# 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
Only with a good overlap and complete coverage of the cave, it is possible to obtain an excellent final result. Lastly, in the post-processing phase, all the acquired data (i.e., benchmarks' coordinates and image sequence) are imported into the processing software where the cave 3D model is computed. The processing, which is subdivided into several stages, starts with recognizing and collimating reference points in every picture manually depicting them. Then, all the images are aligned according to their relative positions and input coordinates. It is essential to point out that, as the acquisition phase is done underground, the images are not provided by a shooting point coordinate. The aligned image set is then used to compute the 3D point cloud and the final model with mosaicked images draped on, resulting in the 3D reconstruction of the cave. Figure 3: Workflow of the proposed methodology. Using the same approach, it is possible to obtain a centimetric-resolution 3D model of the most interesting elements located inside the cave. In this case, the acquisition phase should be done by providing an ultra-high-resolution photo sequence of the target, obtained with a high-end digital camera. Increasing the resolution of the acquired data and processing settings it is possible to create a more detailed 3D model of the interior elements of the caves and provide a double-resolution model: a general medium-scale representation of the caves (1:500-1:1000 scale) with ultra-detailed models of the main elements of the cave at a centimeter resolution. The medium resolution approach is mainly finalized to the acquisition of a continuous 3D model of the cave. This model's final resolution is sufficient for a detailed reconstruction of the morphology of the cave but not so clear to obtain a correct visualization of the speleothems or the other essential elements that often enrich caves. The approach adopted for the ultra-high-resolution model acquisition is similar. Still, the number of images should be sufficient to guarantee both the complete description of the object and the capture of its character details. Their Ground Sample Distances (GSD) should be lower (up to millimeters) [64]. These are the principles that must guide the operator in the images capture to assure the correct survey resolution of specific areas or objects within an underground cave: indeed, it is not possible to define a general acquisition scheme for these types of features, given their irregularity and variability in the morphology of the features, but the survey must be adapted from time to time, trying to cover the entire area of the object. To scale the model, as previously stated, it is necessary to know the relative position of some spread materialized REF points on the study area. In the case of 3D acquisition of natural elements of particular value, it is helpful to exploit removable markers to guarantee the preservation of the surveyed elements or speleothems. Due to the limited space that often characterized the caves, the measurement of these points may be difficult, if not impossible, even with a portable range finder: in these cases, it is appropriate to take advantage of templates of known size to ensure proper scalability of the model to reduce the number of required reference points.