Some old and new ways of working for aerial investigation and mapping in the time of Covid-19.
This article explores some old and new ways of working by the Historic England Aerial Investigation and Mapping team in response to changing technology and the major changes in office working due to the Covid-19 pandemic.
One of the major themes of recent months is social distancing. Aerial archaeology is by its very nature ‘at a distance’ – both physically and temporally. Aircraft, from balloons to satellites, can take us far above the ground, and even into space, so that we can observe from the bird’s, or even astronaut’s, eye view.
Cameras record the aerial experience capturing recent views but also creating a library of images that can take us back in time. The Historic England Archive and other repositories have vast collections of aerial photographs. A few images date from the 19th century, and many more from the last 100 years. We can trace landscape change and discover archaeological sites by analysing and comparing aerial photographs taken at different times. The development of digital technology also allows us to share images and conclusions with people all over the world.
Each new piece of equipment or software does not necessarily replace what came before, rather we adapt them to suit our purposes and use them in a variety of different situations. Our aerial reconnaissance programme carries out a number of functions:
Each situation requires a different approach in terms of the type of photography and the kit used.
We use high-winged light aircraft, Cessna 172s, that allow us to get around the country quickly so we can go where the weather and ground conditions are right. We lease two aircraft, based in Yorkshire and Oxfordshire. These small aircraft are easy to manoeuvre and allow us to position the aircraft to take oblique or vertical photographs with a hand held camera through an open window. This technique for specialist aerial photography is something that is essentially unchanged since the early 20th century: it was used, for example, by Aerofilms Ltd in the 1920s. Oblique aerial photography is a core part of what we do as it provides the right angle for a range of purposes, such as close-ups of potential threats to scheduled monuments and wide views used for historic town centres and 18th-century designed landscapes.
The digital technology we use includes high-specification digital cameras. These are fitted with an Inertial Navigation System (INS) attached to the base of a Nikon camera. The INS we use was developed by a PhD student, Martin Weiser, from the University of Vienna. It comprises an Inertial Measurement Unit (IMU) attached to a global positioning system (GPS), the data from which is used to calculate the geographical position of the footprint and centrepoint of each oblique aerial photograph. This enables a digital flow line to the Historic England Archive and we are currently working on a system to allow people to explore the digital aerial photography archive online using a map.
Safety is a primary consideration in planning and executing flights and our flying archaeologists liaise with the pilot to ensure this. Planning where to go and why relies on a combination of the experience of the aerial investigator and digital technology. This includes an understanding of the right ground and weather conditions for different kinds of photography, combined with use of a tablet with a geographical information system (GIS) loaded with air navigation charts and the latest information on the historic environment.
This means that our aerial archaeologist can plan safe flight paths with the pilot, and that they have information to hand which allows them to navigate to targets, to identify whether a site is a new discovery, and to adapt flight plans if weather or ground conditions are not as expected. We usually fly with the pilot and together they may adapt the route during a flight in response to changing weather conditions or air restrictions. Therefore, we continue to use a mix of well-established and new techniques and equipment for aerial reconnaissance.
Our latest project has been to experiment with the use of an automatic camera fixed to the aircraft to allow us to carry out vertical aerial photography over individual sites or larger areas. Vertical is used as a descriptive term because the camera is pointing straight down at the ground rather than at an oblique angle. This technique will not replace our hand-held photography but we want to be able to take overlapping vertical photographs relatively easily and in a controlled fashion. Vertical photography has been taken since the early days of balloon flight and has been used extensively since the Second World War. However, it tends to involve large specialist aircraft and is not usually taken for archaeological purposes but might be taken for military training, Ordnance Survey mapping or commercial survey.
Historic England uses light aircraft and remote-controlled small aircraft (commonly called drones) for its aerial photography. We are continuing to successfully explore the use of drones, establishing that, due to their limited range of operation, they are mainly suitable for recording single sites. Our Cessna aircraft, however, have the advantage that we can get around to different sites or areas across the country quickly and undertake both oblique and vertical photography, covering large areas in a single flight.
