What’s on : Lectures

Aerial Archaeology in England – Discovering Past Landscapes from the Air

Lectures
Date
25 Jan 2022
Start time
2:30 PM
Venue
Priory Street Centre
Speaker
Matthew Oakey, Historic England
Aerial Archaeology in England - Discovering Past Landscapes from the Air

Event Information

Aerial Archaeology in England – Discovering Past Landscapes from the Air
Matthew Oakey, Interim National Aerial Investigation & Mapping Manager
Archaeological Investigation, Historic England.

Since the early the 20th century aerial photography has been used to discover, record and understand archaeological sites and landscapes. Over this period there have been considerable advances in technology – from glass plate cameras and hand-transcription to the use of digital technologies to produce complex 3D datasets. This talk will explore how archaeologists use airborne remote sensing techniques to investigate landscapes. We’ll see how sites are identified from the air and how analysis of modern and historical aerial photographs, and airborne laser scanning can produce maps of vast archaeological landscapes. We will also look at how online digital dissemination allows us to give greater access than ever to these datasets and communicate aerial archaeology to the wider public.

Image caption: Roman camps and later features on Hadrian’s Wall © Historic England. Lidar source: Environment Agency.

Member’s Report

The first lecture of 2022 in the Covid-19 era was held in the main meeting room of Priory Street Centre where Matthew Oakley of Historic England (HE) and Interim Manager of the National Aerial Investigation and Mapping Team, spoke to an audience of roughly 40 YPS members via Zoom with a further 20 people attending by Zoom.

Matthew outlined the role of HE as UK government’s statutory adviser for the historic environment. Researching archaeological landscapes is done by flying 200 hours annually taking and interpreting new aerial photos. The Northern team flies from Sherburn in Elmet and is based in the HE York Office.

YPS member Anthony Crawshaw received a special mention because he has piloted many HE photographers in his light aircraft all over the north for over 30 years and is himself a highly respected aerial photographer.

Most photos are taken with hand-held digital SLR cameras, but increasingly aerial laser scanning and other hi-tech photographic methods are used. The photographs are just the start; the bulk of the work is spent interpreting photos and mapping them using Geographical Information Systems (GIS).

One British pioneer was Osbert Guy Stanhope Crawford, the first aerial officer for OS who often flew with Alexander Keiller. Crawford’s books Air survey and archaeology and Wessex from the air are seminal works. Another prolific contributor to the HE archive was Derrick Riley, a Nottingham based Mosquito pilot in WW2 who wrote the important well illustrated Aerial photography in archaeology.

We were given a potted history of the development of aerial photography and the role of the military reconnaissance. In total HE curates over 40 million photographs which cover the entire UK, with over 4 million images in its Swindon office. The benefits of aerial photographs over maps was well illustrated with views over the landscape of Sledmere on the Yorkshire Wolds. They are a particularly effective medium for recording large scale industrial complexes which often have a limited life span such as the coal fired power stations of Drax, Ferrybridge and Eggborough.

Natural threats such as coastal erosion continue to threaten important archaeological sites and HE monitors agricultural activity at protected sites such as prehistoric barrow fields where animal burrowing and over zealous ploughmen cultivate too close to monuments causing ‘arable clipping’. Occasionally deliberate damage to ancient monuments is spotted.

In addition, they are also actively seeking undiscovered new sites of archaeological importance. We were shown a 2012 image of a triangular enclosure of late Iron Age  or Roman date showing traces of round houses in Cumbria, hitherto unknown.

The interpretation of aerial photos requires experience. Earthworks, particularly banks and ditches under pasture show up best when the sun is low producing long shadows. Ploughed arable landscapes frequently reveal soil marks, ancient deposits brought to the surface by ploughing. We were told how the colour of the soil varies and how emerging crops grow at different rates on the same field; the germinating crop providing a positive crop mark on deeper more humic ditch fills, while buried walls inhibit plant growth producing negative crop marks.

Matthew’s comprehensive talk covered new, more technical surveys involving lasers (LiDAR) which have been recently used on the Roman fort at Newton Kyme. Digital photogrammetry is now, with the use of powerful computers, used to stitch together up to 2000 separate digital images to produce ‘Orthomosaic’ pictures. These systems involve much data processing and can be used to remove tree-cover and examine the land surface to produce accurate Digital Elevation Models of large areas of landscape. This is remarkably accurate (2-3 cm) and much faster than ground based survey techniques. Near Infra-Red imaging can further enhance photographic images.

HE is now making many of these data available via the website ‘Aerial Archaeology Mapping Explorer’ which was largely developed during recent Covid-19 periods of lock down. Another useful source is the 97,000 aerial photographs available through the ‘Britain from above’ website. Finally we were urged to consult ‘Heritage gateway’ kept by local authorities.

Questions were wide ranging, covering aspects of privacy, licences, the role of Google Earth and other matters. The meeting closed after repeated rounds of applause at 4.20 pm.

In sum: this was a well illustrated highly informative talk enjoyed by all. The role of Zoom was central to the event.

Andrew Jones