# Survey Solutions for 3D Acquisition and Representation of Artificial and Natural Caves
## 2 Material and Methods
### Development of a Methodology for the Acquisition of a 3D Model of a Natural Cave  
The comparison of the acquired 3D models, as reported in Section 3.1, shows that the solution based on the use of a photo camera and SfM can be adopted for the acquisition of a sufficiently accurate 3D model, making it suitable for the reconstruction of the geometry of the studied area. Furthermore, this solution limits the use of expensive and heavy instruments which can hardly be carried in tight spaces. For this reason, SfM can be considered a good solution for the acquisition of the 3D model of natural caves, where the use of more complex topographic instruments can be complicated (in several cases nearly impossible). Starting from the experience using the SfM approach in different environments (e.g., [6; 61; 62; 63]), we defined and tested a methodology dedicated to the survey of natural caves for their 3D modeling at different scales. The proposed method is divided into three different phases (Figure 3): (i) REF Polygonal survey; (ii) photographic survey; (iii) post-processing. Firstly, the cave needs to be located and framed by a topographic survey. This step is fundamental for the correct orientation and georeferencing of the photo sequence. The operation starts outside the cave by properly defining two, or more, reference benchmarks to refer the whole survey to a geographical reference system rather than a local one. Subsequently, measurements continue inside the cave by selecting a sequence of reference points on cave walls and features. The materialization of inner benchmarks should be carefully realized without damaging cave features and by minimizing the impact; for these reasons, a small varnish mark with color in contrast with the surroundings (e.g., red or orange) can be sufficient because the distance between the target and the photo camera is always limited. Additionally, to maximize its accuracy, the polygonal survey should be carried on twice in both directions to close the polygon. The survey phase of benchmarks is performed using a portable range finder capable of measuring and storing angles and distances contemporarily. The position of external reference points can be acquired using GNSS systems. \begin{table}
\begin{tabular}{c c c} \hline \hline
**TLS** & **PLS** & **Photo Camera (SfM)** \\ \hline RIEGL LMS-Z420i & Kaarta Stencil 2–16 & Canon EOS 100D camera \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Adopted instrumentation for the acquisition of the 3D model of the test site. The subsequent phase is dedicated to the capture of the cave by the camera. Images should be captured in accordance with the principles of SfM thus varying the capture position in every picture and, contemporarily, assuring an adequate overlap. As the operation is carried out in a completely manual way, the capture should follow a squiggly path (Figure 4) with the camera framing the cave in direction of the operator path. Due to the extreme variation of the cave's size and shape, it is impossible to strictly define a priori acquisition parameters. The most critical elements are the required overlap (more than 70%) and the illumination of images; according to our experience, the use of flash is strongly discouraged as it doesn't assure a diffuse illumination necessary for an adequate rendering of the cave in post-processing. Concerning the camera settings, they should vary according to the lighting conditions and size of the cave. The best way to illuminate the cave is to exploit the light mounted on the cave's helmet and, when it is possible, to place another operator ahead illuminating in the opposite direction. The device used in the current experiment is a superkikko (kikkolamp.com) set at 200 lumens in the corridors and 1400 lumen in the main chamber. In any case, the illumination should be indirect to avoid blinding effects on the camera sensor. The two presented phases are tightly connected and of high importance for the correct acquisition of the image sequence.