Enhancing Methods for Under-Canopy Unmanned Aircraft System Based Photogrammetry in Complex Forests for Tree Diameter Measurement

Abstract

The application of Unmanned Aircraft Systems (UAS) beneath the forest canopy provides a potentially valuable alternative to ground-based measurement techniques in areas of dense canopy cover and undergrowth. This research presents results from a study of a consumer-grade UAS flown under the forest canopy in challenging forest and terrain conditions. This UAS was deployed to assess under-canopy UAS photogrammetry as an alternative to field measurements for obtaining stem diameters as well as ultra-high-resolution (~400,000 points/m2) 3D models of forest study sites. There were 378 tape-based diameter measurements collected from 99 stems in a native, unmanaged eucalyptus pulchella forest with mixed understory conditions and steep terrain. These measurements were used as a baseline to evaluate the accuracy of diameter measurements from under-canopy UAS-based photogrammetric point clouds. The diameter measurement accuracy was evaluated without the influence of a digital terrain model using an innovative tape-based method. A practical and detailed methodology is presented for the creation of these point clouds. Lastly, a metric called the Circumferential Completeness Index (CCI) was defined to address the absence of a clearly defined measure of point coverage when measuring stem diameters from forest point clouds. The measurement of the mean CCI is suggested for use in future studies to enable a consistent comparison of the coverage of forest point clouds using different sensors, point densities, trajectories, and methodologies. It was found that root-mean-squared-errors of diameter measurements were 0.011 m in Site 1 and 0.021 m in the more challenging Site 2. The point clouds in this study had a mean validated CCI of 0.78 for Site 1 and 0.7 for Site 2, with a mean unvalidated CCI of 0.86 for Site 1 and 0.89 for Site 2. The results in this study demonstrate that under-canopy UAS photogrammetry shows promise in becoming a practical alternative to traditional field measurements, however, these results are currently reliant upon the operator’s knowledge of photogrammetry and his/her ability to fly manually in object-rich environments. Future work should pursue solutions to autonomous operation, more complete point clouds, and a method for providing scale to point clouds when global navigation satellite systems are unavailable.

Handheld Laser Scanning Detects Spatiotemporal Differences in the Development of Structural Traits among Species in Restoration Plantings

“…Due to the nature of the trees studied here (i.e., young age and mostly multi-stemmed), the application of 3D object modelling, such as cylinder fitting to the lower part of the tree stem [31,32], was not possible in this study. This method, which is mainly used in TLS studies, or more recently in below-canopy UAV flights [31,33] is very promising, but we anticipate it would have provided extremely poor results under our present circumstances. Nonetheless, we imagine that as the trees grow taller and stems become more pronounced, a 3D object modelling approach could provide better results than those estimated from allometric equations.…”

Comparison of Backpack, Handheld, Under-Canopy UAV, and Above-Canopy UAV Laser Scanning for Field Reference Data Collection in Boreal Forests

“…However, if under-canopy UAV can be operated in a fully automatic mode in the future by applying anti-collision and navigation systems [35], the collection of field data can be several times faster than than with a human operator. The previous work related to under-canopy UAV data collection with either laser scanning -based techniques or photogrammetric methods include, e.g., [36][37][38][39][40].…”

Improved Accuracy of Riparian Zone Mapping Using Near Ground Unmanned Aerial Vehicle and Photogrammetry Method

“…The Unmanned Aerial Vehicle (UAV) used in this study was a DJI Phantom 4 Pro (Da-Jiang Innovations, SZ DJI Technology Co., LTD; Figure 2). This UAV was chosen due to its recognized camera stability which is considered to be a key factor for precision agriculture and forestry purposes [34][35][36]. The Phantom 4 Pro is a vertical takeoff and landing aircraft with total payload of 1388 g, battery and propellers included.…”