## 1 Introduction
However, the former are not for exclusive indoor use since outdoor applications can be also carried out with IMMSs. The situations in which GNSS data are not available urge a different approach to be used in solving the problem of instantaneous navigation. The analytical methods used to solve this _chicken-or-egg_ problem often belong to the class of so-called \"simultaneous localization and mapping\" (SLAM) algorithms, originally developed in robotics. \"Localization\" involves the estimation of the position and attitude of a (measuring) sensor with respect to a certain coordinate system, while \"mapping\", the obvious fundamental goal of any geomatics technique, is intended as the final output from the sensor, generally in the form of a digital representation of the surveyed environment. To sum up, with a SLAM algorithm it is possible to simultaneously build a map and localize the sensor within the map: obviously the essential aim is the mapping (for the surveyors', not the robotics community), but the localization problem is also solved, almost in real time. In the last few years, the number of IMMSs put on the geomatics market has increased greatly, with a fairly diversified range of solutions: the aim of this paper is to evaluate the potentials and limitations of three different systems commercialized in Italy over a diversified test field, characterized by the widest possible variety of conditions (indoor, outdoor, different elevations, stairs, wide and narrow spaces, trees, moving objects) in order to adequately reproduce very general surveying situations. As detailed further on, the point clouds acquired by a traditional static TLS were considered as reference values, that is, the so-called \"ground truth\" for testing these IMMSs. Very interesting examples of IMMS comparisons are reported in [10; 11; 12]. In 2013, [PERSON] et al. [10] investigated two very different products: the Viametris i-MMS trolley without GNSS/INS sensors, and the handheld 3D Laser Mapping/CSIRO ZEB1, while taking indoor scans acquired by means of a Faro Focus 3D as a reference. [PERSON] et al. [11] compare two similar systems, the handheld GeoSlamZeb-Revo and the Leica Pegasus Backpack, the latter equipped with a GNSS receiver. One of the tests was performed inside a two-floor building scanned with a Leica HDS7000 and the other outdoors in a big (80 m \(\times\) 70 m) city square, previously surveyed with a \"classical\" MMS, namely a Riegl VMX-450 mounted on a van. The comparison carried out by [PERSON] et al. [12] is quite impressive since it makes a comparison of no fewer than eight different systems: five commercial ones--the innovative Matterport 3D camera, the NavVis trolley, the spring-mounted handheld Zebedee (the oldest Zeb model), the handheld Kaarta Stencil, and again the Leica Pegasus Backpack--and three interesting research prototypes--the Aalto VILMA \"rotating wheel\", the FGI Slammer, and the Wurzburg backpack. Furthermore, these systems were analysed in three test areas (however, in truth, not all of them were): a hallway and a car park with a sloping floor (with both sites surveyed by a Leica ScanStation P40), and an industrial hall set out as a co-working space, which was scanned with a Faro Focus 3D. Of course, IMMS models can be compared with models from SfM-based photogrammetric surveying, while keeping statically acquired TLS data as a reference, as reported for example in [13]. In the examination proposed in this paper, the models analysed are the Kaarta Stencil, the Pegasus Leica Backpack, and the GeoSlam Zeb-Revo, while the reference is given by a Z+F TLS.