The Discovery Programme worked in collaboration with the National Monuments Service of the OPW to undertake the 3D recording of a selection of the Clocha Breaca, decorated ‘cursing’ stones from early medieval church site on Inishmurray, Co. Sligo. Originally located on the leacht at the Cashel the stones (worked sandstone) had been removed from the island for conservation and protection from the impact of erosion. Eight stones (with two stone ‘stoppers’) were selected for scanning, ranging from 15 – 50cm in diameter with a variety in the level of carved detail.

The objective of our survey was to generate 3D data at an appropriate accuracy and resolution to enable surface models to be generated showing the subtle carved designs on the stones. A complete surface for each stone was required as a long term objective of the project is to create physical replicas which could be placed in the original outdoor setting.

The Discovery Programme has a NextEngine 3D Scanner HD scanner (Model 2020i), a triangulation laser scanner which has been successfully used in the past to record extremely fine stone work and particularly carved stone art. In this case the stones presented a different challenge than previous decorated stone projects in that they had to be considered as complete 3D objects, with a seamless surface model the required final result. They also presented scans with a greater depth range than encountered when scanning flat rock surfaces. The practical impact of these issues was the need to plan the position of each scan carefully – ensuring the area to be scanned was well within the depth of field setting, and that the overlap was carefully planned. A larger than normal overlap (approx 30%) was used given the complex geometry on the scan registration.

 

When all the scans have been registered the residual errors can be reduced using an automatic refinement function known as a global registration. For this project our objective was to ensure the final residual did not exceed 0.3mm for any of the stones. Some tidying of the pointcloud data - functions such as removal of outliers and noise reduction were performed before the clouds were merged into single pointclouds for each stone. A surface wrap was then generated - triangles fitted between neighbouring points to create a surface. This is an automated process with controls to sample the data further to define the density of triangles. For this project, to ensure the highest surface definition the no sampling of triangles was applied.

 

 

 

 

 

 

 

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The 3D surface models generated for the Clocha Breacha are of an excellent quality and should further the understanding of their form and designs. The Surface wrap files provide a 3D data source to generate replicas if the project progresses in that direction.

The two reports submitted to the OPW can be downloaded using the following links

 

 Clocha Breaca Survey Report (763.93 kB),  Clocha Breaca Modelling report (1.02 MB)

 

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