## 1 Introduction  
Mobile mapping systems, based on terrestrial/aerial vehicles or on human-carried devices, have been used in the past dozen of years to map and monitor areas of interest for a wide range of applications [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]. These systems are equipped with navigation and imaging sensors. The former ones are usually based on the integration of positioning with the global navigation satellite system (GNSS) and inertial navigation with an inertial measurement unit (IMU), the latter ones consist of laser scanners and cameras. Despite their high reliability, the use of mobile mapping systems in indoor environments is usually limited (due to vehicle size and difficulties in moving it in such environments). During the decade, several portable mobile mapping systems have been developed in order to enable accurate surveys also in environments that are difficult to reach with (large terrestrial/aerial) vehicles [11; 12; 13]. A recent example of a commercial solution allowing accurate surveying also in difficult/indoor environments is the Leica Pegasus backpack [14]. It is a wearable pedestrian mapping system enabling almost ubiquitous 3D data capture, i.e., outdoors, indoors, underground. Given its limited size (\(73\times 27\times 31\) cm) and weight (11.9 kg), its nominal reconstruction accuracy (relative error 3 cm, in local coordinates; absolute error 5 cm, in georeferenced coordinates) [15], it can be considered a very interesting solution for quick and accurate surveys of small-/medium-sized sites. The primary goal of this paper is to compare accuracies between the state-of-the-art portable laser scanner system described above and an alternative mobile mapping system. The proposed system is based on the use of photogrammetry [16; 17; 18; 19] and ultra-wide band (UWB) sensors [20; 21], whose principal characteristics are its low-cost and very high portability. A GNSS receiver can be optionally employed if data georeferencing is required [22; 23]. The rationale is that of developing a low-cost system for indoor surveys, which requires only minimal human expertise and effort in its use. The imaging sensor is a standard consumer camera, i.e., Canon G7X, whereas positioning in local coordinates is obtained by means of a set of Pozyx UWB devices [24] (Figure 1a). Metric reconstruction is achieved by combining a self-calibrated photogrammetric reconstruction [25; 26; 27; 28; 29] computed with Agisoft PhotoScan [30] with UWB measurements. Georeferencing is obtained by using a Topcon HiPer V GNSS receiver [31], positioned before the bastion entry. The considered system is lightweight, easy to use and cheaper than the Pegasus backpack. Indeed, most of the weight is given by the GNSS receiver, 1 kg approximately, so that all the required instrumentation can be carried with a quite small backpack. Data collection simply requires properly spreading UWB devices inside the environment, taking photos with the camera and a UWB sensor attached to it and acquiring GNSS measurement outdoors, if needed. The overall cost of the photogrammetric system is relatively low, about $6k, out of which $400 is for the camera, $150 per Pozyx device and $4k for the (geodetic grade) GNSS receiver. The considered case study for validating the proposed low-cost system is the survey of the Impossible bastion (the \"Impossible bastion\" is a defense post, which historically was considered unconquerable, thus the term \"impossible\"), a medieval structure (XVIth century) located within the ancient city walls of Padua, Italy (Figure 2a). Figure 1: (**a**) UWB Pozyx device (beside a centimeter ruler). (**b**) Leica Pegasus backpack. This historical building (nowadays almost completely underground) was surveyed by the Interdepartmental Research Center of Geomatics (CIRGEO) of the University of Padua in 2016 with a terrestrial laser scanner (TLS), within the framework of a fellowship (Culturalmente 2015). The 3D model produced from the TLS survey is shown Figure 2b.