Short Introduction to Aerial Archaeology
Aerial photographs of archaeological features in Britain, Europe and in other parts of the world, show there to have been occupation of much of the land from Neolithic times onwards. In some places this is represented by apparently isolated sites, in others there are networks of tracks, fields and settlements that show there to have been a higher population than at present.
In 1858, Gaspard Felix Tournachon took aerial photographs from a captive balloon, having already applied for a patent on the idea of using the photographs for the production of maps. Henry Negretti took the first recorded aerial photographs of Britain in 1863, during a balloon flight over London. The military potential of aviation was soon recognised, however, and this resulted in many technical developments related to flight. The first aerial photograph of Stonehenge was taken from a military balloon in 1906, and archaeologists began to realise the potential of aerial photography during these very early days of aviation. The 1914-18 war saw considerable developments in aviation and produced the pioneers of aerial archaeology, such as O.G.S. Crawford and G.W.G. Allen, who were among the first individuals to regularly record archaeological sites from aircraft.
Besides vertical cameras, the early photographers also used hand-held cameras and simply took oblique photographs, at an angle from the aircraft. While this type of photography results in distortion of features caused by perspective, it is still regularly employed by archaeologists as it is flexible and cost efficient. The Second World War also saw great developments in terms of aircraft, camera, and film technology. The use of vertical photography was developed, which results in what amounts to photographic maps of large blocks of the landscape. This is achieved by mounting cameras, which point directly down to the earth, onto an aircraft and taking a series of overlapping images at regular intervals. This technique also allows any specific image on a flight path to be viewed stereoscopically, giving a "pop-up", three dimensional view of the landscape. While this technique was developed for reconnaissance over enemy territory during the war, the RAF subsequently photographed almost all of the country in the immediate post-war period as part of their National Survey. The survey created what is effectively a "snap-shot" photographic Doomsday Book of the country in 1946. The photographs taken during this, and subsequent surveys, offer a unique and valuable resource which records the evolving landscape of the country in the early post-war era.
In 1945, J.K. St Joseph, who was then a lecturer in Geology, began a programme of aerial reconnaissance for Cambridge University which was to result in the establishment of the Cambridge University Committee for Aerial Photography (CUCAP). The survey was multi-disciplinary in nature, recording natural landscapes as well as archaeological and historical sites. The CUCAP collection is extensive and now contains over 400,000 photographs of the British Isles, Ireland and parts of the continent. Many of the recorded cropmark sites in Essex were first discovered by CUCAP during regular flights over the county between the 1950s and 1970s.
The Royal Commission on the Historical Monuments of England (RCHME) curates a national record of information, in various forms, relating to ancient monuments and historic buildings. The National Monuments Record (NMR) contains a collection of around 3,000,000 individual vertical aerial photographs and some 600,000 oblique photographs which cover all of England. The vertical collection consists of material taken by the RAF, the Ordnance Survey and various commercial companies, while the specialist oblique photography, which embrace archaeological, architectural and landscape subjects, results from the work of the RCHME’s ongoing programme of reconnaissance and various regional aerial photographers. The collection is held at the National Monuments Record Centre in Swindon.
That we are now able to identify these features is due to cause and effect. Most of what we now record from the air was originally made by digging into the ground – ditches that defined ownership boundaries, pits cut for storage or burial, and post holes that supported uprights for houses and other structures – although foundations and walls are sometimes also visible. These old holes in the ground, now backfilled, levelled, and invisible to the ground observer, comprise a huge percentage of the surviving sample of past communities that is available for study.
The remains of past human activity can be seen from the air in a number of ways. In addition to churches, castles and other old buildings, partly eroded remains of past structures, in the form of earthen mounds and banks, can also be viewed. These sites are known as "earthworks". More elusive, however, are the remains of buried archaeological sites which, in certain conditions, are best viewed from above. When arable fields are ploughed, it is often possible to view the flattened remains of old buildings, roads and field boundaries because of the difference in the soils and materials which make up the remains.
In arable land, the fill of these holes may hold a higher moisture content which can affect the growth of some crops above them, and this differential growth may be recorded on aerial photographs. On some soils, past features may be seen in winter as different coloured soils. The fill of a ditch, for example, may be a dark organically rich soil that shows against a light-toned natural background.
For example, the site of a demolished house might be recognisable from the air due to the concentration of brick, tile and other debris which is visible because of variations of colour and texture. These sites are known as "soilmarks".
Perhaps the most remarkable type of site, however, are those known as "cropmarks". In this instance, variations in the sub-soil caused by buried archaeological features results in differential crop growth.
In early summer, as crops begin to ripen, the ditches, walls and pits of past settlements, fields and worship places affect the rate at which the crops change colour, and the speed and height to which they grow. For example, the crop over a buried ditch will result in a taller, larger plant because the ditch will contain additional moisture compared to the soils around it. Conversely, the buried remains of a wall will encourage water to drain from the soil, and possibly interrupt the root growth of the plants, therefore resulting in smaller, weaker crops. While most crops can produce marks, cereal crops, and wheat and barley in particular, give marks of especially good definition and resolution. The appearance of cropmarks is enhanced by dry weather when the ripening crops are short of water, and the differences in the condition of the crop can become very marked in drought conditions. From the air, these ditches and pits appear as lines and dots of differently coloured crops, which represent past, and usually hidden landscapes of the county.
Cropmarks can also be formed, in much the same manner, by geological features, however. In addition, modern agricultural practices can also leave marks and patterns which add confusion to a field containing cropmarks. Specialist interpretation of cropmark features is therefore crucial if archaeologists are to get the best information available. While geological cropmarks can often mask or confuse archaeological features, they can also give useful information about the location of a prehistoric site in the natural landscape in which it existed.
If you wish to learn more about aerial archaeology, mapping and how archaeological sites become visible from the air, we recommend you consult the following web sites compiled by members of AARG:
- Aerial Archive – Aerial archaeology in Austria
- Royal Commission on the Ancient and Historical Monuments of Wales (Aerial Survey pages)
- Irwin Scollar’s solutions to problems caused by different national map projections and their incorporation in Airphoto.