# Survey Solutions for 3D Acquisition and Representation of Artificial and Natural Caves
###### Abstract
Thanks to technological advances over the last decade, several 3D surveying techniques for generating full 3D and multi-resolution models of underground cavities are now available. Besides topographical instruments, such as Total Stations (TS), it is pos  
Figure 1: Example of a cave survey based on the laser range finder approach; the post-processing software adopted by caves can be used for a coarse 3D reconstruction of the cave. The model presents the Gazzano cave that is the case study that is described and discussed in this manuscript. sible to get 3D information about the environment in the form of point clouds quickly and easily and with high levels of details thanks to a wide range of passive and active sensors [9]. These sensors are now commonly used in different application fields (e.g., cultural heritage, monitoring, etc.) for the reconstruction of scenes or 3D objects and there are several studies aimed at comparing the use of different techniques to highlight their advantages and criticality and to define the best practice to follow [10; 11; 12; 13; 14; 15]. To achieve more accurate and complete surveying results, it is crucial to be aware of the properties that certain technique needs to respond and of the requirements of the desired results [16]. Firstly, the selection of the most appropriate survey approach should consider not only the precision and the reliability of the resulting products, but also the size and manageability of the final datasets. Then, from the operational point of view, it is fundamental to evaluate the portability of instruments, especially in underground sites with limited accessibility. Other important elements are the required acquisition time and the autonomy of the power system of selected survey solutions. Finally, the conditions of the survey environment greatly influence the choice of the most appropriate technique, especially in the case of underground caves, where the difficulty of access, humidity, temperature, and lighting conditions strongly reduced the number of effective solutions. Both active and passive remote sensing techniques are, nowadays, widely exploited in underground environments surveys for 3D modeling purposes. Active remote sensing-based instruments include all the scanning devices, such as Terrestrial Laser Scanners (TLS), emitting laser pulses [17]. A surface or object reflects each pulse, and the instrument is capable of computing the distance and position, in polar coordinates, of the point. The intensity of the echo is registered, too. According to its performance level and its settings, the instrument can repeat this sequence from 10,000 to 500,000 times per second. Moreover, to acquire a complete survey of the surroundings, laser beams are projected in several directions using rotating and/or oscillating mirrors. The color information can also be recorded thanks to the integration of the system with a digital camera. For each scan station, raw output data is generated in the form of a point cloud. The data registration is carried out by positioning reference targets (natural or artificial), which were previously surveyed by a topographical survey, to georeference the resulting point cloud in a specific reference system [18]. TLS is now a consolidated approach to obtain a 3D measurement. Despite its encumbrance and weight, some tentative approaches to use it in cave surveys were attempted. They were especially tried in caves open to tourists and thus equipped with stairs and footbridges, easing the transport of the instrumentation, as evidenced by several research works [19; 20; 21; 22; 23; 24; 25] or in sites relevant for scientific purposes such as the Naica cave [26]. TLS allows capturing highly accurate data even at long distances but, in indoor environments and complex areas characterized by limited lines of sight and many shadow areas, it often cannot be an efficient method given the need for a high number of station points.