Aerial archaeology is the study of archaeological sites from the air. It is a method of archaeological investigation that uses aerial photography, remote sensing, and other techniques to identify, record, and interpret archaeological features and sites. Aerial archaeology has been used to discover and map a wide range of archaeological sites, from prehistoric settlements and ancient roads to medieval castles and World War II battlefields.

Aerial archaeology involves interpretation and image analysis of photographic and other kinds of images in field research to understand archaeological features, sites, and landscapes. It enables exploration and examination of context and large land areas, on a scale unparalleled by other archaeological methods. The AARG (Aerial Archaeology Research Group) boasts that "more archaeological features have been found worldwide through aerial photography than by any other means of survey".

Aerial archaeological survey combines data collection and data analysis. The umbrella term "aerial images" includes traditional aerial photographs, satellite images, multispectral data (which captures image data within specific wavelength ranges across the electromagnetic spectrum) and hyperspectral data (similar to multispectral data, but more detailed).

A vast bank of aerial images exists, with parts freely available online or at specialist libraries. These are often vertical images taken for area surveys by aircraft or satellite (not necessarily for archaeological reasons). Each year a small number of aerial images are taken by archaeologists during prospective surveys.

History

Late-19th and early-20th century observations

The origins of aerial photography, which led to the rise of aerial archaeology, began in the mid-19th century with early experiments in capturing landscapes from above. The French photographer Nadar (Gaspard-Félix Tournachon), who is credited with taking the first aerial photograph from a balloon in 1858 over the outskirts of Paris. In America, a man named James Wallace Black, in 1860, became the first person to successfully take aerial photographs from a hot air balloon over Boston. Around the same time, Thaddeus Lowe, a scientist and aeronaut, also used balloons for military reconnaissance during the American Civil War.

In the years leading up to the First World War, several early attempts were made to use aerial photography for archaeological purposes, though they often faced significant technical and practical challenges. One of the earliest figures involved in this effort was Sir Henry Wellcome, a British–American pharmaceutical entrepreneur and philanthropist. In the early 1900s, Wellcome financed several archaeological expeditions in Sudan, where he employed aerial photography using a box kite to document excavations in Jebel Moya, Sudan.

Nevertheless, while the products of these efforts proved the potential value of aerial photographs for archaeological research, these early attempts to obtain usable photographs fell short. Camera technology of the time was insufficient, let alone the erstwhile methods for getting cameras into the air.

World War I and the birth of aerial archaeology

The outbreak of World War I marked a critical turning point in the development of aerial archaeology. British and Belgian air forces pioneered the use of aerial photography for reconnaissance as early as 1914, marking the beginning of a systematic approach to aerial observations. Aerial photography advanced rapidly due to its essential role in military reconnaissance. Both the Allies and Central Powers relied heavily on aerial photographs to map enemy positions and monitor battlefield activity. It was during this time that aerial photography techniques improved dramatically, particularly in terms of camera technology, altitude, and photographic film quality. The use of specialized reconnaissance aircraft, which were equipped with advanced cameras, allowed for the collection of high-resolution images on a scale and precision previously unattainable.

Although aerial photography during the war was primarily a military tool, its application to archaeology was soon recognized. The thousands of aerial photographs captured over Europe revealed previously unknown archaeological features, such as crop marks, that were visible from the air but invisible from the ground. These images, primarily intended for military strategy, revealed unexpected archaeological features, especially in Europe. This discovery highlighted the potential of aerial photography not just for military purposes but also for archaeological discovery and documentation.

After the war, many of the techniques and technologies developed during the conflict were adopted by archaeologists. Figures like English archaeologist O.G.S. Crawford, who is widely considered the "father of aerial archaeology," were instrumental in applying wartime advances to peacetime archaeological research. Crawford recognized the potential of aerial photography for archaeological research and conducted extensive surveys of the English landscape from the air. He published numerous articles and books on the subject, laying the foundation for the development of aerial archaeology as a scientific discipline. Crawford's systematic use of aerial photography in the 1920s marked the beginning of modern aerial archaeology, building on the foundations laid by earlier pioneers and the technological leaps made during the war.

O.G.S. Crawford and the systematic use of aerial photography

In the 1920s, Osbert Guy Stanhope Crawford emerged as a key figure in the development of aerial archaeology. Crawford recognized the potential of aerial photography for systematically documenting archaeological sites. His work in Britain and the Middle East demonstrated that aerial surveys could reveal features such as crop marks and soil disturbances that were undetectable from ground level. His pioneering efforts helped establish aerial archaeology as a legitimate and essential method within the broader field of archaeological research.

