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State of Knowledge and Current Debates
Aerial archaeology (AA) uses photographs, and other kinds of image acquisition, in archaeological field research. It involves taking photographs of the land from above, examining them for pertinent information, interpreting the images seen there and making the resulting data available in a variety of forms to develop archaeological knowledge about past people and the conservation of archaeological sites and landscapes (Bewley and Rączkowski 2002).
Why Can We See Variety Types of Sites?
Since people first learnt to fly, it has been appreciated that traces of early human activity can be observed from the air, recognized from their curved or linear shapes. Humans have always exploited and adapted the environment to their own needs. The surface of the ground has been disturbed and altered by generations of previous occupants, who have dug into it to create foundations, ditches and pits, and raised structures upon it, in the form of stone buildings or earth ramparts and mounds. All this activity has caused “injury” to the land. Much of it has been subsequently covered over or levelled by later exploitation, particularly agriculture. Today, in the majority of such places, there is little sign of this past human activity on the surface, but the “scars” remain beneath and these may show up from the air (Wilson 1982).
Development of Techniques: History
The first known aerial photographs of archaeological sites were taken between 1899 and 1911 (Forum Romanum, Tiber delta, Pompei, Ostia) and in 1906 (Stonehenge). The hot air balloons were used to take cameras into air. The First World War advanced the development of both aeroplanes and cameras. The number of pioneers using aerial photographs to search, identify, and document archaeological sites increased (T. Wiegand, L. Rey, G. Beazeley, A. Poidebard, C. Schuchhardt). O.G.S. Crawford (1923) made a significant contribution to the methodology and its application in research. In the 1920s and 1930s, photographs were taken of archaeological sites (e.g., hillforts) across Europe, excavation work in progress was documented from balloons and aircrafts (e.g., Biskupin in Poland), and aerial surveys led to the discovery of new sites (e.g., Woodhenge – UK, Ipf near Bopfingen – Germany). Similar surveys were also successful in the USA (C. A. Lindbergh, N. Judd), Mexico (A. V. Kidder, P. C. Madeira Jr.), and Peru (G. Johnson).
Developments in both technology and the interpretation of aerial photographs (e.g., the Allied Central Interpretation Unit) during the Second World War enhanced the technique and established it after the war as a primary research tool in archaeology. Although political regimes in some European countries severely restricted overflying, it developed without major interference in the UK (J.K. St Joseph, A. Baker, J. Pickering, D. Riley, D. Wilson), France (J. Baradez, R. Agache), West Germany (I. Scollar, R. Christlein, P. Filtzinger, O. Braasch, K. Leidorf), Belgium (C. Leva, J. Semey), and Denmark (H. Stiesdal). The 1994 Klienmachnow conference (in Germany) was a key moment in raising awareness of AA among archaeologists from Central, Eastern, and Southern Europe.
Stereoscopy was a successful technology especially applied during WWII. Nowadays, it is frequently used in AA when working with vertical photographs to give an illusion of depth. A 3D effect can be achieved using two photographs offset by 60%. The stereoscope shows the left eye one photograph and the right eye the second, the brain then creates a 3D image of the area.
Since the late 1960s, AA has seen dramatic technical advances. In addition to the traditional platforms (e.g., kites, model planes, balloons, aircrafts, helicopters), remote sensing now makes use of multispectral imagery captured by satellites to explore past landscapes and features at a wide range of scales. Satellites (since 1960s) orbiting at 600–1200 km from the earth’s surface have recorded a wealth of information.
The declassification by the USA in 1995 of an archive of images acquired by the first generation of US photo reconnaissance satellites (CORONA – 1960 and 1972) and the KH-7 GAMBIT and KH-9 mapping camera programs in 2002 was a milestone for archaeologists who quickly recognized the potential of these archives for extensive survey coverage of the Earth, including territories currently lying in no-fly zones (e.g., Turkey, Syria, Armenia) (Ur 2003). The ERTS satellite (later renamed LANDSAT) was launched in 1972 to continually photograph the Earth’s surface. Many countries and organizations have sent satellites equipped with cameras and sensors into orbit to acquire information on surface events by using electromagnetic radiation across the spectrum (Parcak 2009; Lasaponara and Masini 2012).
