## 1 Introduction  
In the geomatics community, the technical requirements and resulting potentialities of the terrestrial surveying of _objects of interest_ (buildings, cultural heritage, civil structures, industrial environments, etc.) are well known. In addition, it is also widely known how these activities are carried out nowadays, that is, by means of terrestrial laser scanning (TLS) and/or photogrammetric images, whose somewhat complementary features are compared by the authors of [1]. In TLS surveying, the scanner instrument is placed on a tripod in a sequence of static scan stations in order to survey \"all\" of the surfaces to be measured, that is, the inside and outside of an element of architecture. These various scans are later registered together, generally by using many artificial targets placed over the scanned object as tie points. It is better if they are surveyed by a total station in order to acquire control points and ensure, after the classical least square adjustment of the topographic measurements, the best geo-referencing of the whole obtained point cloud. On the other hand, photogrammetric surveying uses many digital images which are frequently acquired these days by means of uncalibrated cameras. The interior and exterior image orientation process is resolved by automatic procedures developed in the computer vision community, following an approach known as \"structure from motion\" (SfM). This last expression derives from the fact that the photographed object (structure) is surveyed by estimating the position and rotation (motion) of the imaging sensor, which is computed in a single automatic step (apart from the scale factor of the model). Although the term \"motion\" is contained in this approach, and the images really can be acquired by a moving charge-coupled device CCD (for instance, mounted on/inside a drone), the analytical solution considers each image to be statically acquired, and the control points have the same role as in TLS surveying (and are essential for the estimation of the model scale factor). The necessity to have known control points is instead overcome once and for all with _indoor mobile mapping systems_ (IMMSs) (e.g., in [2]), a new form of surveying technology developed in recent years. These systems are also referred to as _human_ (e.g., in [3]), _handheld_ (e.g., in [4]), _backpack (wearable)_ (e.g., in [5]), _portable_ (e.g., in [6]), or _trolley_ (e.g., in [7]) (mapping) systems in the literature. Within the cited references, not only are many instrumental details and configuration differences found, but important theoretical issues regarding the complex analytical models are also addressed. Nevertheless, all of these systems perform the surveying in a similar way, using a linear scanner as the measurement sensor, although some instead make use of imaging or range cameras. In other words, an IMMS is a moving multi-sensor system which surveys the surrounding environment in a kinematic manner. From the geomatics point of view, IMMSs are very similar to the _mobile mapping systems_ (MMSs) devised for road cadastres more than 20 years ago [8]. At that time, these vehicles, usually vans, were developed with photogrammetric imaging sensors, while nowadays they are used with more advanced surveying devices to quickly capture the geometry of a road (and the buffer area around it) thanks to a global navigation satellite system (GNSS) receiver and an inertial navigation system (INS) platform mounted onto the vehicle [9]. Some analytical details on the MMS surveying principle are explained in Section 2.1, but it is evident that GNSS/INS navigation sensors enable an estimation of the position and the attitude of the vehicle, and then of the CCD or the video-camera, in each instant. The revolution introduced by MMSs was the direct orientation of the imaging sensors thanks to the navigation sensors, thus avoiding the classical indirect method which instead used control points and required topographical surveys. The term \"indoor\" differentiates IMMSs from MMSs.