# Survey Solutions for 3D Acquisition and Representation of Artificial and Natural Caves
## 5 Conclusions  
The establishment of new technologies, instruments, and algorithms for the execution and processing of three-dimensional surveys have expanded the possibility of performing such surveys in underground environments, like caves. The traditional approach was based on a series of reference points along the cave measured by clinometer and compass and led to a discrete representation of the cave. This method is currently improved by using laser portable range finders, which allow faster execution of measures and their storage in built-in memory or in an app. The methodology proposed and tested in the work is aimed to achieve a full three-dimensional reconstruction of the cave while assuring its accuracy in terms of positioning. To test the methodology, before the cave survey, a benchmark site was surveyed. A stretch of a tunnel was surveyed by TLS, PLS, and camera shooting. All the three-point clouds were registered to the reference targets with centimeter-level accuracy, i.e., 0.015, 0.010, and 0.012 m, respectively, allowing the following phases of the research. The tests provided that the point clouds obtained by PLS and by SfM processing of the shooting are in good agreement with the TLS one, selected as the reference data set. PLS-TLS comparison provided an average residual of 0.053 m while SfM-TLS 0.040 m. The tunnel was also selected as a test site thanks to the easy accessibility, which allowed to employ of bulky instrumentation. According to the test site result, we developed a methodology to survey natural caves adopting only a photo camera and SfM processing. The proposed methodology is tested in the Gazzano cave, located in the Garesso municipality (Piemonte region, northwestern Italy). The cave is 105 m long, and it has several chambers, with a diameter that ranges from 37 m up to less than one meter. The cave was provided with several reference points surveyed by the cave that were used as ground control points. According to the proposed methodology, the image acquisition was carried out in both directions from the entrance to the end of the cave and backward. The SfM processing provided the point cloud reconstructing the entire cave except for the final chamber whose ceiling was 25-m-high and images were not enough light. During the survey, a peculiar stalagmite was surveyed, too. This survey aimed to reconstruct a single object with a higher resolution if compared to the whole cave model. The reconstruction of this speleothem allowed to confirm the validity of the proposed method. The registering of the whole cave was carried out with an overall accuracy of 0.331 m; the scaling accuracy of the speleothem reached an overall accuracy of 0.0135 m.  
By analyzing the benchmark and the cave survey, it was also possible to summarize the different survey methods' pros and cons in underground contexts. Namely, SfM resulted in the most manageable and affordable, both in terms of equipment and processing software. On the contrary, PLS and, particularly, TLS are expensive and with less maneuverability. A possible solution, for cost reduction, is the use of available open-source alternatives of processing software which can, even if often lacking user-friendly interfaces, provide similar functionalities. Conceptualization, D.G. ([PERSON]) and M.P; data curation, N.G.and R.Z. ; investigation, D.G. ([PERSON]), M.B., N.G.and R.Z. ; methodology, D.G. ([PERSON]), [PERSON] ([PERSON]), M.B. and R.Z. ; supervision, M.P. ; validation, M.B. ; writing--original draft, [PERSON] ([PERSON]), [PERSON] ([PERSON]) and N.G. ; writing--review and editing, M.P. All authors have read and agreed to the published version of the manuscript. * [PERSON] et al. (2011) [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] [PERSON] Laser scanning technology for the hypogean survey: The case of Santa Barbara karst system (Sardinia, Italy). _Acta Casrologica_**2011**, _40_. [CrossRef]
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