Skellig Michael, Co Kerry, Ireland

The archaeology of Skellig Michael consists primarily of upstanding stone-built monastic structures in contrast to the earthwork features which define most heritage applications of lidar.

The remote nature of the island and the extreme topographic conditions present many challenges to conventional survey. There is also an archaeological challenge, with most features being stone built structures, many still standing to a significant extent. The primary objective of this project was to provide an accurate baseline survey for the whole island as a resource for the ongoing conservation and research programme on the Skellig Michael.

Only a DSM data set was provided as the output due to the absence of significant vegetation and the fact that upstanding walls and structures constituted the core archaeological monuments. The Discovery Programme was commissioned to generate the GIS layers and map outputs to facilitate the data interrogation. To maximise the visibility of archaeological features a number of different processing algorithms were applied (see Lidar DEM visualizations) with sky-view factor proving most effective.

Working with the lidar data

Skellig Michael is owned by the Minister for the Arts, Heritage and the Gaeltacht on behalf of the Irish people, with the Office of Public Works having responsibility for the management of the site. It has been the subject of long-term conservation and stabilisation programmes since it was taken into state guardianship in 1880, processes which have intensified and been subject to greater scrutiny since 1996 when the site was inscribed on the World Heritage list.

Insufficient detail in base mapping, even when supplemented by a large scale (1:1000) photogrammetry project, was the driving force behind the decision to commission the lidar survey. The lidar survey has been used by the archaeologists of the National Monuments Service (NMS) of the DAHG to further both the research and management agenda. Examples include:-

(i) The extraction of detailed and accurate plans of the main archaeological complexes of the island. These have been used as a base map to accurately position old survey plans and drawings.

(ii) Detailed sections generated from the DSM 'on demand' have proved an extremely valuable additional resource, identifying structural components of the buildings and associated terraces, tracks and steps.

(iii) The DSM has been scrutinised in great detail and a number of new discoveries made, such as previously unidentified continuations of steps, and improved definition of many existing archaeological features. The ability to scrutinise the model in the GIS environment cannot be over-emphasised as in many cases access on the ground is impossible or highly dangerous.

(iv) Management of the site has benefitted from access to the lidar survey, with the models being used to identify and map areas requiring routine maintenance. This information, stored as GIS data, will establish those areas most at threat from erosion caused by tourists and enable action to be taken to reduce their impact.

Data Issues

The exceptionally steep and rugged terrain of Skellig Michael presented more survey problems than had been anticipated. The survey had to be flown twice as the first attempt contained a large data gap at the South Peak – one of the major zones of archaeological interest but also one of the most inaccessible parts of the island.

Interpolation errors also occurred during the initial processing of the raw lidar data that only became apparent in some areas of the model with the creation of the DEM surface. These appeared as an interference pattern in the model, but when viewed in profile looked as if they were associated with extreme slope angles or perhaps overhanging cliffs.

In other areas of the model some unusual issues occurred on some of the modern features which were accounted for as reflectance from standing water. Issues of this nature had not been encountered in previous projects where the remarkably clean data was a notable feature. These issues are still being investigated but have not had a significant impact on the usefulness of the lidar survey.

Conclusions

Working with this lidar data set has provided great benefits to the archaeologists researching Skellig Michael, taking the mapping and plans to another level of precision and accuracy. However, it is only by viewing the relief models in conjunction with other resources such as photographs and physically visiting the site that the most complete interpretation can be achieved. Since many of the archaeological features are stone built structures and the definition of these was a primary objective of the scanning project, the lidar-based relief model GSD of 12.5cm may not be adequate in certain areas. Here the application of terrestrial based scanning systems with a resolution of better than 5mm may be necessary. To complete the 3D documentation of Skellig Michael a terrestrial laser scanning project is planned with the intention of creating a seamless integration with the lidar data. Looking to the future, the fragile nature and inaccessibility of Skellig Michael point to the need to control and manage the number of visitors and offer alternative engagements with the site through the development virtual tours of the island in a digital environment. The level of detail in the relief model generated from the lidar survey would provide an excellent basis for such a development.

Acknowledgements

This project has been undertaken by the National Monuments Service of the Department of Arts, Heritage and the Gaeltacht. Special thanks to Dr Ann Lynch and Ed Bourke for there assistance in preparing this article, and to Con Brogan for accessing photography.