# Risk of Tree Fall on High-Traffic Roads: A Case Study of the S6 in Poland
## 2 Materials and Methods  
### Airborne Laser Scanning  
Airborne laser scanning (ALS) is a technique for collecting terrain data using laser scanners mounted on aircraft boards. Two segments can be distinguished, making up the ALS system: airborne and ground-based. The airborne system includes a laser range finder, a trajectory positioning system based on GPS (Global Positioning System), an inertial navigation system, a camera, a data logging block, and a flight planning and management system [26; 27; 28]. The ground part of the ALS includes a GPS reference station and a workstation for processing and generating the resulting products. The use of laser scanning, unlike classical photogrammetry, has a key advantage: this method is not dependent on natural sunlight. This means that the analysis of these data is not affected by shadows caused by clouds or surrounding objects. A key advantage is that the laser pulses can penetrate small gaps in tree crowns, making it possible to collect information about the ground structure. The laser works in pulses, scanning the area at a high frequency. Some of the energy that is reflected from the object's surface is then picked up and recorded by the scanner's optics. Acquiring a densely sampled three-dimensional surface with a single laser beam requires a scanning mechanism that moves the beam across the object's surface. Several basic scanning patterns can be distinguished in this process, including [29; 30]:  
* Oscillating mirror--Creates a characteristic zigzag pattern on the surface. The distances between laser points in the scan line are variable values, resulting from the mirror's constant acceleration and deceleration. Larger distances between points occur in the middle part of the lane, while they are smaller at the end of the lane, where the direction of the mirror's movement changes.
* Spinning polygon--Points are created only when scanning in one direction, forming parallel scan lines with a uniform distribution of points over the measured area.
* Palmer scanner--The laser beam deflecting device is designed so that the mirror surface and axis of rotation form an angle other than 90\({}^{\circ}\). Systems based on Palmer scanning for aerial systems achieve an elliptical pattern on the ground.
* Fiber-optic system--A laser beam is directed through a rotating mirror onto a bundle of optical fibers, outputting energy in the form of a line perpendicular to the flight.  
During the measurement, when a single laser pulse is sent, it is possible to record several reflections. A pulse that hits a wooded area first partially reflects off the crown of the tree, and then the energy passes through the crown and is reflected off the ground surface. Intermediate reflections can occur between these two extreme reflection points. Different types of laser systems allow only the first reflection, the last reflection, or both reflections (known as echoes) to be recorded. In addition, some systems record intermediate reflections, recording up to 5 reflections [31; 32; 33]. Some systems record the full waveform, making it possible to extract multiple reflections at the data-processing stage (Figure 1). Using this information, tree heights can be determined or a normalized numerical land cover model can be obtained.  
Figure 1: Discrete and full waveform LiDAR system, Reprinted/adapted with permission from Ref. [34].