3D digitization of the Sirkeli rock reliefs

With the advent of digital technology, aspects related to the study, understanding and enhancement of cultural heritage have undergone a radical change, shifting the limits of perception and critical schemes beyond the conventional evaluation procedures, imposing a joint knowledge of the humanities and technologies. Digital data generation processes require a clear strategy for their use, offering vast possibilities in terms of extending the quantity and quality of information. While the development of three-dimensional digitization systems, especially linked to the use of Structure from Motion technology and increasingly intuitive photogrammetry software and to the spread of high-performance and low-cost digital cameras and drones, has led to a strong increase in the applications of 3D surveying in numerous fields of the professions and research, there has not been an equal development of theories and methods of data management within their extended life cycle. More and more the post processing phases stop at the production of point clouds or mesh models as automatically generated by photogrammetric processing software, from which metric information, images or simple animations can be extracted. All this stems from the ordinary approach characterized by a total absence of a survey project and even more so of a strategy for using data, which, if well structured, allow complex processes of analyzing models, extracting information not visible in the real environment, managing and interrelating data in GIS or SIM platforms (Repola 2018), and representing and communicating data through new languages that now extend the complex spaces of the virtual. Furthermore, the performance afforded by 3D scanners (quantity of points recorded per scan and speed of execution) allows even complex geometries to be studied and modelled far beyond the limits of conventional surveying techniques. The need to reconstruct surfaces with varying curvatures entails a continuous surveying of points in space that only a three-dimensional scan can offer through the so-called 'point cloud', which represents the real mathematical model of the object to be surveyed. This process therefore appears to be the opposite of the classic method, in which a number of fixed points are measured and then two-dimensional drawings are made and finally the volumes are reconstructed in 3D, whereas in the case of the 3D scanner survey, the starting data is a model in the form of points in its entirety, from which one starts to obtain the set of information sought, including, for example, orthogonal projections (the classic plans, elevations and sections). The three-dimensional survey can be performed with active or passive systems, to the former belong three-dimensional scanners in their different types, time-of-flight, phase-difference, optical triangulation, structured light; to the latter belong photogrammetric systems, based on the processing of images acquired according to specific procedures. The schedule for the three-dimensional digitization of the Sirkeli archaeological site involved the use of both systems, specifically ground and drone photogrammetric systems and Riegl's LMS-Z420i time-of-flight scanner and Artec Eva structured light scanner.