# Smartphone-Based Photogrammetry for the 3D Modeling of a Geomorphological Structure
## 7 Conclusions  
The aim of this work was to analyze and propose a low-cost technology and methodology in order to realize 3D representations of objects of geomorphological interest. The aspects examined concern the entire process, which, from the acquisition phase, leads to the analysis of the final result accuracy. The issue to be tackled was to find solutions able to guarantee a certain accuracy of the data but at the same time able to fulfill the requirements of portability, low-cost, speed in data acquisition and flexibility in order to adapt their use in complex environments. Nowadays, smartphone devices are accessible to everyone in the mass-market, and although they have not been created for metric purposes they solve some problems related to the acquisition of 3D data. However, due to these devices being generally realized for amateur purposes, it has become crucial to study their characteristics, analyze their behavior and test performances in depth, before using them in surveying operations. The calibration problem was one of the addressed topics in this research, because in photogrammetry a good accuracy of the final model cannot ignore the correct calculation of the internal parameters of the camera. Attention was dedicated to the planning of the acquisition procedures, in order to guarantee the capture of a limited number of images for the three-dimensional reconstruction, sufficiently overlapping between them. Moreover, it has been demonstrated how the acquisition geometry influences not only the accuracy but also the resolution of the 3D model, through the estimation of the GSD. In particular, the proposed methodology was, first, assessed in a controlled environment, on a single small object (large scale) and, subsequently, tested on real cave walls (small scale). Many tests were performed in order to identify the limits and potentialities of the proposed solution in the generation of three-dimensional models of a rock sample, combining the use of different image datasets with the potentialities of the Structure from Motion approach, implemented in many pieces of software, both commercial (AMP) and open source (VSFM), and in a smartphone application (SCANN3D). The used software tools offer ease of use and automation in the data processing and are able to realize suitable products in quite a similar way, although there are some slight differences in the results, including in the free and open source software. The potentialities of the proposed approach were evaluated by analyzing the RMSEs with respect to topographically measured control points and check points and by comparing the dense point clouds with the reference model obtained from a professional camera. The obtained results demonstrated that the behavior of the different software tools in terms of performance is quite different. On the one hand, the data processed with software running on computer devices allow us to reach accuracies of about 1 mm on control points, which was also confirmed by the comparison with the reference point cloud; this makes them suitable models to be exploited for scientific analyses. On the other hand, it was proven that the algorithms exploited by the SCANN3D application must be optimized in order to be able to locally generate a 3D model on a smartphone platform. However, it is clear that \"mobile\" photogrammetry strongly depends on the computational power of the devices, and, above all, we still need hardware that is more powerful than that of the current generation of high-end smartphones, if the goal is to generate 3D models in real-time. Given the good results of this first attempt to use smartphones for the generation of 3D models of objects of geomorphological interest, some preliminary tests were conducted in order to investigate how the proposed methodology behaves in different environmental conditions, such as in a real underground cave, in which the various operating conditions could affect the results: