# Smartphone-Based Photogrammetry for the 3D Modeling of a Geomorphological Structure
## 1 Introduction
[32], who demonstrated the effectiveness of close-range photogrammetry for rapid mapping, using images acquired from tablets, in post-earthquake scenarios. [PERSON] et al. [33] investigated the potentialities of the acquisition of high-resolution topographic and terrain data using hand-held smartphone technology for the generation of digital terrain models (DTM) of a river bank and an Alpine alluvial fan. These studies highlighted that an image-based approach that exploits mass-market devices allows for the reconstruction of geometrically correct models at different scales with a centimeter accuracy, or even higher. Moreover, the discussed methodology allows one to extract, in addition to the geometric properties of the object of study such as the shape and volume, its radiometric information [34], which characterizes each material; furthermore, this a fundamental aspect to take into account in order to define, for some applications, its properties and values. The aim of this work is to investigate the potential of a smartphone device in the 3D reconstruction of objects of geomorphological interest and to propose a procedure that can be easily used, even by non-expert operators, based on the principles of photogrammetry. This refers in particular to researchers and experts interested in the geological and geomorphological evaluation of the different conformations present in an underground cave. Before applying this technique in an underground cave, the performances obtainable with a smartphone device are tested in a laboratory environment, in order to verify the precision and accuracy that are obtainable with these low-cost instruments. In addition, it is therefore important to define an operative methodology to be proposed in the field to non-expert users and which allows for a correct acquisition of images for the generation of 3D models; these are fundamental in order to have a good geological, geomorphological and tectonic knowledge of the subsoil, both for purely speleological or geological purposes and for the stability analysis of cavities. The proposed methodology is designed to allow for acquisitions with easily transportable instruments (commercial cameras, tablets and smartphones) that are also available locally and that are used in limited spaces: often the elements of interest and the areas to be detected are located in areas that are difficult to reach, where it is possible to stop only for a short time, so that it is impossible to operate with LiDAR instruments. Moreover, if on the one hand the methods based on active sensors ensure rapidity in the survey and processing of dense clouds, on the other hand image-based procedures allow for the use of less expensive equipment, resulting in the same final product. The main novelty of this contribution is the use of the computer vision algorithms through the use of images acquired from a smartphone, exploiting both known software solutions and, above all, a smartphone application, which is quite a new approach for this type of application. Only the data from smartphone images were considered, without any integration into the model of other sorts of data (e.g., TLS point). Dense image matching techniques were then used to build a dense 3D point cloud, to be used as a support for geological and geomorphological investigations and any other activity, which could be supported by a dense and high-level-of-detail 3D model. With the aim of also involving non-expert users in the image-processing phase, low cost or open source tools were also tested: the acquired imageswere processed using some of the best known software tools implementing CV algorithms, which are now also implemented in some smartphone applications (e.g., SCANN3D) [25], in order to evaluate their behavior in the processing and results.