Trialling an Accessible Non-Contact Photogrammetric Monitoring Technique to Detect 3D Change on Wall Paintings

Abstract

ABSTRACT The visual comparison of 2D images, or ‘photographic monitoring’, is commonly used in cultural heritage conservation to assess rates of deterioration and the impact of interventions. While it is an accessible technique, three-dimensional changes can be difficult to discern and identifying change can be subjective. Structure from Motion (SfM) photogrammetry, which generates 3D point clouds from digital photographs, has the potential to make monitoring more objective and more informative by recording three dimensions of change rather than two. A methodology was tested on the Romanesque wall paintings of St Botolph’s church, Hardham, UK to develop a SfM photogrammetric condition monitoring system that is affordable, portable, non-contact, and repeatable. Commercially available camera equipment was used for capture, and readily available or free software was used to generate and detect change between two 3D point clouds. Time-SIFT, an in-software workflow of point cloud alignment, was used as a reliable non-contact method to replace adhered or embedded registration points on the wall to align the clouds from different capture epochs. The technique was able to identify change in the wall paintings, with the smallest verified changes at a scale of 0.5 mm. The results are discussed in relation to their advantages over 2D imaging, the avenues for further research, and the limitations of applying the technique on site. This research demonstrates the potential of photogrammetry as an accessible in situ condition monitoring tool using a workflow and equipment tailored to working on sites with wall paintings.