**2. Related Work**

Mobile mapping systems are becoming popular as they can build 3D point clouds of any type of environment rapidly by using a laser scanner that is integrated with a navigation system. The laser scanner is able to record millions of points. The files with the point clouds can be viewed, navigated, measured and analyzed as discrete 3D models. The evolution of the laser scanner as a surveying technique has resulted in this tool being used not only to obtain an optimal geometric reconstruction of the scene, but also to assess changes in a particular state.

Mobile LiDAR (Light Detection And Ranging) technology presents some advantages: high-speed data capture through reduction of time and cost, remote acquisition and measurement increasing survey in e fficiency and safety, high point density data ensuring a comprehensive representation of the detected scene, an abundance of data acquired in movement. The light weight of these MLSs makes them flexible and versatile instruments, so they can be mounted on any mobile platform. In places with complex topography, the use of MMS, following a continuous path, is more advantageous than a tripod-based laser scanner that requires multiple scan positions to cover all the areas of the survey [6]. Moreover, integrating MMS with other geomatics techniques, such as Digital Photogrammetry (DP), allows giving an added value and greater richness of the acquired data providing a high detailed DEM or DTM (Digital Terrain Model) of the selected area [8].

Alternative purposes to use mobile LiDAR technologies, in addition to the assessment of the geometrical state, concern change detection, deformation analysis [9], hazard assessment and structural and infrastructural health monitoring [4] in di fferent types of natural environment. In technical terms, mobile mapping solutions contemporaneously allow users to acquire geometrical aspects for geological studies and geomorphological analysis, to operate mapping of all the elements present in the detected area (e.g., vegetation, road, etc.), and to define basic modeling for monitoring operations (e.g., rockfalls, coastal erosion, river dynamics, etc.). Examples described by literature are various, depending on natural e ffects as geological and atmospheric actions or anthropogenic consequences of the built environment which compromise the stability of the natural landscape.

The major case studies that include the use of MLS for mapping and monitoring purposes are those related to geomorphology detection and landscape dynamics such as landslides and rockfall displacements. The combination of MLS and DP can greatly improve the realization of detailed modeling of the geometrical discontinuity of a slope to analyze a landslide susceptibility and potential rockfall mechanisms [9]. Examples of slope stability assessment through the kinematic method are applied in rock outcrops in British Columbia, Canada, and Carrara marble quarry, Italy [10]. The use of MLS was presented by Francioni et al. [11] in a geological study of landslides applied to an interesting case study in Normandy. They used a boat-based MMS to scan 3D point clouds of unstable coastal cli ffs to detect rockfalls and erosional deposits. A similar case is proposed by [12] who described a new approach of coastal cli ff monitoring, in Poland, which is based on a combination of MLS from the sea with the geotechnical stability calculations. A 3D mapping was carried out by an airborne MMS, installed on a helicopter, to monitor a small landslide in the North Yorkshire coast in the UK. As a result, more accurate modeling of the terrain was obtained, especially areas covered by vegetation [13]. A ground-based approach was evaluated by James et al. [5] using a handheld MLS to collect topographic data in complex terrain as a gully site at the coastal cli ff in Sunderland, UK. To carry out the survey, the hand-held mobile device is walked around the site following a close loop path to facilitate accurate 3D reconstruction and avoiding problems associated with drift.

Landslides happen also inside residential areas causing cracks to buildings and drainage, for example, the study area located in a Malaysian city. Here, to monitor an active landslide, researchers used MLS to assess the movements of the land, in both vehicle-based and human-based mode in a complementary way to acquire completely the surface of the study area [14].

Mobile mapping solutions have been also applied in change detection and deformation analysis for sandy dunes and river courses. The adoption of MLS was illustrated by Nahon et al. [15] that provides an accurate and robust method to obtain high resolution space-time datasets along a Dutch beach useful to understand the changes of dunes volume, under the influence of both marine and aeolian processes. The MLS was carried out mounting the device on a car and using airborne LiDAR to process several DEMs of the sandy dunes. The monitoring of beach dunes is needed to improve the scientific observation of their dynamics. A test was realized in Cap Ferret, France, where researchers combined MLS with aerial photogrammetry to deliver accurate 3D reconstruction of the dunes. They confirmed the good results of using mobile mapping devices for their high level of detail and greater spatial coverage [16]. The best results were also achieved in monitoring the lagoon area of Padre Island National Seashore, located in Texas, where mobile scanning devices may be preferred for the detailed and comprehensive final DEMs useful to detect 3D terrain features and so to monitor geomorphic changes [17].

These challenging trials prove that a mobile LiDAR system, in recent years, has emerged as a viable alternative for surveying coastal beaches and foredunes, but also for riverine topography. Measurements were conducted by Vaaja et al. [18] using an MMS mounted on a boat and mounted on a manually-operated cart to define the riverine topography and modeling in 3D of the coarse fluvial sediment along the river Tenojoki, in Finland. The application of a MMS was tested by Williams et al. [19] for a complex relief and the following reconstruction of fluvial surface sedimentology and topography of river Feshie, in Scotland.

Monitoring actions also occur to control the actual state of civil infrastructures, for example, the maintenance of pavement condition of roads to detect instabilities and to assure safe conditions. It is possible to detect the road surface on 3D models derived from a dense point cloud acquired by MLS. LiDAR instruments are suitable to collect data with adequate accuracy and high resolution for mapping and inventory purposes, and, also, the surveyors can make surveys safely with minimal interruptions of the traffic flow [9]. The use of accurate and dense point cloud data along a route corridor enables the detection of surface distortion, joints, cracks and other roughness conditions [20]. Also retaining walls along roads needs to be monitored to assess their stability, especially in mountainous regions to support either roads or slopes adjacent to roads. An efficient method was proposed by Lienhart et al. [21] based on a mobile mapping system to detect a retaining wall used to construct a highway, in Austria. The MMS, mounted on a car, generated a high-density point cloud where tilt changes of the structure can be calculated to define the current state of the structure itself. The 3D model helped to recognize forms of damage and their distribution on the surface of the wall, thus compensating for the typical punctual analysis carried out with the total station. Moreover, referring to the case study presented in that article, among the monitored infrastructures they also include water reservoirs and hydroelectric power plants. A boat-based MMS was selected by Brazilian researchers to scan the progression of marginal erosion in different reservoirs of hydroelectric plants. The processed point clouds and rendered meshes and the creation of cross-sections were used to compute the rate and the dynamics of the erosion phenomenon [6,22].
