Editor’s Note: We at RIEGL would like to thank the orginal author Jakub Karas for his well written words about his experience with RIEGL ULS LiDAR and how it has benefitted him and his team in the Czech Republic. Please enjoy the article below or click the link below to access the article at GeoConnexion!
In the Czech Republic, Charles University and UpVision have been conducting testing of low-altitude drone laser scanning for several research tasks for detailed mapping of the digital terrain model in hard-to-reach areas.
Two examples include detailed mapping of the perpendicular slopes of the banks of the dam, or the steep cirque (after glacier) below the highest mountain in the Czech Republic.
Acquiring high-quality laser scanners that can be placed on commercially available drones that have an acceptable size and weight and mainly parameters for high quality outputs is a huge step forward in the use of laser scanning from drones. This type of work with laser scanners on drones now allow for the filtering of point clouds to create a quality digital terrain model, even when covered with vegetation as opposed to detailed drone mapping using photogrammetry, where the main output is a digital surface model with the vegetation.
The great potential of laser scanning from the drone is primarily for the scanning of vertical objects such as rocks and steep slopes in inaccessible or dangerous locations where classical ground laser scanning cannot be used or where vegetation coverage makes it difficult to create an accurate digital terrain model using classical mapping from a drone using photogrammetry. For mapping of digital terrain models of experimental areas in the Czech Republic, UpVision has been using the DJI Matrice 600 Pro drone, which has excellent flight characteristics and load capacity up to 6 kg for a flight time up to 16 minutes and the possibility of replacing various sensors with regard only to size, weight and power supply. The RIEGL miniVUX-1UAV laser scanner with weight 1.6 kg and hung directly under the drone without the use of a gimbal was chosen for laser scanning with regard to a LiDAR weight and size and data acquisition efficiency. This LiDAR sensor has a 360 ° FOV, 100,000 measurements / seconds and a classic operating height is up to 100 meters above the ground.
In good conditions, it is possible on average with such a drone and LiDAR system to achieve a flight time of 20 minutes. To map with a high point density for the most accurate and detailed digital terrain model, it is necessary to fly relatively close to the mapped location or object where the density can be quickly calculated and set up on LiDAR and adapted to the drone flight plan itself. If high cloud density is needed, it is effective to execute the flight plan in lines with overlaps, which we set depending on the flight height, scanner range and the required point cloud density. In order to increase the density of the point cloud and to best cover the terrain, it is most effective to fly lines in two directions perpendicular to each other – crisscross (see Figure 1.1).
When you are facing situations where you are mapping difficult, inaccessible, or dangerous locations, it is not possible to rely on a pre-scheduled flight plan in low altitude or close to the vegetation, especially trees. This is because it doesn’t show the exact altitude model at the location and therefore, it is needed to fly the lines manually with the drone in lines. For these occasions, you need an FPV camera on the drone to increase safety where it is possible to check in real-time whether the pilot is moving at a safe distance above terrain or vegetation. One of the topics selected for experimental purposes in the Czech Republic is the creation of a detailed map of part of the bank of the Nechranice Dam, which is the longest rock-fill dam in Central Europe (3.3 km) and where part of the bank is formed by perpendicular slopes up to 15 meters high, which are entrainment into water. Therefore, it is necessary to focus on and create a detailed digital model of these walls, where they will be laser scanned repeatedly at certain time intervals. These models will be compared to see in which parts there is the greatest risk of landslides or other locations that require safety adjustments. These walls are often covered with various roots and remains of vegetation and therefore laser scanning is the best choice for obtaining the best digital model of these shores. Since the walls are between the water surface and the terrain above these walls, it isn’t possible to use terrestrial laser scanning from the shore. The only other option would be to scan the walls directly from the water surface, which would be very complicated in terms of the difficulty of data acquisition and in terms of data alignment. The drone can very easily perform laser scanning from a distance of about 20 meters from the shore above the water surface. This then provides a high point cloud density from which it is possible to create a very accurate digital model with a density of several hundred points per square meter. This process is then repeated and is mapped out at certain time intervals where it is then possible to determine where the landslide occurs and which places are most vulnerable to the landslide. A manual flight with laser scanning of the shore line of about 300 meters length can be accomplished in two flights about 40 minutes in total without any risk of movement at the edge of the shore or directly on the water surface.
The second typical example of using laser scanning from a drone in hard-to-reach terrain was mapping a part of the Úpská pit, which is a glacier cirque directly below the highest mountain in the Czech Republic – Sněžka, near the Polish border in the Krkonoše National Park. The valley walls are up to 600 meters high and are dangerous to move around especially for mapping, such as with a terrestrial laser scanner. At the same, time the walls of the cirque are very steep and slippery and covered with vegetation. In order to obtain an accurate digital terrain model, it is necessary to use laser scanning, which is possible here only from a drone in such difficult terrain. Even flying with a drone is challenging, because there are different air swirls in the cirque and it is necessary to choose the most suitable climatic conditions with minimum wind. The upper boundary of the cirque can be reached on foot with a drone from where you can see the main part of the cirque valley.
The flying of the drone itself in such a sharp abyss was by manual flights, which were in rows perpendicular to each other first in one direction down and then cross lines up to the edge of the cirque. The flying of the drone was below the top edge of the cirque under constant control of the onboard FPV camera by the pilot to keep the drone at a sufficient distance from the terrain, at least 20 meters, because the view of the descending drone against the terrain was very distorted. In this way, the drone flew more than 120 meters below the pilot’s position on the edge of the cirque and thus, moved at a high minus altitude against the take-off point. By crossing the flight lines, which alternated two laser scanning positions simultaneously, a high density of scanned points was achieved, which was up to 1000 points per 1 square meter. Scanning itself in such an environment is important not only in terms of creating detailed terrain characteristics where changes in time can be monitored but also in vegetation mapping, due to the many rare vegetation types for monitoring. This, in combination with the acquisition of hyperspectral data research from drones, there are important outputs that cannot otherwise be obtained.
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