Aerial Survey

techniques aerial 1Toby Driver carrying out an aerial survey from a light aircraft across the Wexford coastline

Aerial photographs have many uses and the phrase that a 'picture is worth a 1000 words' is never more true than for aerial photographs. They provide the landscape view and site context, the building blocks for broad-brush landscape characterisation and understanding the historic landscape. The bird's eye view is a powerful way of exploring sites and landscapes, and for certain types of sites (e.g. cropmarks) is the only effective way of discovering monuments and placing them on record. Beyond the archaeological uses for recording during primary reconnaissance, interpretation and mapping, they provide excellent materials for teaching and illustration.

The use of aerial photographs in archaeology has a history extending back more than 100 years and is recognised as one of the most effective ways of recording sites and landscapes. Archives of aerial photographs are a rich source for identifying otherwise unknown monuments and can provide unique records of landscapes and sites that have been changed or destroyed, while new aerial photography provides a means of recording during primary archaeological reconnaissance. Aerial photographs result from two ways of recording the ground, firstly routine survey to photograph a pre-defined area of land (e.g. area-coverage vertical, usually for planning/cartography/military intelligence) and secondly archaeological reconnaissance by an airborne observer who photographs objects seen and understood to be of interest.

Aerial photography

Aerial survey image of Skomer Island, WalesAerial survey image of Skomer Island, Wales

Aerial reconnaissance is widely used around the world and is part of the wider discipline of ‘remote sensing’, surveying archaeology in the landscape without actually touching it, as one would do in an excavation. ‘Aerial archaeology’ encompasses a wide variety of survey and recording activities, from observing the landscape from above and actually taking the pictures to interpreting and mapping sites from the photographs taken. Aerial photography captured from a fixed-wing aircraft remains one of the most powerful tools to document and monitor the coastal heritage of Wales and Ireland. ‘Oblique’ aerial photographs taken at an angle to the ground give a more realistic landscape view of sites and monuments. ‘Vertical’ aerial photographs are taken looking straight down and look more like a map.

Carrying out surveys using a light aircraft means that hundreds of miles of coastline can be covered during periods of just 3-4 hours. The elevated perspective helps to clarify the layout of complex monuments, or show up features on a site which may be hidden from view or difficult to access at ground level.

Times of flights will vary with the seasons. Winter and spring is ideal for the photography of upstanding earthwork monuments, when low vegetation and low light allows all the details of a site to be picked out. Flat light or overcast conditions are preferred for recording monuments for Structure from Motion 3D modelling. Flights in summer droughts can reveal ‘cropmarks’ of buried or lost elements of an archaeological site, often with remarkable clarity.

There is plenty to see when flying over the coastal, intertidal and maritime zone. As well as reconnaissance for, and discovery of, timber and stone built fish traps or wrecks and hulks, the search can be successfully extended for some distance offshore through shallow seas on very calm days when coastal waters may be remarkably clear. This is particularly important for recording wrecks which may show well against sandy sea beds.

Photos taken within CHERISH of eroding coastal archaeological sites will stand as a record of the condition of a monument long into the future, allowing comparison with historic aerial photographs taken from the 1940s onwards and charting future change. Powerful software also allows individual aerial photos taken from a drone or light aircraft in orbit around a site to be combined into a highly accurate 3D rotatable model (a process known as Structure from Motion).

Cropmarks

aerial survey crop marksWhen archaeological features are buried they can affect the growth rate of the crops above them. The presence of features such as buried wall foundations or compacted floor surfaces produce a reduction in the soil depth and lower moisture levels than the surrounding land. Crops immediately above these features tend to have reduced growth rates in comparison to the plants above of no archaeological activity, producing “negative cropmarks"

In contrast areas where ditches, pits and other features have been dug into the subsoil become filled over time. This relative increase in soil depth and the potential to provide increased soil moisture enables the crops above to grow higher and ripen later than the plants around them, producing “positive cropmarks”. Both negative and positive cropmarks are more easily detected from the air and are usually visible during times of drought when crops are at maximum stress.

Soilmarks

soil marks cross sectionSoilmarks Over time human activity has the potential to disturb the local soil profile. As humans dig pits or ditches into the soil or introduce new stone structures they can affect the viable appearance of the soil at the surface. Features such as pits and trenches over time become in-filled with material often different in nature than the surrounding undisturbed soil, including differences in texture (e.g. grain size) or colour. Buried structures such as walls and compacted stones can be brought to the surface by ploughing and are often brighter that the surrounding soil. Soilmarks are usually present after ploughing in the autumn or spring.