# Preface of Special Issue on Laser Scanning
## 2 Featured Papers  
Multi-wavelength LiDAR is a technology that expands the typical single-wavelength LiDAR, usually having a wavelength in the near infrared range (e.g., 1064 nm [2]). It provides important information on the reflective properties of targets by using more channels, such as green, near-infrared, and mid-infrared channels, and using them for the improved classification of land cover [3] and for accurate land/water discrimination [21].Unmanned, as well as manned, aerial systems, equipped with laser scanners are topics of active investigation. Precise system calibration improves point position accuracy, which is a matter of foremost importance for accurate models. Authors in [17] proposed an outdoor calibration procedure for UAVs using linear and planar features scanned from different flight lines, allowing them to accurately estimate bias between the components of the sensors (i.e., boresight and lever-arm angles). LiDARgrammetry is the reverse engineering of aerial laser scanning products to extract infrared stereo pairs (i.e., greyscale imagery). The stereopairs are used in the typical photogrammetric procedure. This allows using well-known photogrammetric workflow and software; operators can align stereo pairs and digitize 3D points, extract breaklines, etc., which adds value to the LiDAR product [20]. Moving the focus from the air to the ground, laser scanners can and are used for indoor and outdoor navigation and modelling. A backpack laser scanner with simultaneous localization and mapping (SLAM) can provide accurate models without information from Global Navigation Satellite System (GNSS) [6]. Lower-cost solutions based on photogrammetry and ultra-wide band (UWB) for relative positioning can be interesting alternatives [5]. Indoor reconstruction can be a complex process when the laser's line of sight is occluded by objects. Authors in [15] proposed the indoor completion of walls with an energy minimization problem using graph cuts, doors, and windows to be detected from occlusions by a ray-tracing algorithm. Authors in [7] provided a full framework for the reconstruction of buildings' indoor environments with added information on converting objects into the Industry Foundation Classes(IFC) model for inclusion in building information modelling (BIM) software. Laser scanning is not only applied to urban and regional scales, but is also common in the accurate modelling of small- and medium-sized objects. In many sorts of manufacturing, precise models like computer-aided design (CAD) are required in the processing stages. Two particular examples are reported in this special issue. One is related to car doors in the automotive industry, where the accuracy of a handheld scanner was tested in [13]. The second application is in oral medicine, where intraoral scanners were tested for accuracy (deviations of inter molar width), providing information on clinical validity [19]. Detecting object position and movement is important for mobile applications, especially in the realm of robotics, where laser scanning can provide this information at a very fast rate. Robot position and moving direction is a key information that can be provided as described in [8]. Elevation maps extracted from laser scanning can also support robot navigation as shown in [11]. Monitoring movement during and after the aircraft landing phase is another application that can be done with laser scanners, allowing the improvement of airport safety thanks to knowledge of accurate position and speed (accuracy of 17 cm and 0.17 m/s, respectively) [9]. Also, human detection and monitoring takes advantage from laser scanning information, together with infrared imagery, leveraging the two technologies to complement one another [14]. A particular application of movement detection is shown in [12], where the vibration of fan blades was monitored through a laser scanner vibrometry (LSV) technology, providing the means for determining harmonics.