Historic stone pavement photogrammetric 3D-survey: a way to get a catalogue to pass on ancient crafts

In Tuscany, the art of paving has never been codified in written form, being already encapsulated in knowledge based on practical experience, with specialized tradesmen and stonemasons relying on direct observation and passing on their knowledge to apprentices through repetition of gestures. The streets of Volterra, a historic town in Tuscany, are still paved with the local stone, the panchina volterrana (Volterran panchina stone). Photogrammetry techniques allow high-resolution reconstruction of 3-D pavement models and geometric representation of the macrotexture component resulting from the chiselling of the stone, as well as gaining high-resolution, high-precision geometry information along with colour information. In this way, it is possible to fully characterize the stone and generate a catalogue, which can become an operational tool for those involved in monitoring and maintaining pavements. This paper presents the methodology of photogrammetric surveying and the possibility of obtaining indices of the macrotexture of the processed surface of the stone, in terms of Virtual Mean Texture Depth (V-MTD), in analogy with the analysis of asphalt surfaces. The classification is also proposed by integrating the geometric macrotexture data with the data derived from a segmentation of the different textures obtainable for the model (diffuse, ambient occlusion). This geometric information could be then enriched by the stonemason's indications, constituting, therefore, a tool for passing on one's knowledge and experience.