# Smartphone-Based Photogrammetry for the 3D Modeling of a Geomorphological Structure
## 1 Introduction
Specifically, the CV approach has been experimented since the years 2008-2009, and its integration in common software tools increased its potentialities [18]. The method permits the collection of very complex and complete information in a short amount of time, which is a great advantage for mapping and documentation purposes: moreover, this method can be applied for different case studies and to obtain information at different scales, both for small objects and for indoor or outdoor environments. Furthermore, the acquisition process is absolutely non-invasive, since it possible to collect data completely, even from remote areas, without touching the site at all. It also permits for the reaching of hardly accessible parts of the environment (such as the higher parts, which are difficult to record by means of other techniques, e.g., Terrestrial Laser Scanner, TLS). In fact, with TLS techniques it is sometimes not possible to extract the edges of an object, while this is possible with a photogrammetric restitution approach. Moreover, nowadays, thanks to the CV algorithms and to the new technologies, we are able to also use amateur cameras for photogrammetric purposes. The advantage of using passive sensors is that they do not require elaborate data acquisition planning, and data processing is also very easy for non-expert operators. Some recent studies described the application of 3D photogrammetric techniques on small areas of dark caves [6,21,22] or as an integration into the terrestrial laser scanning (TLS) approach [23]. A solution for the 3D mapping of natural caves can be found in terrestrial digital photogrammetry, using mass-market high-diffusion and low-cost technologies, such as smartphones or amateur commercial cameras for image acquisition. The limited size of image sensors represents an additional advantage, which allows one to install them in commonly used devices such as smartphones and tablets, giving anyone the ability to acquire data and extract 3D information. Thanks to the significant development of these devices and the embedded passive sensors, they represent a valid alternative to the classic survey technologies, which can be exploited for photogrammetric and 3D modeling purposes. Despite this, their performances are not yet comparable with high-end cameras, due to the quality of the sensors and the resolution of the images, which could affect the photogrammetric block alignment and, therefore, the 3D reconstruction; however, accurate results can be obtained, especially when performing a careful estimation of lens distortions [24]. In addition, one of the major advantages of using a smartphone is the ability to directly perform data processing with specific applications installed on the mobile, now widely available in stores [25; 26; 27; 28; 29]. For example, today there are smartphones on the market, such as the Sony Xperia XZ1, sold with an integrated 3D modeling application [30]. In the literature, several studies are approaching 3D reconstruction methodologies involving the use of images acquired by smartphone devices or tablets for different analyses in multiple domains. In [PERSON] et al. [24] and [PERSON] [31], first attempts at exploiting the close-range photogrammetric technique using smartphone images were conducted, with the aim of reconstructing morphologically complex architectural elements, both for the documentation and the scientific dissemination of the cultural heritage and for the digital preservation of historical buildings and smaller objects of historical, architectural and artistic relevance. A further advantage derives from the easy availability of these tools, which makes them suitable for use in emergency situations when the goal is to obtain a complete description of the object in the shortest possible time. Since it is not possible to create a full object model with TLS or with photogrammetry, it is in both cases possible to interpolate the collected points in order to have a 3D model, removing the so-called shadows. This aim was pursued by [PERSON] et al.