# Smartphone-Based Photogrammetry for the 3D Modeling of a Geomorphological Structure
## 1 Introduction  
Geomorphological and geological observation in underground environments such as caves could still represent a challenge. These environments are often prohibitive, given the difficulty of access and the conditions, even extreme, of humidity and temperature inside them. The continuous evolution of sensor-based 3D surveying and modeling techniques and the development of increasingly efficient systems for the visualization of digital data highlight the added value given by the use of these methods in the geological field. Indeed, the technological development of surveying instruments allowed passing from subjective descriptions to illustrate the environment [1], influenced by human perception, to full three-dimensional models. However, it is only in the last few years that lighter, cheaper and easier-to-introduce versions of surveying instruments have been sold on the market. The most recent electronic surveying equipment allows one to obtain a complete description of the environment and to represent all those morphological details that characterize the floors, walls, and roofs, which are fundamental for drawing conclusions on the evolution of the underground environments. To date, beside instruments typical of classical topography, such as Total Stations (TS), both active sensors and passive sensors can be mainly used [2]. Active sensors are now widely used in underground caves to extract 3D information [3; 4; 5; 6; 7; 8; 9; 10; 11], and include all the scanning devices that emit a signal (in the form of a coded structural light pattern or through electromagnetic radiation in the form of a modulated laser light) and that record information from scene observation. Laser scanners are the most popularsolution, and today it is possible to find cheaper and smaller versions, such as profiler [12] and handheld laser scanner devices [13,14]. The active optical sensors [15] allow to directly obtain the spatial position of the detected points, sometimes coupled with the color information; the latter can be recorded by the sensor itself or by an external digital camera integrated into the instrument. These type of instruments have the main advantage of directly and quickly acquiring large amounts of data related to a complex geometry, with a very high accuracy. The raw output data of laser scanner sensors are unstructured point clouds, one for each acquisition point (in the case of a classical laser scanner), which need to be registered in a unique reference system to produce a single point cloud of the scene. Typically, in terrestrial laser scanners (TLS) a mirror rotates vertically and the whole instrument rotates horizontally, allowing a sequential scanning of the scene in both horizontal and vertical directions. There are also handled versions of laser scanners, but they do not always allow for the acquisition of the radiometric information of the object, or they are still very expensive and require some experience, especially in the post-processing of data. These instruments, however, can be easily transported and introduced in restricted access environments [16], but they have limited capabilities when it comes to describing details and edges, which become curved and approximate. Finally, 3D data delivered by active sensors are generally affected by the light and surface characteristics of the scanned materials, by the glass and by specular materials [2]. On the other hand, passive sensors attribute the radiometric information of the environment to a 2D matrix of pixels. The transition from the 2D data to the 3D model takes place thanks to different strategies, using a classical photogrammetric technique [17] or the more recent integration of Computer Vision (CV) algorithms [18], through stereo matching [19] and the Structure from Motion (SfM) [20] approaches. Image sensors such as digital cameras, action cams, and webcams, allow one to capture input data for a 3D structure reconstruction with these approaches.