Archaeological fieldwork is not limited to the ground. Aerial photography or reconnaissance is a way of investigating sites from the air without the need for test trenches and mapping. It is a useful way of revealing buried or partially destroyed features that are only visible from above.
Photographs taken from the air help make sense of anomalies and reveal previously undiscovered sites. Although often limited by seasonal or weather conditions, new technology is helping to expand the scope of this method of non-invasive fieldwork.
What is Aerial Photography?
Aerial photography was developed as fieldwork technique in the 1920s. Planes had been first used for aerial reconnaissance during the First World War when it was discovered it was possible to obtain a full and detailed views of large areas of land from the air. It was quickly realised that this method could be used effectively in archaeology, particularly in rural areas as features hidden on the ground were often revealed from above.
In 1929 OGS Crawford published the first manual for archaeological aerial photography, establishing aerial reconnaissance as an established method of fieldwork. But the technique required more than the ability to take a good black and white photograph. The photographer needed a basic understanding of the geology, topology, history and archaeology of the land he was surveying so that they could spot any anomalous features. These anomalies revealed themselves in different ways, according to the nature of the site, the time of year and the quality of light.
Crop Mark Sites
As the name suggests, crop mark sites are generally on agricultural land where ploughing has removed surface features. Beneath the ground hidden features such as pits and walls may remain. These features reveal themselves in an aerial reconnaissance through differences in the growth rate of plants. A pit will allow for deep root penetration and high moisture content, resulting in taller, lusher plant growth. Buried features such as walls mean a shallower depth of soil leading to stunted crops.
Such sites can only be investigated during the summer months and over several years to ensure patterns are consistent.
Soil Mark Site
Ploughed out features also reveal themselves from the air in other ways . Fill from pits or burnt fragments and fragments of stone can all reveal themselves on aerial photographs as variances in soil colour. Viewed from the air, these soil marks can indicate the location of potential features. Investigating soil mark sites has helped archaeologists identify many roman villas that may otherwise have been lost.
Shadow Sites
Often archaeological features remaining on the surface are indistinguishable from the contours of the land-unless viewed from above. In the right light conditions, bumps can be highlighted and pits revealed in shadow, allowing archaeologists to trace the remains of buildings and human activity.
Low, winter sunlight is the optimum for revealing the features of shadow sites. Frost and snow also help create a greater contrast. Once again, these essential features limit the timeframe in which such sites can be photographed.
Shadow sites are often known sites too large to be fully appreciated from the ground-such as roman forts. Photographs of shadowy aerial features have also helped identify lost sites which may otherwise have remained hidden in the landscape- such as the medieval village of Wharram Percy.
Modern Developments
Many aerial photographs are still of the same basic monochrome first taken in the 1920s. But modern technology also now plays a part. The advent of digital photography means features can be enhanced on computers to make them clearer.
But aerial photography is also advancing in other ways. Satellite images are useful for producing photographic images of large sites by recording the intensity of light and infrared radiation from the earth’s surface. These readings are then translated into images with strongly contrasting colours that reveal features. Using this method, LANDSAT revealed many 112 previously undiscovered Mayan sites in South America. It also helped prove that the Mayan system of farming was a settled one by identifying walled field systems and house mounds previously unnoticed during ground investigations.
Sources
•Greene, K (1995). Archaeology an Introduction. BT Batsford Limited: London
Renfrew, C and Bahn, P (1991). Archaeology: theories, Methods and practice. Thames and Hudson: London
Natasha Sheldon - A writer since 2000, Natasha Sheldon holds a BA Hons in ancient history and archaeology and MA in ancient history and historiography.
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