy of the control that the photo is rectified against, usually Ordnance Survey maps and height data, at 1:10,000 or 1:2,500 scale. For the highest possible accuracy, new vertical photography of the site is taken using a calibrated camera, and control is surveyed on the ground.

A computer software programme automatically generates height information, by comparing adjacent photo frames (remember that overlap?), and a composite orthophoto (a photo-map accurate to a few centimetres) can then be produced to the desired scale (see the image of Cawthorn camp). This is made possible through the use of a device called a stereoscope.

Stereoscopy involves two photographs, vertical or oblique, taken in close succession, each frame capturing a slightly different view of the same subject. They can be viewed stereoscopically, using no props, to recreate in the mind's eye a 3-d model of the landscape - or you can use a stereoscope to make it easier. The Victorians had stereoscopes in their parlours and bought views of famous places to marvel at, and the 'magic eye' craze of a few years ago worked on the same principle.

A stereoscope, at its simplest, is a pair of lenses mounted on a folding stand and costing less than £30. The device helps make it easier to see earthwork archaeology on vertical and oblique photographs, as things appear to 'stand up' out of the photo. Anyone interested in this process can take their own 'stereo pairs' of photographs on the ground. After taking one photo, you just take a couple of steps to the side and take a second photo. Try it next time you are out with a camera. The only tricky part is learning how to view them if you don't have a simple stereoscope, but it isn't impossible.

Satellite images

The streets and buildings of Piercebridge, a Roman town  © Conventional photography is the mainstay of aerial archaeology, but there is tremendous potential for archaeological surveys in other, less familiar, remote sensing techniques.

A lot of interest is being focused on satellite imagery that can offer sub-metre resolutions (ie can show fine detail). This means that most archaeological earthwork remains could be made visible, as could many cropmark sites, providing the imagery is captured at the right time of year, time of day, and in the right conditions. Satellite or airborne multispectral imaging of this kind, used a lot by geologists and environmental analysts, holds great potential for archaeology - but to date its use has been limited.

Another exciting recent development is LIDAR (Light Detection and Ranging), which uses an airborne laser to measure the distance between the aircraft containing the equipment and the ground. This effectively produces a high-resolution model of the landscape that the plane has flown over, which can be lit and viewed from any position. This mapping technology is being deployed by a variety of organisations, such as The Environment Agency and the Unit for Landscape Modelling at Cambridge. The system is so accurate that height differences of a few centimetres can be detected, revealing the most subtle archaeological earthworks.

Yet another technique is that of thermal imaging, or Infrared Line Scanning technology, mainly in military use, which is also successful at detecting archaeology. None of these techniques, however, is likely to replace conventional or digital photography in the immediate future.

Future directions

Dave MacLeod (on the right) with colleague Pete Horne and the Cessna 172  © In the last 80 years aerial photography has done more than any other survey technique to change our perception of the distribution of archaeological remains in Britain. In Europe too, there have been tremendous advances in archaeological knowledge, due to aerial survey.

Indeed, since the collapse of the Soviet Union, vast regions of eastern Europe have opened up to civil aviation, a prerequisite for archaeological reconnaissance. These are being explored from the air for the first time, with rich results. In contrast, some European countries, for example Spain and Greece, still do not allow aerial photography by civilians. Things are changing though. In December 2000, Italy responded to requests from the aerial archaeology community and removed long-standing restrictions that banned aerial photography. Let's hope more countries follow this lead soon.

Aerial survey is one of the most cost-effective methods for the archaeological survey of landscapes. What we must remember, though, is that aerial survey, like any of the other ways of collecting archaeological data, does not hold all the answers. Integration is the key, and in future the most effective results will come out of projects that routinely employ a suite of survey and investigative techniques, tailored to the specific needs of each site or landscape.

Find out more

Books

Ancient Landscapes of the Yorkshire Wolds: Aerial Photographic Transcription and Analysis by C Stoertz (Royal Commission for Historic Monuments of England, 1997)

Air Photography and Archaeology by DN Riley (Duckworth, 1987)

Air Photo Interpretation for Archaeologists by DR Wilson (Batsford, 1982; Tempus, 2000)

About the author

Dr Donald MacRaild is head of history at the university of Northumbria at Newcastle. He is author of Irish Migrants in Modern Britain (Palgrave, 1999) and co-author with Jeremy Black of Studying History (Palgrave, 2000) - as well as a number of books and articles on aspects of Irish migration and labour history.