# Survey Solutions for 3D Acquisition and Representation of Artificial and Natural Caves
## 4 Discussion
### Pls  
PLS were developed for dynamic use in indoor spaces [37]. This class of instruments evidenced its interesting potential in the survey of the tunnel segment. The limited range of the instruments is compatible with the tunnel section's size and the acquisition of a sequence of close paths allows the progressive exploration and acquisition of the studied area. The main advantage given by the use of this technology, to the others (Table 8), is the real-time generation of complete point clouds describing the surveyed environment, which takes place using mapping and registration algorithms integrated into the acquisition system. One of the critical points of this system is the continuity of the survey that is \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Cost (Instrumentation)** & **Cost (Software)** & **Weight** & **Manageability** \\ \hline TLS & From \(\sim\)30 k \(\epsilon\) to \(\sim\)150 k \(\epsilon\) & From \(\sim\)3000 \(\epsilon\) to \(\sim\)8000 \(\epsilon\) for annual licenses (1) & From \(\sim\)5 kg to \(\sim\)13 kg & Low \\ \hline PLS & From \(\sim\)30 k \(\epsilon\) to \(\sim\)60 k \(\epsilon\) & Embedded software providing real-time solutions & From \(\sim\)800 g to \(\sim\)13 kg (e.g., backpack solutions) & Medium \\  & & & \\ \hline SfM & From less than 100\(\epsilon\) (e.g., webcams) to \(\sim\)3000–4000 \(\epsilon\) & From \(\sim\)2500 \(\epsilon\) to \(\sim\)4000 \(\epsilon\) for annual licenses (1) & From \(\sim\)200 g to \(\sim\)500 g & High \\  & & & \\ \hline Today there are free/open versions of processing software, although they cannot always guarantee the same usability (e.g., graphical interface) of commercial software. & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Characteristics of the analyzed instrumentations and peculiarities of the employed techniques. required for a good result. This continuity could be guaranteed in a tunnel, but it is challenging to assure in a natural cave, where some corridors are very narrow and difficult to cross. The best practices in the use of PLS highlight the need to retrace at least part of the path several times to optimize the registration, taking advantage of the matching of common features between the clouds [66]. Additionally, it is recommended to start and finish the acquisition at the same point, to exploit loop closure algorithms to correct global drift errors [68]. Again, these operations may not be feasible in these complex natural environments. The operator's experience in handling and managing the instrument also has a strong influence on the acquisition process's final result [41]. It is necessary to avoid moving the instrument abruptly or to make specific movements that can negatively affect the attitude estimation and data processing algorithms, such as vertical movements, rotating the instrumental origin or turn, and tilt at the same time [69]. Often, acquisitions too close to surfaces (<30-50 cm) can create problems in the point clouds registration phase, as can happen in tight quarters. The active sensor of PLS is an important feature of this class of instruments that can be vulnerable in natural caves where the level of humidity is very high. As described for TLS, the moisture can hamper the use of PLS because the water on the sides of the cave reduced the reflectivity of the material, and, with intensive use, it can damage the instruments. In general, these environmental conditions are unfavorable for all active sensors. Unlike the individual scans acquired with TLS, it is impossible to manually remove any errors in PLS products. This limitation increases the noise of the final point cloud generation and could also create some registration errors [67]. These critical errors cannot be easily removed even with post-processing software.