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  • Subject area(s): Engineering
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  • Published on: 7th September 2019
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Historically aerial photography has only been in existence since 1891 before the invention of the aeroplane, when it was first suggested that vertical photographs taken from a great height would reveal archaeological features that were not observable from the ground was first put into practice. In this first instance of aerial photography, balloons were used to photograph Calcutta (Kolkata) for the Surveyor General of India. Following this the next major leap was 1920 with the production of aerial photos by a Dr Theodor Weigand. This was the result of working in what was then Southern Palestine and the Sinai Desert for the German forces during WW1. This produced a series of images showing abandoned cities including vineyards under the desert sands. Three years later a photograph taken by the RAF in 1921 brought greater interest to the subject. With an image of the processional way towards Stonehenge was captured, this clearly showed the full extent of the Great Avenue.

In 1975 when David Wilson wrote ‘Science and Archaeology’, he stated that ‘there have been many uses of aerial photographs in the service of archaeology since then (referring to the early days), but despite the dramatic results of air surveys and the clear pictures they have been presented, there has been a strange unwillingness to make the partnership between the two into a regular marriage. Perhaps it has been the perennial lack of funds that always affects the archaeologist.’ (David Wilson p190, Science and Archaeology). In hindsight he should have waited until the invention of the internet and the access to satellite imagery from ‘Google Earth’ which changed the face of aerial archaeology overnight. Rather than look at the supposed traditional methods of obtaining aerial photos by fixed wing aeroplane I’m going to look at more modern methods of obtaining images of the ground.

Firstly however first we must discuss what is to be captured, the type of image required, the camera to be used, the type of platform used, and what we are trying to achieve. In other words, will what we are using answer the questions set about the site we are investigating. The main issues are the types of photograph to be taken and the platform- aerial vehicle- being used to do the work. Early efforts to gather imagery of archaeological sites were costly and time-consuming. A plane had to be hired, a pilot acquired and paid, along with associated fuel costs. Photographs taken might not be adjusted for problems with scale arising from the camera’s look-angle, or influences of terrain. Storage of film materials becomes problematic over time, as film can degrade over time and rendered useless for our purposes. There are two main types of photographs used in aerial archaeology; vertical and oblique photographs. Vertical photographs are taken looking directly (vertically) down at the ground below, giving a map-like view of the ground on a uniform scale. They are rarely taken specifically for archaeological purposes, being taken as part of large-scale survey for military, cartographic (map making) or civil engineering purposes (route planning for road and rail contracts). Oblique photographs are normally specifically for archaeological purposes. They are taken at an ‘oblique’ angle, from the window of an 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. This type of photograph is taken at much lower altitudes than vertical photography, so more detail and definition can be seen. Using this method the visibility of features is dependent on the time of year, weather conditions and light levels.

Looking at the aerial platforms for the taking of photographs for archaeological use the most current are satellites in orbit around the Earth and Drones or ROV’s. Both platforms are now becoming the mainstay of aerial reconnaissance in the field. The most impressive and potentially most far reaching is that of satellites which have produced data images that are slowly being released into the academic and commercial world. These data downloads are expensive and are usually sold under licence to others. However such satellite data, from agencies as NASA, and other commercial satellite companies, have had a great impact in recent years. For example a BBC documentary from 2011 showed the Scientists at the University of Alabama, led by Dr Sara Parcak had found 3,000 ancient settlements using a new technique of satellite infra-red imaging. NASA\'s ‘Aster’ satellite takes images of the Earth in 15 different wavelengths, Dr Parcak and her team processed the data using computer techniques that showed fields as red, cities as blue and ancient ruins green. The results having been confirmed on the ground with field archaeologists with picks and shovels. Dr Parcak said, she couldn\'t believe that they could locate so many sites all over Egypt. Her team analysed images from satellites orbiting 400 miles above the Earth, equipped with cameras so powerful they can pinpoint objects less than a yard in diameter. This technique is so powerful that it can be used to monitor archaeological sites that are being looted across war zones. Through this technique she and her team have located 17 unknown pyramids and over 1000 building complexes which may be tombs. These discoveries were highlighted in a BBC documentary, ‘Egypt’s Lost Cities’ and in the National Geographic magazine.

