Scanning the High Crosses of Iona

The Discovery Programme was invited by the School of Humanities at the University of Glasgow to record and model in 3D the collection of sculpted High Crosses on Iona, a small island in the Inner Hebrides of Scotland. These are some of the earliest and best examples of early medieval crosses in Scotland and are housed in the recently renovated museum, the old infirmary building of Iona Abbey, which is managed by the public body Historic Environment Scotland (HES). Access to the museum was kindly given by HES for four days working early in the morning and late afternoon and evening when the museum was closed to the public.

The projects objective was to use 3D scanning technology to record in high resolution the surface detail of three High Crosses, St John’s, St Matthew’s and St Oran’s, along with a selection of carved grave slabs of interest to the research team in Glasgow. These are extremely challenging objects to scan, not only in terms of scale and complexity of geometry, but also the museum setting, shown in Figure 1.

Figure 1 – Iona museum high crosses. St John’s in the foreground, behind left is St Oran’s, behind right is St Matthew’s

Special mounts have been used to allow the stone fragments to be displayed upright, presenting the crosses standing as they would have appeared, up to 4.2m high.  This creates a wonderful experience for the visitor, but the steel frames and clips are difficult challenges for the scanning, obscuring some important detail.

Figure 2 – The rear and side views of the crosses of St Oran and St Matthew show the impact of the frameworking

The Artec Eva was the 3D scanner of choice for the survey, providing the ideal combination of flexible hand-held scanning operation with a sub-millimetre resolution result, matching the level of detail the intricate stone carving demanded. Each of the crosses was divided into smaller scan jobs to retain manageable project sizes, for example the shaft and head of the cross of St John in the foreground of Figure 1 were scanned as separate jobs. To allow the detailed Artec scans to then be placed in their correct position with respect to one-another a terrestrial scan was also taken of the museum space and lower resolution models of the crosses generated.

Figure 3 – Artec EVA scanning St John’s cross, Iona

Ladder access was used to reach the higher elements of the crosses, and for the extreme high parts the scanner was used on an extendable pole. During these more awkward scanning positions a small 7” monitor was connecter to the scanner to enable the operator to retain a visual of the scan process.

Data processing followed our standard pipeline. This begins on-site in Artec Studio software with a check that quality thresholds have been achieved, that the initial alignment of scans is successful, and that coverage is complete. This gives confidence that the more complex, time consuming processing that takes place back in the office will be successful.

Figure 4 – Views of the final model of St Oran’s cross taken from Meshlab, rendered using radiance scaling shader

The Editor and Tools functions of Artec Studio are used to delete unwanted scan data, complete the registration process and then finally create a fused surface of the cross (or section of cross). This is effectively the final, full resolution model and is exported as a standard .obj or .ply file.

Our processing pipeline takes the .obj file into Geomagic Studio to finalise the surface model – running mesh doctor functions to ensure a high quality, high resolution surface, the primary objective of the survey, and this is the end product for each of the crosses. Effective visualisations of these surfaces are achieved in Meshlab, using the radiance scaling function to highlight the ornate carvings of the crosses. Figure 4 and Figure 5 illustrate the quality of models achieved with a selection of views of the final models, visualised in Meshlab.

For dissemination purposes further processing steps are followed which were developed through our involvement in the 3D-ICONS project, resulting in low polygon, lightweight models for web-viewing. Our work is hosted by Sketchfab and the Iona collection is accessible through, https://sketchfab.com/discoveryprogramme/collections/iona.

Figure 5 – Views of the final model of St John’s cross taken from Meshlab, rendered using radiance scaling shader