Monitoring Selective Logging in a Pine-Dominated Forest in Central Germany with Repeated Drone Flights Utilizing A Low Cost RTK Quadcopter

Abstract

There is no doubt that unmanned aerial systems (UAS) will play an increasing role in Earth observation in the near future. The field of application is very broad and includes aspects of environmental monitoring, security, humanitarian aid, or engineering. In particular, drones with camera systems are already widely used. The capability to compute ultra-high-resolution orthomosaics and three-dimensional (3D) point clouds from UAS imagery generates a wide interest in such systems, not only in the science community, but also in industry and agencies. In particular, forestry sciences benefit from ultra-high-structural and spectral information as regular tree level-based monitoring becomes feasible. There is a great need for this kind of information as, for example, due to the spring and summer droughts in Europe in the years 2018/2019, large quantities of individual trees were damaged or even died. This study focuses on selective logging at the level of individual trees using repeated drone flights. Using the new generation of UAS, which allows for sub-decimeter-level positioning accuracies, a change detection approach based on bi-temporal UAS acquisitions was implemented. In comparison to conventional UAS, the effort of implementing repeated drone flights in the field was low, because no ground control points needed to be surveyed. As shown in this study, the geometrical offset between the two collected datasets was below 10 cm across the site, which enabled a direct comparison of both datasets without the need for post-processing (e.g., image matching). For the detection of logged trees, we utilized the spectral and height differences between both acquisitions. For their delineation, an object-based approach was employed, which was proven to be highly accurate (precision = 97.5%; recall = 91.6%). Due to the ease of use of such new generation, off-the-shelf consumer drones, their decreasing purchase costs, the quality of available workflows for data processing, and the convincing results presented here, UAS-based data can and should complement conventional forest inventory practices.Drones 2020, 4, 11 2 of 26 Remote Sensing. The title is based on the idea that, after the implementation and operationalization of airborne and spaceborne platforms, UAS emerged as a new source of valuable Earth observation data. These data are capable of closing the gap between in-situ and far range remote sensing data, and thus allow for new developments of data scaling approaches [2]. Most UAS are equipped with optical camera systems providing imagery with ultra-high (i.e., centimeter scale) spatial resolution. Well-established photogrammetric processing chains, often summarized as structure from motion (SfM), exist to compute three-dimensional (3D) point clouds and orthomosaics based on overlapping UAS imagery [3]. Their high flexibility and accessibility (data acquisition at almost any time), scalability (choice of spatial resolution and areal coverage), and ease of use has led to an increased use of UAS for many applications and to...

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