# Survey Solutions for 3D Acquisition and Representation of Artificial and Natural Caves
###### Abstract  
A three-dimensional survey of natural caves is often a difficult task due to the roughness of the investigated area and the problems of accessibility. Traditional adopted techniques allow a simplified acquisition of the topography of caves characterized by an oversimplification of the geometry. Nowadays, the advent of LiDAR and Structure from Motion applications eased three-dimensional surveys in different environments. In this paper, we present a comparison between other three-dimensional survey systems, namely a Terrestrial Laser Scanner, a SLAM-based portable instrument, and a commercial photo camera, to test their possible deployment in natural caves survey. We presented a comparative test carried out in a tunnel stretch to calibrate the instrumentation on a benchmark site. The choice of the site is motivated by its regular geometry and easy accessibility. According to the result obtained in the calibration site, we presented a methodology, based on the Structure from Motion approach that resulted in the best compromise among accuracy, feasibility, and cost-effectiveness, that could be adopted for the three-dimensional survey of complex natural caves using a sequence of images and the structure from motion algorithm. The methods consider two different approaches to obtain a low resolution complete three-dimensional model of the cave and ultra-detailed models of most peculiar cave morphological elements. The proposed system was tested in the Gazzano Cave (Piemonte region, Northwestern Italy). The obtained result is a three-dimensional model of the cave at low resolution due to the site's extension and the remarkable amount of data. Additionally, a peculiar speleothem, i.e., a stalagmite, in the cave was surveyed at high resolution to test the proposed high-resolution approach on a single object. The benchmark and the cave trials allowed a better definition of the instrumentation choice for underground surveys regarding accuracy and feasibility. structure from motion; LiDAR technology; underground survey; speleology; GPS denied environment; 3D cadastral map of caves +
Footnote †: journal:capable of measuring angles and distances such as the total station. Still, the space complexity limits the use of some instruments for 3D surveying or requires the operator's great experience. Additionally, the three-dimensional representation of the caves is even more complex. The task is complicated by extreme environmental conditions, such as low/variable illumination, humidity, and limited accessibility [6]. Traditionally, the mapping procedures in underground cave environments were carried out by tracing a three-dimensional polyline by using clinometers and compass to store the different angles featuring the survey; each segment implied the marking of its vertices on cave walls to serve as a reference. The cave outline was then manually plotted by the cave with a remarkable level of subjectivity (Figure 1) [1; 7], including systematic and random errors. Recently, the use of clinometers and compass was substituted by the spread of handheld topographic instruments such as portable laser range finders equipped with digital clinometers and compass [1; 7]. The measurements acquired with the laser range finder are stored in the device or, currently, in a phone app (e.g., TopoDroid) linked to the range finder where the cave can visualize the entire survey [8]. Moreover, in every station point, the survey procedure implies a series of circular measures to trace a transverse section of the cave obtaining a discrete three-dimensional representation. The app allows the cave to manually draw the cave outline adding details to the survey (Figure 1). The obtained final result is a subjective representation of the cave done by the cave on a network of measured points. The resolution and accuracy of the three-dimensional model are often highly heterogeneous and rely on the cave's ability that connects the sequence of dots and creates the representation of the surveyed area.