The application of SfM µ-photogrammetry to the understanding of material decay and morphology

In Cultural Heritage studies the documentation and 3D graphical representation of the artefacts is an important issue both for operative and archiving reasons. Instruments such as Laser Targeting Systems (LTS) have been continuously improved, reaching unprecedented resolution down to 1 mm, becoming standard tools for fast 3D acquisitions of vast volumes or objects with complex shapes that couldn’t otherwise be measured. Nonetheless, LTS sensitivity can be not sufficient for the study of material decay throughout time and the description of the surface morphology. In order to fill this gap, in our research we tested Structure from Motion μ-photogrammetry, a completely non-invasive technique deepening the concept of photogrammetry [1]. The method has been applied on cemetery headstones (CWGC Cemetery, Padua, Italy) made of a compact white limestone (Botticino Stone, Italy). The chosen headstones were characterized by medium decay and were not surrounded by vegetation which would have impeded a rigorous multi-view image acquisition. An APS-C Reflex camera (Nikon D-300, equipped with a 24-85mm F/2.4-4 lens) set at 50 mm focal length was used and a special holding rack has been designed in order to minimize focusing error: for a slate of about 76x38x7.6 cm, about 600 shots have been taken. SfM images were then elaborated with Photoscan software to obtain 3D models with resolution of about 25 μm (Figure 1), allowing measurement of volume loss caused by surface recession and the identification of small-scale heterogeneities which are more prone to recession. We have thus shown that this innovative technique is an ideal tool in the study of decay of carbonate rocks and in the determination of surface recession rates.