# Survey Solutions for 3D Acquisition and Representation of Artificial and Natural Caves
###### Abstract
Thanks to the high level of detail obtained with this technique, TLS has been used in many studies to detect limited portions of caves, to conduct morphological and structural analysis [27; 28; 29; 30]. Only in some works, as mentioned by [31], TLS were used to make surveys of entire caves, with extensions greater than a few hundred meters. Although these surveys allowed to obtain detailed models, they demonstrated that, due to the huge amount of final data, the point clouds must be divided into parts to be analyzed or sub-sampled, with a consequent loss of resolution. To overcome these applicability issues, in recent years, a new generation of range-based profilers was pushed on the market. This new generation of instruments is represented by integrated and handheld solutions called Indoor Mobile Mapping Systems (IMMS) [32]. These systems implement multiple sensors, such as profilers, Inertial Measurement Units (IMU), Distance Measuring Instruments (DMI), and, in some cases, camera sensors, in a unique platform. The portability and flexibility of these solutions make them particularly suitable for indoor mapping applications [33; 34]. These instruments are generally based on Simultaneous Localization and Mapping (SLAM) technology, including specific algorithms to solve the problem of localization, which allow to, simultaneously, es timate the instrument position and generate a 3D digital model of the environment [35; 36]. Consequently, SLAM technology makes IMMS a competitive possibility in terms of time consumption in data acquisition and data processing. Several IMMS solutions are now available on the market. Among the different types, many of them are integrated on vehicles, trolleys, backpacks, or handheld platforms [33; 37], and several researchers analyzed their performances [37; 38; 39]. The maneuverability of these tools makes them particularly suitable for indoor applications and narrow spaces [40; 41], even in environments with limited access, which may require a certain speed in the acquisition phase, as could be the case of caves or mines [42; 43]. Among passive solutions 3D information can be extracted, thanks to different approaches involving classical photogrammetric technique [44] or Structure from Motion (SfM) algorithms [45; 46] from a sequence of images. The SfM approach is based on the capture of multiple shots that portrays the surveyed object or area. The algorithms developed in the computer vision field and widespread in RPAS (Remotely Piloted Aircraft Systems) applications [47] exploit the photogrammetry principles and allow the 3D reconstruction of the surveyed target [48]. The algorithm's capability allows reconstructing the surveyed object even from pictures taken for other aims (i.e., reconnaissance photos) sufficiently overlapping (70-80% of overlap), allowing to exploit archived dataset and obtain unprecedented datasets [49]. SfM can be considered a valid and low-cost solution for the acquisition of 3D models that can be considered when it is impossible to use other well-known systems like TLS or other topographic solutions [50]. Furthermore, xxx by knowing the internal and external orientation of a digital image, using a dense 3D digital model, it is possible to associate the value of the distance between the center and the object to the same pixel. This procedure allows realizing a solid image, which is, therefore, the result of the integration of the RGB digital image with distance, allowing the restitution of the third dimension [51]. The reconstruction of the photogrammetric point cloud can also be registered with reference points to be plotted in geographical coordinates. The advances in sensors miniaturization and optimization have allowed developing portable instruments that are currently at an experimental level trying to substitute the current ones. Today, image sensors are integrated into commonly used devices, such as action cameras and smartphones, giving, even to non-expert operators, the ability to exploit collected data for the generation of 3D models [6].