Use of the wider spectrum of different bands of wavelengths of electromagnetic radiation (daylight, infrared, ultraviolet, thermal radiation) means that AA can be classed as a method of remote sensing. Radiation of different wavelengths detects different physical features. The majority of satellite survey work in archaeology has focused on the band of visible light to detect archaeological features and past landscapes. However, visual data are only a small proportion of what cameras and other sensors can detect. A multispectral scanner registers a small number of bandwidths. By comparison, a hyperspectral scanner registers 100 or more bandwidths – including those which are beyond the visible spectrum, e.g., radar, ultraviolet, thermal radiation.
For assessing what can be detected, two parameters are especially important – spectral resolution and spatial resolution. Spectral resolution denotes the detection that is possible owed to the chosen wavelength. The range of visible light is from 0.380 to 0.780 μm (panchromatic image). If a sensor registers visible light, then it covers four channels (spectrum bands) – blue (0.45–0.52 μm), green (0.52–0.60 μm), red (0.63–0.69 μm), and infrared (0.76–0.90 μm). The spectral response pattern of soil is generally governed by the properties of the soils: color, texture, structure, mineralogy, organic matter, free carbonates, salinity, moisture, and the oxides/hydroxides of iron and manganese. Thus, analysis of results from parts of the spectrum provides information about the physical-chemical characteristics of any detected features. For example, analysis of green and red bands may give information on the contents of iron (Fe) in soil. Normalized Difference Vegetation Index (red and infrared bands) is a method for measuring vegetation vigor, which may indirectly infer the presence of archaeological features.
Ground (spatial) resolution measures the minimum size of a feature detectable on the ground. A feature larger than the spatial resolution will be visible on the image, while a feature appearing smaller than a pixel on the image will not be seen. High ground resolution therefore is extremely important. Images of 80 m resolution (Landsat series MSS 1, 2, and 3) or 30 m (Landsat TM 4 and 5) are sufficient to determine geological or geographical aspects but not to detect archaeological features. Current resolutions can be achieved down to 1 m, which enables individual features such as storage pits, barrows, or sunken houses to be identified. The IKONOS satellite’s panchromatic imaging (the whole visible spectrum, which means more energy reaches the sensor) provides a surface resolution of 1 m, although multispectral imaging resolution falls to 4 m. Satellite images (mostly optical, high resolution) are currently used in prospection of archaeological features, study of their environmental contexts, spatial analysis, past landscape studies, 3D modeling, preservation assessments, protection, and management of archaeological heritage.
LiDAR data also provide additional information on the intensity of the reflected light, as the emitted signal is usually in the near-infrared (NIR) spectrum. It is therefore possible to use it to analyze moisture, chlorophyll content, and other factors that characterizes cropmarks.
AA has been a key branch of field archaeology for more than a century. Its initial success was applied to generating culture history, featuring interpretations based on evolution and diffusion. This took the premise that a photograph is neutral and objective in its representation of the world. As image registration is “mechanical” in nature, photographs were seen as recording real anomalies devoid of a subjective human factor. These anomalies added to the world’s stock of sites and monuments, from which the narratives of prehistory and history can be written. By the same token, repeat visits to certain landscapes led to the realization that the sites were disappearing. A Matter of Time (published in 1960 by the Royal Commission on Historical Monuments of England) established a role for AA in conservation practice in the UK, leading to The National Mapping Programme and the development of set standards.
Processual archaeology was a major factor in the technological “revolution” in AA. It emphasized the objectivity of the research process and the consequent importance of the precise measuring of cultural and natural features. This mission was aided by new analytical technologies, particularly computerized data bases (including mapping) and Geographic Information Systems (GIS). Gaps in the record raised questions about the visibility of sites, and prompted research into formation processes as applied to cropmarks and soilmarks. Results obtained from remote sensing influenced the classification and construction of models describing the relation between cultural systems and the natural environment and the application of technological innovation.
Postprocessual archaeology questioned the “realism” of aerial photographs and emphasized the role of perception and interpretation in the creation of the record. Interpretation issues are the subject of intense discussion on the way the cultural context affects aerial survey, the photoreading process, and their role in forming how we imagine the past (Brophy and Cowley 2005).
