Mapping ancient landscapes: how archaeologists use aerial photographs

 


Introduction

Higher Treworder, Egloshayle. A prehistoric enclosure appears on an aerial photo as two concentric cropmarks. The entrance is on the left hand side, where there is a gap in the circuit of the enclosure. Photo © Cornwall County Council Historic Environment Service
Higher Treworder, Egloshayle. A prehistoric enclosure appears on an aerial photo as two concentric cropmarks. The entrance is on the left hand side, where there is a gap in the circuit of the enclosure. Photo © Cornwall County Council Historic Environment Service

The term aerial archaeology is used to describe the various processes relating to the discovery and recording of archaeological sites from the air. These activities include both the actual taking of photographs and the mapping and interpretation of archaeological sites visible on aerial photos. Aerial survey is one of the most important tools for the discovery of archaeological sites in this country. Each year hundreds of previously unknown sites are discovered and photographed through aerial reconnaissance.

 

Many of the archaeological sites mapped by aerial archaeologists have been levelled by ploughing and are identified from the photos as cropmarks or soilmarks. Cropmarks are patterns in vegetation caused by differences in the rate of growth and ripening of crops such as wheat and barley. These differences can be brought about by the presence of buried archaeological features. A ditch cutting into bedrock, for example, will contain a greater depth of soil than the surrounding area. Crops growing over the ditch will grow taller and ripen later than elsewhere because they are able to tap into extra reserves of moisture.

 

The well preserved earthwork remains of an abandoned medieval longhouse settlement at Trewortha Marsh on Bodmin Moor. Photo © Cornwall County Council Historic Environment Service
The well preserved earthwork remains of an abandoned medieval longhouse settlement at Trewortha Marsh on Bodmin Moor. Photo © Cornwall County Council Historic Environment Service

Soilmarks are visible when ploughing brings to the surface buried archaeological deposits which are a different colour to the surrounding topsoil.

 

Archaeological sites which have not been ploughed down generally survive as low earthworks or slight stony banks. From the air these are best viewed in low light conditions, especially in the early morning or evening.

 

There are two main kinds of aerial photograph; oblique images which are taken specifically to record archaeological sites and vertical photographs which are usually taken for other purposes. Vertical photography generally covers large areas of landscape; oblique photographs are usually taken at a much lower height than verticals and show much more detail. However they are taken of specific sites rather than large areas and are therefore far more selective in their coverage. In the past most aerial photographs were taken in black and white, but the development of digital photography means that most modern photography is shot in colour.

 

Aerial archaeology has a hundred year history. The development of aviation, initially in balloons and later aeroplanes, allowed unique birds-eye views of familiar places to be captured. From this new perspective archaeological features that had not been recognised before on the ground were first identified and aerial archaeology was born.

 

Aerial reconnaissance developed during the Great War of 1914-1918 to meet the needs of military intelligence. At this time O G S Crawford, an observer in the Royal Flying Corps, first recognised the real potential of aerial reconnaissance for archaeological purposes and became the pioneer of aerial archaeology. After the Second World War a growing number of archaeologists continued to take aerial photographs. Notable among these was Professor J K St Joseph who carried out systematic reconnaissance work which forms the basis of the important collection of aerial photos held at Cambridge University.

 

An example of mapping from Cornwall’s National Mapping Programme. This example shows prehistoric , medieval and industrial features in the Godolphin area.
An example of mapping from Cornwall’s National Mapping Programme. This example shows prehistoric , medieval and industrial features in the Godolphin area.

 

In 1965, the Royal Commission on the Historic Monuments of England (RCHME) established a national archive for aerial photographs which was taken over by English Heritage in 1999. This archive contains three million photos, making it the largest collection of aerial photographs in the country.

 

Recognising the need to interpret and map the mass of archaeological information contained in this vast collection of photos, RCHME and English Heritage initiated an ambitious programme of archaeological mapping from aerial photographs - the National Mapping Programme. The overarching aim of the Programme is to map, interpret and record all archaeological features visible on aerial photographs to a consistent standard.

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