This means our aircraft set-up is ideal for extensive monuments, such as those along the ridge of the Malvern Hills or where there are continuous archaeological features. The latter includes the settlements and field systems that extend across large parts of areas such as Dartmoor in Devon or the Yorkshire Dales. Therefore, we aim to use the aeroplane or the drone for different circumstances and sometimes for work on the same site or area. So far, our trials suggest that the light aircraft photography can achieve about the same Ground Sampling Distance (a measure of scale and accuracy) as drone footage, depending on the height flown and lens used. The GSD for the Malvern Hills examples is 5 centimetres, an acceptable level of tolerance for most archaeological purposes.
By taking our own vertical photographs, we can ensure we have the right conditions and resolution for archaeological illustration and survey. We will use them to create digital 3-D visualisations to help illustrate sites and landscapes but their main application is in monitoring the condition of large archaeological monuments.
We have two different vertical automatic-camera set-ups attached to each of the Cessna-172’s that we lease.
Emma Trevarthen from our York team covers the north of England from Sherburn-in-Elmet. The aircraft there uses a camera placed over a hole in the floor of the plane.
Damian Grady from our Swindon team covers the south from Oxford Airport (Kidlington) and uses a camera pod fixed to the wing strut of the aircraft. We use a Digital SLR camera (Nikon D850) linked to an Inertial Measurement Unit (IMU) and a Global Positioning System (GPS). During a vertical survey, the aircraft flies a proscribed straight course with constant height and direction and the camera automatically takes pictures at set intervals to create overlapping images.
For vertical photography using an automatic camera, a skilled pilot is required to fly the plane at a steady height and speed but it is inevitable that there will be some variation. The IMU and GPS provide the information to allow us to geo-reference the images – that is, to relate the view to an exact location on the ground.
We use photogrammetric software (Agisoft) to process the photographs, using a process sometimes called ‘structure from motion’, to create a seamless composite from the overlapping images. We can also model this in three dimensions. We aim to use this kind of photography to aid with monitoring monuments and landscapes. They will provide an accurate and measurable benchmark of possibly encroaching vegetation, erosion and other potential issues such as animal burrowing. They also provide an engaging illustration and different viewpoint of some of our iconic sites.
We also aim to use the vertical camera set-up during drought conditions if we identify areas with multiple cropmarks revealing buried archaeological remains. We were planning to do this anyway over the spring and summer but the lock-down caused by the Covid-19 pandemic also meant that we had to rethink our approach to aerial reconnaissance.
When airfields opened in May, social distancing still meant it was impossible for our flying archaeologists Emma or Damian to go up in the plane with the pilot. However, they could travel to the airfield or use video-conferencing to communicate with the pilots.
Applying social distancing, stringent cleaning and using protective clothing, we were able to set up the camera in each plane. The pilot was then briefed and sent to areas or sites on their own where they could start up the automatic vertical camera. This method relies even more than usual on the expertise of our pilots who are not only highly skilled at flying but also have an interest and knowledge of aerial archaeology through their work with us.
The vertical set-up is still in the early stages of development but it has been a great help in allowing us to carry out some limited reconnaissance during social distancing. When the lock down eases and we return to something nearer to our normal operation we will once again be able to go up in the plane with the pilot and use a mix of hand-held and automated cameras, sometimes in the same flight.
The rest of the Aerial Investigation and Mapping team (our ground crew) and our colleagues working on grant-aided aerial projects have also had to adapt to the new situation created by Covid-19. We usually work in teams based in offices where we use archive aerial photographs and have the chance to discuss the archaeological remains and work on mapping projects together. We typically work on projects where we create archaeological maps using information from aerial photographs and airborne laser scanning (lidar) to promote understanding of the historic environment. This informs strategic planning and management through rural and urban casework and large-area projects. All our mapping and recording is digital and is available from the Historic England Archive or local Historic Environment Records.
Like so many we have adapted and moved our specialist equipment and software into our homes, and are concentrating on projects that make the most of digital sources.
We can communicate through video conferencing and our two teams in York and Swindon are probably seeing more of each other than usual. At home, digital technology allows us to review the results of our own aerial reconnaissance and produce records of discoveries to go into the historic environment record. We are also trialing a map-based platform that provides access to all Historic England digital aerial photographs. This is so that staff (and in future the public) can easily find and use aerial photographs and therefore have a virtual visit to the archive. We are also compiling the results from all our mapping projects into a single dataset so these can be shared online too.
