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29 September 2004
Airborne survey of ancient Irish sites

One of Irelandís most noted archaeological landscapes has yielded new secrets to an innovative airborne survey. Using aerial lasers and computer imaging, an Anglo-Irish team has found an even richer concentration of sites than previously known.
     The Loughcrew Neolithic cemetery, a complex of prominent stone-built mounds enclosing megalithic passage graves, lies on the hilltops of Slieve na Calliagh in western Co Meath. Below the hills is "a rich archaeological landscape with a stone circle, henge monument, standing stones and a possible cursus monument", Colin Shell and Corinne Roughley report in Archaeology Ireland.
     "Although the passage graves themselves, with their megalithic art, are well known and accurately documented, the surrounding landscape has only begun to be considered in detail in recent work," says the report. This archaeological component of the project is being carried out by Elizabeth Shee Twohig, of University College, Cork, while Dr Shellís Cambridge team are carrying out the aerial survey.
     They are using the recently-developed technology of Lidar (light detecting and ranging), which uses airborne lasers to scan the landscape and pick up details of relief. "The beam is scanned over the ground in a zigzag manner as the aircraft flies along an accurately planned set of paths," says Dr Shell. "The flight speed and scan frequency determine the average distance between readings, in our case 60 cm."
     Using a base station in Kells, the team were able to calculate each of the 83 million observations within an area of 5 by 6 kilometres and to within a tolerance of 20cm on the Irish National Grid. The team also obtained conventional aerial photographs to a scale of 1:12,500, which could be used to provide a digital terrain model.
     "By digitally illuminating the surface with a low oblique light, features become clearly visible," Dr Shell says. "Being able to illuminate the Lidar surface with a digital sun from any direction offers considerably greater flexibility than is available in the real world. We have the best means for revealing the often slight variations in the ground that mark past human activity. The Lidar not only provides information on the form and preservation of known sites, but can also reveal potential new sites and define ancient field boundaries underneath the modern ones."
     Some 160km, or 100 miles, of apparent cultural features have been documented, of which only 10 per cent are sites already listed. The new ones cannot be dated from the survey directly, although their form and interrelatedness may provide clues. Dense concentrations of interesting sites have shown up in the lands around Ballinvally and Summerbank, where Dr Twohig has identified what may be a previously unknown henge monument, a banked enclosure of ritual function, and also additional burial mounds close to a known barrow at Drumlerry.
     The Loughcrew survey has shown serious erosion of the cultural landscape by agricultural improvement in the past forty years, although it has also revealed the survival of parts of sites thought destroyed.

Sources: Archaeology Ireland Vol.18 no.2, The Times (23 September 2004)

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