## 3 Data Acquisition
### Mobile Mapping System  
After the acquisition phases performed by EDM and CRP campaigns, a survey with the MMS was conducted. As previously stated, despite the high accuracy reached with the aforementioned methods, visual occlusions and vegetation prevented the whole mapping; mainly the streets and the lower parts of the rock walls could not be surveyed. Kaarta Stencil 2 is a stand-alone, lightweight instrument, with an integrated system of mapping and real-time position estimation. An MMS consists  
Figure 4: Images acquired from the same spatial position with two fixed lenses: (**a**) normal 50 mm; (**b**) telephoto 200 mm.  
Figure 5: Set of imagery used for the bundle adjustment of the normal and tele cameras and visualization of the measured points on geodetic pillars and check points.  
mainly of three components: mapping sensors for data acquisition, a positioning and navigation unit for data localization and a time referencing unit for data recording [30]. Kaarta Stencil 2 depends on LiDAR consisting of a Velodyne VLP-16, connected to a low-cost MEMS (micro electro-mechanical system) IMU (inertial measurement unit) and a processing computer for localization and real-time mapping. VLP-16 has a maximum laser range of 100 m and 360\({}^{\circ}\) horizontal FOV (field of view) with a 30\({}^{\circ}\) azimuthal opening with a band of 16 laser beams. The LiDAR has a centimetric accuracy with a value variation of \(\pm\)30 mm and a speed of 300,000 points per second. The acquisition phase was carried out using the appropriate configuration parameters, set for outdoor environments, that include values for the voxelSize, namely the resolution of the point cloud in the map file, for cornerVoxelSize, surfVoxelSize, sorroundVoxelSize, those indicate the resolution of the point cloud for scan matching and display, and for blindRadius, that is the minimum distance of the points to be used for the mapping (Table 3).  
In realizing this ground-based survey characterized by a long path on foot, walking along the edge of the road, the laser scanner was mounted on a small pole held by hand (Figure 6). Human-based LS has not been widely used in the monitoring of rock slopes, but in this case study was useful to detect areas of the surveyed region not clearly visible through the CRP. The MMS needed to fill the gaps of the photogrammetric 3D model, such as roads or parts obstructed by the crown of trees, that the photogrammetric camera cannot reach from the different points of view to detect all the features.  
The data acquisition operation was performed following a closed path (close-loop) [31; 32], to facilitate accurate reconstruction of the surveyed region and avoid problems associated with drift, where the beginning and end of the route coincide [3]. In Cortes de Pallas test site the surveyor completed a trajectory (backwards and forwards) of 2.9 km long in total, at an average distance of 3-5 m from the rock wall, acquired in 1 h and 43 min, collecting over 480 billion points (Table 4).  
\begin{table}
\begin{tabular}{c c} \hline
**Parameters** & **Default Value [m]** \\ \hline voxelSize & 0.4 \\ cornerVoxelSize & 0.2 \\ surfVoxelSize & 0.4 \\ surroundVoxelSize & 0.6 \\ blindRadius & 2.0 \\ \hline \end{tabular}
\end{table}
Table 3: Parameters setting of Kaarta Stencil 2, optimized for outdoor scenario.  
Figure 6: Two sequences of the MMS (Mobile Mapping System) survey with Kaarta Stencil 2. (**a**) Lower part and (**b**) upper part of the rockwall.  
A tracker camera, integrated to the mobile mapping device, needs to show and save the trajectory made during the acquisition operations. The progress of the scanning can be monitored in real-time via an external monitor attached with a USB cable. At the end of the acquisition phase with Kaarta Stencil 2, information about the configuration setting, the 3D point cloud characteristics, the estimated trajectory is stored in a folder automatically created by the MMS processer at every operation of the survey.