Understanding that there is an interpretative process by which the information from aerial photograph becomes an archaeological record is crucial. The interpretation of archaeological features and landscapes is a skill built on experience and knowledge, where intuition and subjective judgment are acknowledged as major factors. The ability of the archaeologist to interpret and depict is as important as the technical processes of rectification and georeferencing.
As archaeological use of digitally recorded data has developed, it has become increasingly clear that it is not an “objective” dataset. Methods of primary data collection and processing parameters have a significant impact on output; the ability to “see” is heavily dependent on software for manipulation and visualization. These factors are a complex mix of objective parameters (e.g., point density) and subjective judgments that are inextricable from the pervasive issue of archaeological interpretation.
Archaeologists decide which platform to use (aerial, satellite, ALS) and which electromagnetic emission to record. Similarly, data-processing, the selection of suitable algorithms, and their mode of visualization are matters decided by researchers. The final image undergoes visual editing and interpretation according to knowledge and interpretation experience and is accepted when a result is deemed to be satisfactory. Thus, like all other forms of archaeological data, the corpus of aerial photographs is the result of reconciling observation and imagination, of matching what we want to know, what has survived, what we currently recognize, and the methods available for their detection and recording. These methods are improving all the time as fresh interpretations raise our expectations and ambitions further.
The enormous dynamics of modern technological changes in the field of acquisition of remote sensing data and their computer processing have influenced the shaping of new trends. On the one hand, these trends are very distant from the traditionally understood aerial photography, and, on the other hand, they do not question its basics, achievements, and identified problems.
Traditional aerial reconnaissance is gradually losing its importance although it does not mean its elimination from the field of interest and reflection of archaeologists. Archaeologists conducting an aerial survey are currently equipped with GPS and also have access to the archaeological heritage database. This allows taking many more rational actions in the field of flight route planning and straight line recognition of selected regions, and as well, making decisions regarding the shooting of visible cropmarks/soilmarks/earthworks. Appropriate software allows quick geotagging of the taken photos and their cataloging. Also aerial photographs taken during a traditional reconnaissance can be used to create 2D orthomosaics and/or 3D models.
Still, one of the most developed methods is ALS. In the applications, special attention is paid to the use of algorithms, which allow us to emphasize specific aspects ALS derived DTM (e.g., Sky View Factor, Local Relief Model, Openness, Cumulative visibility) (Kokalj and Hesse 2017). Variety of ways to visualize data in a greater degree highlights the complexity of their interpretation process. The new challenge is to develop a method of generating DTM for coastal zones (hydrographic airborne laser scanning systems – Doneus et al. 2013) allowing identification of submerged archaeological relics.
Technological changes in many fields of aerial photography applications and remote sensing data open the possibilities and the need for systems which allow for creation of total coverage surveys with the usage of aerial photographs, ALS data, or satellite images. As a result, acquiring large amounts of data requires automated and semi-automated tools allowing image correction and their enhancement leading to management and interpretative mapping (e.g., Verhoeven and Sevara 2016).
Undoubtedly, technology opens new possibilities in recording traces of the past and allows constructing new narratives. However, it cannot be forgotten that in studies about the past, data visualization is something other than traditional, analogue aerial photography. Remote sensing users must be aware of the complexity of the process, including data acquisition → data processing → data visualization → interpretation. This is the subject discussed in the community, and it is essential for conscious and critical studies about the past.
Regardless of the interest in new technologies, the community of aerial archaeologists recognizes the value of historic aerial photographs. Historic aerial photographs, mostly from the World War I, but also taken later during World War II and by military spy missions, are an extremely important source of information about relics of past human activities as well as past landscapes. A great deal of interest about this potential can be attributed to numerous conferences devoted to this topic and publications (e.g., Cowley et al. 2010; Hanson and Oltean 2013; Stichelbaut and Cowley 2016). In addition, historical aerial photographs play an important role in the new currents of archaeology (e.g., archaeology of contemporary past, conflict archaeology).
In Europe, the Aerial Archaeology Research Group is a main platform for discussions on archaeological applications of aerial photographs as well as other airborne and satellite remote sensing data (http://www.univie.ac.at/aarg/).
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