Research continues into old and new sources of information. Many of our sources were produced for non-archaeological purposes but have potential benefits for archaeological survey. For example, we find archaeological sites and track urban and rural landscape change using RAF or Ordnance Survey photography taken from the 1940s onwards. Our own reconnaissance and Google Earth coverage provide a contemporary source for archaeological discovery.
We are now examining that most remote of sources, satellite imagery. Many people are under the mistaken assumption that the imagery that they look at on Google Earth, or similar platforms, is satellite based, when in fact the vast majority of it is captured from aircraft. Very High Resolution (VHR) satellite imagery does exist, with resolutions as high as 30 – 40 centimetres. However, it generally requires up-front payment and, for England, with its particular weather, it is by no means comprehensive in its coverage of the ground due to clouds. This combination of reasons is why Historic England, like most bodies within England, has previously been limited in their investigation of the resource.
This has changed recently with the establishment of the 'Space for Smarter Government Programme' (SSGP). This was set up by the British government to help the public sector save money, innovate and make more effective policy decisions using space-enabled services. Part of this programme, begun in 2019, aimed to look at making VHR data available, free of charge, to a selection of government bodies to assess how it might be used to improve efficiency.
A project is underway within Historic England looking at a sample of this satellite data to determine its usefulness for archaeological prospection and monitoring and how it might be incorporated into future mapping projects as an additional resource. This will inform changes to our workflows to ensure that we use satellite data in a cost effective and efficient way. The project will assess the extent to which this data complements or supplements current standard aerial sources. This will contribute to guidance and standards for use of satellite data in the wider historic environment sector.
This will be achieved by using the SSGP satellite data to compare archaeological results with those from current standard sources in a sample of different types of landscapes. The satellite imagery examined will include not only natural colour (RGB) imagery, but also data from the Near Infra-Red (NIR). This waveband can be used to recognise stress in plants and might therefore reveal traces of buried features before they appear in the visible spectrum.
The satellite data will not replace the current sources that we use but will become one among the many sources of information at our disposal. Because of the increasing number of digital sources, many of them publicly available on the internet, we have been able to adapt to home working and continue our archaeological research at a distance from each other. However, the Historic England Archive aerial photographs (analogue and digital) remain the core source for our projects so we are looking forward to when we can re-introduce the full range of sources and techniques at our disposal.
Like all other approaches to archaeological research and survey, aerial photography adapts to new circumstances and advances in ideas and techniques, exploring new technology and identifying how it can be applied to our discipline. Old and new complement each other, and the crucial element in all of our work is the person making the decisions on how to collect and interpret data.
Covid-19 has offered an unexpected opportunity, in of course unwelcome circumstances, to experiment with different methods of working, and this will help us to develop new ways of presenting stories from the distant past to the present.
Helen is a landscape archaeologist and air photo interpreter. She has worked for almost 30 years on multi-period sites and landscapes across England using aerial photographs and airborne laser scanning (lidar) as the main source. She has a special interest in how aerial photographs show changes in military, urban and rural landscapes.
Damian joined the Royal Commission on the Historical Monuments of England in 1990 to map archaeology from aerial photographs and from 1998 became responsible for managing the aerial reconnaissance programme.
Simon is a landscape archaeologist and air photo interpreter at Historic England, with over 25 years’ experience of mapping and interpreting features of archaeological and historical interest visible on aerial photographs and other aerial imagery. He has worked in many areas of England including the World Heritage Sites of Avebury and Stonehenge. He has a special interest in “new” technology such as lidar and satellite imagery and its application to archaeological research and investigation.
To find out more about the history of aerial archaeology see: Martyn Barber, 2011: A History of Aerial Photography and Archaeology: Mata Hari's glass eye and other stories, English Heritage
Find out more about the History of of the Historic England Aerial Photo Archive
Find out more about aerial archaeology methods at Historic England
Find out more about terrestrial structure from motion (using the same techniques as aerial SfM)
You can download this article in PDF format as part of Historic England Research magazine Issue 16.
Explore our research reports with this map which is an on-going project that allows access to the majority of research reports produced for place-based projects. It covers most types of non-invasive surveys, including scientific analysis, such as tree ring dating and archaeobotany.
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