Major G. W. G. Allen was an English engineer who, after learning of the work Crawford was doing, he was inspired to use his own airplane around Southern England, taking photographs of the landscape. His work in documenting prehistoric landscapes and Roman roads in England marked a significant advancement in the application of aerial methods for archaeological surveys.

Post-war expansion

Technological advances

Following the Second World War, advancements in aerial photography and imaging techniques significantly broadened the scope of aerial archaeology. The introduction of infrared photography and multispectral imaging enabled archaeologists to detect subtle variations in vegetation and soil, allowing for the identification of buried structures that would otherwise remain hidden. These technological innovations made it possible to conduct more detailed and accurate surveys, further enhancing the effectiveness of aerial archaeology.

Global expansion of aerial archaeology

In the post-war period, aerial archaeology expanded beyond Europe, becoming an invaluable tool for researchers working in regions with vast and difficult-to-access landscapes, such as the Americas, Africa, and Asia. In these areas, aerial surveys uncovered ancient civilizations and revealed the locations of significant archaeological sites, many of which had been previously unknown. The ability to survey large areas quickly and efficiently made aerial archaeology a critical tool for understanding diverse archaeological landscapes across the globe.

Modern aerial archaeology

Drones and UAVs

In recent years, the advent of drones, or unmanned aerial vehicles (UAVs), has revolutionized aerial archaeology. Drones offer greater flexibility and lower operational costs compared to traditional crewed aircraft. The increased use of drones has made aerial archaeology more accessible to a broader range of researchers and has led to more frequent and detailed studies of archaeological landscapes.

LiDAR and photogrammetry

Technological advancements such as LiDAR (Light Detection and Ranging) and photogrammetry have further enhanced aerial archaeology. LiDAR, in particular, is capable of penetrating dense vegetation to reveal features hidden beneath forest canopies, making it a valuable tool for studying heavily forested regions. Photogrammetry, which allows for the creation of precise 3D models of archaeological sites, has enabled researchers to document and analyze sites with increased speed and accuracy. Together, these technologies have expanded the potential of aerial archaeology, allowing for more detailed and comprehensive analyses of archaeological sites.

Notable people in the field of aerial archaeology include:

  • O. G. S. Crawford (England): Pioneered the use of aerial photography for archaeological survey.
  • Roger Agache (France): A leading figure in the development of aerial archaeology in France, known for his work on Gallo-Roman sites.
  • Antoine Poidebard (Syria): Used aerial photography to study Roman limes in the Middle East.
  • Dache McClain Reeves (United States): A pioneer in the use of aerial photography to study Native American archaeology in the United States.
  • Henry Wellcome (Sudan)
  • Lionel Rees (Jordan)
  • Giacomo Boni (Italy)

Applications of aerial archaeology

Aerial archaeology plays a crucial role in discovering new archaeological sites and mapping their extent. By providing a bird's-eye view, aerial images can reveal subtle features and patterns that are often invisible from the ground. This is particularly valuable in areas with:

  • Dense vegetation: Aerial photography can penetrate tree canopies and reveal hidden earthworks, structures, and other features obscured by foliage.
  • Large-scale features: Extensive archaeological landscapes, such as ancient field systems, roads, or settlements, are often best appreciated and understood from the air.
  • Inaccessible terrain: Aerial reconnaissance allows archaeologists to survey remote or difficult-to-access areas, such as mountains, deserts, or wetlands, where ground surveys would be challenging.

Identifying and recording archaeological features

Aerial images provide a wealth of information for identifying and recording archaeological features. These include:

  • Earthworks: Banks, ditches, mounds, and other earthworks often create subtle variations in topography that are readily visible in aerial photographs, particularly when illuminated by low-angle sunlight.
  • Cropmarks and soil marks: Variations in plant growth or soil color can reveal the presence of buried features, such as walls, ditches, or pits.
  • Stone structures: Walls, buildings, and other stone structures can often be identified by their shape, shadow, and texture in aerial images.
  • Ancient land use: Traces of past human activities, such as field systems, roads, and canals, can often be detected in aerial photographs.

By carefully analyzing aerial images, archaeologists can identify, document, and interpret a wide range of archaeological features, providing valuable insights into past human activities and settlement patterns.

Creating site plans and maps

Aerial photographs provide the foundation for creating accurate and detailed site plans and maps. This involves:

  • Georeferencing: Aligning aerial images with real-world coordinates, ensuring accurate spatial referencing.
  • Orthorectification: Correcting for geometric distortions in aerial images, creating a planimetrically accurate representation of the ground.
  • Mapping features: Tracing the outlines of archaeological features identified in the imagery, creating a digital record of the site.
  • Creating topographic maps: Using aerial photographs and elevation data to generate contour maps that illustrate the shape and relief of the terrain.