The other new method being used again comes from the world of the military and that is ROV’s (Remote Operated Vehicles) or UAV’s (Unmanned Aerial Vehicles) or usually called Drones. The original drones had only military uses, however with reduction in size and advances in technology they have become an affordable means of carrying out quick and easy aerial surveys. They are light, reasonably easy to use for the generation of people brought up on computer games, they can be fitted with CCTV capacity to take live footage and video. However those same advantages are also disadvantages, they are light so flying conditions have to be good, bad weather conditions such as high wind could damage them. They have limited air time as they are dependent on battery life, the cameras are small and if damaged will not produce and images and as they are usually integral with the drone, if they are broken the whole thing becomes unusable.

In 2012, research sponsored by the Field Museum, Canada, and conducted by Durham University and the Bulgarian University, gathered surveys of the Mirkovo Basin (Bulgaria). The Mirkovo Basin has evidence of over 8,000 years of human settlement, from the Neolithic to the modern times (HRAR/QuestUAV). Another example of the use of drones is in archaeologists surveying parts of New Mexico. This revealed an ancient Native American settlement called Blue J, this is a 1,000-year-old village and was first identified in the 1970s. It’s near the Chaco Canyon site in north western New Mexico that contains 60 Pueblo Indian houses. Jesse Casana from the University of Arkansas and John Kantner of the University of North Florida, (Archaeological Aerial Thermography) had previously excavated at the site and the drone images showed stone compounds that had not previously been identified. They used LiDAR (Light Detection and Ranging) using lasers to map terrain very accurately. Subtle changes in terrain, when the elevation data is processed and mapped, can be used to analyse historical human landscapes, by measuring the height of the ground surface and other features in large areas of landscape with a very high resolution and accuracy. Casana and Kantner highlighted that in some cases, both in Central American and in Southeast Asia, they have literally walked over archaeological sites without realizing. Only after mapping and surveying, either using ground-based or aerial techniques, did they realise how they had underestimated the size of their study areas.

Such information produced by LiDAR was previously unavailable, except through labour-intensive field survey or photogrammetry. It provides highly detailed and accurate models of the land surface at metre and sub-metre resolution. This provides archaeologists with the capability to recognise and record otherwise hard to detect features. Commonly airborne LIDAR operates by using a pulsed laser beam fired from a plane. The beam is most commonly scanned from side to side as the aircraft flies over the survey area. It measures between 20,000 to 100,000 points per second to build an accurate, high resolution model of the ground and the features upon it. This speed of capture has been consistently growing over the last decade and will no doubt increase further. In the United Kingdom the Environment Agency Geomatics Group has used LiDAR for over a decade for the production of cost-effective terrain maps suitable for assessing flood risk. They therefore have data available for large areas of the country. In more recent years many other bodies have acquired the capability to carry out LiDAR surveys (Crutchley and Crow, ‘the Light Fantastic’). This has been particularly true for utilities companies, highway agencies and other developers. English Heritage (now Historic England) was also closely involved in working with the former Unit for Landscape Modelling (ULM) at Cambridge and the Forestry Commission looking at the potential for LiDAR to penetrate wooded terrain. Because LIDAR uses light beams it has the potential to penetrate gaps in the woodland canopy and so record the ground surface under the trees. This can reveal features that would not otherwise be seen. But be able to use a Drone/UAV/UAS with LiDAR has the potential to revolutionize archaeology. When coupled with ground survey information, using levels and transits – traditional survey tools, UAVs can provide unparalleled detail for archaeological studies. GIS (Geographic information systems), as well as other remote sensing methods, promise to transform the field of archaeology from the stereotypical man in a hard hat to a technology-driven, cutting-edge 21st century digital discipline.

New technology in the field of aerial archaeology is becoming more accessible and cost effective to the Field Archaeologist. The image quality enhanced by computer interpretation of images taken and the reducing cost of access to this commercial imagery from satellites or drones should benefit all. To be able to live stream an aerial survey of a hilltop in the middle of winter or desert feature in the Sahara to specialists in landscape, stone age, bronze age or iron age to Roman archaeology, where they are sitting in front of monitors hundreds of miles away is immeasurable. The time saved in field surveys, using satellites or drones to cover in a few hours what takes weeks, again immeasurable. There will always be a place for a digger, but now a lot of the guess work where to dig could be removed by this new technology.

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