Mapping Forest Structure Using UAS inside Flight Capabilities

Area of Science:

• Forestry
• Remote Sensing
• Photogrammetry

Background:

• Accurate 3D forest structure mapping is crucial for forest management and research.
• Traditional methods can be labor-intensive and may not capture detailed interior stand structures.
• Unmanned Aerial Systems (UASs) offer a promising alternative for efficient forest data acquisition.

Purpose of the Study:

• To evaluate the efficacy of two UAS models (DJI Phantom 4 Pro and DJI Mavic Pro) for 3D mapping of forest stand interiors.
• To assess the accuracy of tree stem modeling and diameter at breast height (DBH) estimation using close-range photogrammetry.
• To compare the performance of different diameter fitting methods for point cloud data.

Main Methods:

• Assisted flights were conducted using DJI Phantom 4 Pro and DJI Mavic Pro UASs in mature Norway spruce and European beech forest stands.
• Geotagged images were captured at a flight height of 8 m to create georeferenced 3D point clouds of tree stems.
• Tree stem diameters at breast height (DBH) were estimated from point clouds using four fitting methods (bounding circle, convex hull, least squares circle, least squares ellipse) and compared to field measurements.

Main Results:

• UAS-based photogrammetry successfully modeled tree stems up to 10 m height, providing detailed 3D structure information.
• The accuracy of DBH estimation varied significantly between UAS models and fitting methods.
• The DJI Phantom 4 Pro with the least squares ellipse method yielded mean errors of -1.17 cm (-3.14%) for spruce and 0.27 cm (0.69%) for beech.

Conclusions:

• UAS-equipped close-range photogrammetry is a viable technique for detailed 3D forest structure mapping, including the forest stand interior.
• The choice of UAS and diameter estimation method impacts the accuracy of DBH measurements.
• The study demonstrates the potential of UAS technology for precise forest inventory and monitoring applications.