These site plans and maps are essential for documenting archaeological sites, planning excavations, and managing cultural heritage resources. They provide a valuable record of the site's extent, features, and spatial context, aiding in interpretation and future research.

Techniques and technologies

Aerial photography

Photography is the most common method used in aerial archaeology. Archaeologists use specialized cameras and lenses to capture high-resolution images of the ground from aircraft or drones.

Oblique vs. vertical photography

Aerial photographs can be captured from different angles, each offering distinct advantages for archaeological investigation:

  • Oblique photography: Taken at an angle, oblique photographs provide a more familiar perspective, similar to what the human eye would see. This allows for easier recognition of features and provides a sense of depth and context. Oblique images are particularly useful for visualizing the relationship between archaeological sites and their surrounding landscape.thumb|Effects of sub-soil features on the growth of crops. This allows for diagnosis of sub-soil features by visual inspection of crops using aerial images.|384x384px

Tiny differences in ground conditions caused by buried features can be emphasized by a number of factors and viewed from the air:

  • Shadow marks: slight differences in ground levels will cast shadows when the sun is low and are best from above.
  • Cropmarks: buried ditches will hold more water and buried walls will hold less water than undisturbed ground, which causes crops to grow taller or shorter, and therefore define buried features, e.g. as tonal or color differences.
  • Soil marks: slight differences in soil color between natural deposits and archaeological ones can often be visible in ploughed fields.
  • Frost marks: frost can also appear in winter on ploughed fields where water has naturally accumulated along the lines of buried features.
  • Water pooling: differences in levels and buried features will also affect the way surface water behaves across a site, producing striking effects after heavy rain.

For a three-dimensional effect, an overlapping pair of vertical photographs, taken from slightly offset positions, can be viewed stereoscopically.

Remote sensing

Beyond traditional aerial photography, archaeologists use a range of remote sensing techniques to investigate sites without physical excavation. These methods involve collecting data from a distance using specialized sensors that detect and record different forms of electromagnetic radiation. This information can reveal subsurface features, variations in vegetation, and other archaeological clues hidden from the naked eye. Digital data, for example, ALS, can be used effectively in "heavily automated workflows," (a process that uses rule-based logic to launch tasks that run without human intervention), e.g. a six-year project using supervised automated classification to survey of Baden-Wurttemberg in Germany, identified as many as 600,000 possible sites. LANDSAT images have helped in identifying large-scale features, such as an ancient riverbed running from the Saudi Arabian desert to Kuwait.

SLAR (sideways looking airborne radar) is a remote sensing technique that records pulses of electromagnetic radiation from an aircraft. Richard Adams used SLAR to identify a matrix of possible Mayan water irrigation systems underneath the dense rainforest from a NASA aircraft.

SAR (synthetic aperture radar) involves radar images that are processed to create high-resolution data. It can be faster and less time-consuming than surface survey.thumb|Lynchet system near Bishopstone in [[Wiltshire. LiDAR technology used to map topography.|300x300px]]

LiDAR

right|thumb|A [[lidar view of Richborough Roman fort and amphitheatre at Rutupiae in Kent and part of Richborough port (HER 1916–1945)]]

Lidar (light detection and ranging) aka ALS (airborne laser scanning) uses laser scanner pulses to measure the distance to the ground and other objects. By emitting thousands of pulses per second and recording the time it takes for them to return to the sensor, LiDAR creates highly accurate 3D models of the Earth's surface. In archaeology, LiDAR is invaluable for:

  • Mapping terrain: LiDAR can penetrate dense vegetation, revealing subtle variations in topography that may indicate the presence of buried features like walls, ditches, or mounds.
  • Creating digital elevation models (DEMs): These models provide detailed representations of the ground surface, allowing archaeologists to identify archaeological features that are not readily apparent on the ground.
  • Visualizing landscapes: LiDAR data can be used to create stunning visualizations of archaeological landscapes, aiding in the interpretation of site layout and land use patterns.

Satellite imagery

In places yet to be documented (or where maps are considered confidential), satellite imagery is vital to providing base maps for excavation. (to be investigated further for a greater understanding). We can thus see the impressive effect aerial methods can have on streamlining archaeological survey, and pushing the limits of what is possible.

Homs, Syria

Homs, Syria provides an example of how different types of satellite imagery can be used in combination. The site is based in an area notorious for its difficulties surrounding archaeological survey, as the diversity of terrain makes the detection of archaeological sites difficult. As a result, Homs is a perfect candidate for aerial reconnaissance. Modern agriculture often obscures remains through practices such as deep ploughing (which removes many levees and low-lying sites from the archaeological record).