Aerial Photographs in Archaeology
The Origins and Development of Archaeological Aerial Photography
The first aerial photographs were taken before the invention of the aeroplane, from balloons, usually filled with hydrogen or coal gas. Although balloons were used to observe enemy positions during the Boer and American Civil Wars, there is no evidence aerial photographs were taken for military purposes until the First World War. The association between aerial photographs and warfare continued during the Second World War, during which the new discipline of aerial photography, and related skills of aerial observation and air photograph interpretation rapidly developed.
The first aerial views of an archaeological site were of Stonehenge, captured from a balloon over 100 years ago in 1906. However, it was after the First World War that the value of aerial photographs for archaeology was recognised. Wartime methods of observation, interpretation and mapping were applied to archaeology in the 1920s by a new breed of professional archaeologist, such as OGS Crawford, who became the Ordnance Survey's first archaeological officer. The discovery of sites such as the 'lost' Stonehenge Avenue, widely publicised in 1923, demonstrated that an aerial observer could confidently and accurately identify archaeological sites not readily visible on the ground.
After the Second World War, more civilian flying took place and increasing numbers of discoveries were made. Aerial photographers discovered thousands of new sites, but also demonstrated the threat posed by agriculture, quarrying and development to buried archaeological sites.
The application of aerial information to the broader historic information is also widening. In addition to finding new sites, regular aerial reconnaissance is a cost-effective way of monitoring the condition of a large number of known and protected sites, has been used in large-scale landscape surveys and has even proved effective in providing new perspectives on historic buildings and cityscapes.
Types of Evidence
(Figure 1: How cropmarks are formed. The archaeology beneath the topsoil can affect how the crop above the ground grows, creating impressions that can be viewed from above)
The early success of aerial archaeology was due in part to the recognition of how buried archaeology presents itself, most significantly in the formation of cropmarks in cereal crop. Cropmark formation is dependant on unpredictable factors such as rainfall and soil-moisture deficits.
In largely pastoral areas, such as Exmoor, cropmarks are rare but not unknown. For instance, cropmarks have revealed prehistoric enclosures around Roadwater as the land has been turned to the plough. Cropmarks remain one of the most important forms of evidence, but other equally variable factors influence the success of aerial photographs for archaeology. For example, ploughing can create soilmarks, differences in colour and texture between archaeological features and the surrounding soil revealing the buried remains. Changing light conditions are important in creating the shadows that reveal and emphasise subtle earthwork sites. Different weather conditions, such as snow, can also highlight earthwork archaeological remains and extreme drought can create parchmarks in grass conditions, revealing sites in unusual locations, such as the site of a probably Romano-British farmstead on Stoneditch Hill, just outside the National Park (see image to left; Copyright Historic England - see below).
Types of Aerial Photograph
There are two main types of aerial photographs used in aerial archaeology; vertical and oblique photographs.
Vertical photographs are taken looking directly (vertically) down at the ground below, giving the observer a map-like view of a landscape at a uniform scale. They are rarely taken specifically for archaeological purposes, instead being taken as part of large-scale surveys for military, cartographic or civil engineering purposes. For instance, the Ordnance Survey has commissioned vertical photograph sorties to update their mapping information. Any archaeological features captured within a frame are entirely incidental. Nonetheless, they are a rich source of information. Most parts of the country have been photographed several times since the 1940s, so verticals offer chronological depth, capturing changes in the landscape that are not reflected in contemporary mapping. In addition, they are usually taken so that each frame overlaps the next by 60%. All parts of the ground are therefore covered by at least two photographs taken from slightly different angles, allowing them to be viewed through a stereoscope. In this way any two adjacent photographs can be combined to form a three-dimensional image, allowing archaeologists to interpret and record any subtle archaeological features visible, or create accurate plans and measurements.
In contrast, oblique photographs are normally taken specifically for archaeological purposes. They are taken at an ‘oblique’ angle, usually from the open window of a high-winged light aircraft. The oblique view is a more familiar way of seeing the landscape, but perspective is enhanced, with features closest to the camera appearing largest and scale varying across the frame. Overlapping obliques can also be viewed through a stereoscope. Archaeological reconnaissance occurs at much lower altitudes than vertical photography, so more detail can be seen in the photograph. As they are taken specifically for archaeological purposes they are also taken from the most rewarding angles for light and shadow. However, the visibility of archaeological features is extremely dependent on the time of year and the prevailing weather and light conditions. As archaeological reconnaissance is a selective process, the photographer only recording what sites he or she notices, aerial photograph interpreters analysing the images at a later date occassionally identify sites on the edges of the photographs that were not seen when the image was taken.
Interpretation, Recording and Mapping
Aerial photographs can reveal a great deal about our historic landscape. They can also reveal features that may mislead the viewer and which must be interpreted with care. For instance fungus rings can look like round barrow ring ditches, geological banding can resemble the ditches of hillforts and even agricultural activity can create the illusion of buried remains which may ‘mislead the unwary’.
Today, aerial archaeologists interpret both new and historic photography, many of which date from the 1940s or earlier, to create synthetic maps of an area's archaeology. Using 'historic' aerial photographs allows the interpreter to record features which may have been lost since the photograph was taken, such as a Second World War searchlight at Holcombe Water, thereby providing a fuller picture than may be seen on any single aerial image. In this way the information held in thousands of aerial photographs is made accessible in a single map. However, previously unseen collections of aerial photographs may become accessible, changing seasons and agricultural regimes mean new cropmark sites are recorded every year, and what is valued as 'archaeology' is constantly changing. The process of aerial archaeology can never be fully completed.
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Image 2: Parchmark of the enclosure on Stoneditch Hill, RAF 82/1281 (F22) 0054 (29 August 1955), © Historic England (RAF Photography)