**4.3. Indoor mobile mapping systems and Forensic**

Indoor mobile mapping systems could be defined as a complex set of sensors that allows spatial data acquisition in movement. This is especially useful in forensic indoor scenarios when some environmental conditions become dangerous for humans (chemical risks, danger of collapse, etc.) or simply in those places really complex where a lot of laser or photographic stations would be required [32]. The set of sensors is composed by two groups of electronics devices working together on a self‐moved vehicle [33]. On the one hand, navigation instruments are in charge of the guidance of the vehicle, providing an automatic motion through a planned route. On the other hand, the role of geomatic sensors, such a digital cameras or laser scanners, is the acquisition of spatial data (images and point clouds), as described in previous sections. Both geomatic and navigation sensors are controlled by a microprocessor, so that measure‐ ments are done simultaneously. Furthermore, each set of spatial data is associated to a time stamp which links it with the positioning settings of the mobile unit in every instant [34].

Navigation devices could integrate high‐precision GNSS, advanced inertial technology (three axis accelerometers and gyroscopes), magnetometers and pressure sensors to calculate orientations and heights. The indoor georeferencing is done by using measures from GNSS outdoors. The inertial measurement unit provides uninterrupted data of the true position, roll, pitch and yaw of the system when moving indoors [35].

Data acquisition is performed when the vehicle is in motion following a planned route whose purpose is to cover all the parts of the scene. There are two possibilities in the route configu‐ ration: making a round trip, or a back and forth displacement. The strategy selected will come as a consequence of the ground characteristics in the area inspected, since obstacles should be avoided in order to have a continuous path. If the last option is not possible, data will have to be acquired in different sequences, and then a registration procedure will put all the sequences in the same coordinate system, considering that there are overlapping areas within consecutive sequences. When data acquisition is finished, the absolute position of each single point is calculated from the data of the trajectory of the navigation unit, making it possible to obtain a complete 3D point cloud of the inspection area.

The time needed for indoor mobile mapping systems during data acquisition is equal to the time that a person needs to walk through the area of interest; with the noticeable time reduction regarding other systems and an important cost decrease [36].

In forensic engineering, indoor mapping systems (**Figure 11**) stand out for their suitability for the acquisition of big and complex indoor scenes, as they perform automatically multiple processes such as dynamic scanning and self‐determination of the autonomous trajectory of the vehicle. As a result, 3D metric information is obtained in real time as the vehicle completes its path.

**Figure 11.** Indoor mapping systems: TIMMS (a), iMMS (b) and CARTOGRAPHER (c). Images courtesy of Trimble, SmartGeometrics and Google, respectively.
