©2023 International Society of Arboriculture Arboriculture & Urban Forestry 49(2): March 2023 77 deployed in terms of accuracy and efficiency of model production, and to identify key limitations to this approach to aerial tree surveying. Research Objectives Our first research objective was to determine the accuracy of photogrammetric models that could be obtained from low-cost UAVs and any key limitations. For the photogrammetric models to be recommended for tree surveys, the accuracy and level of detail that can be achieved needs to be quantified and objec- tively determined to be at an acceptable level for the purposes of the survey. Our second research objective was to investigate which UAV flight path might provide a more accurate photogrammetric model of an FOI in a tree. We trialled 3 different flight strategies for capturing images of trees and processing into 3D models with the aim of determining if UAVs and photogrammetric models can be successfully used in the assessment of tree FOIs and thus inform management recommendations for trees. MATERIALS AND METHODS This study was designed to make use of low-cost, easily accessible hardware and software so that the results can be utilised by practitioners without the need for specialist, expensive equipment. The UAV used for this study was a DJI Mini 2® (DJI, Shenzhen, China) equipped with an integrated 12-mp camera with manual exposure controls. It is a hobby aircraft which currently costs less than £600 (USD $736) (Figure 1). This UAV weighs less than 250 g which places it in the lowest level of restrictions in the UK, and it does not require a professional qualification to be flown. The software used was the lite version of 3DF Zephyr (3Dflow SRL, Verona, Italy) photo- grammetry software currently costing approximately £150 (USD $184) at the time of writing (3Dflow SRL 2022). The software used for the assessment of the dense point clouds from the field study was Cloud- Compare (EDF R&D, Los Altos, CA, USA), which is an open-source program (EDF R&D 2022). This study assessed the quality of information that could be obtained from UAV-derived photogramme- try models of FOIs set above eye level. To do this, a 2-step approach was devised. First, we carried out an accuracy trial to assess the accuracy and reliability of the photogrammetric models for isolated tree branches in an artificial setting. The outcome of the first trial then fed forward to a field trial, where FOIs of 4 mature oak trees (Quercus robur L.) on their stems, crowns, and canopies were recorded using the UAV- mounted camera to capture images for processing into digital 3D models using photogrammetry software. Accuracy Trial This first trial tested the accuracy of digital 3D models processed from UAV-obtained digital images captured under ideal conditions. Using photogrammetry soft- ware, 3D models were created of tree branches, allow- ing direct comparisons between measurements taken from the 3D photogrammetric models and the subject branches. Ten samples of variously sized fallen branches from broadleaf tree species were collected locally. These samples were elevated from the ground using a timber gantry crane, ropes, and pulleys (Figure 2A). Two photographic scales were attached to each upright leg of the gantry crane to allow for the photogram- metric models to be scaled. The UAV was then flown around the subject branches, collecting images from multiple angles at 2-second intervals, with an orbital radius of approximately 1.5 m. The camera has a 24-mm equivalent lens and a fixed aperture of 2.8F. The ISO of all images was set at 200, and the expo- sure was adjusted to the available light by changing the shutter speed to provide well-exposed pictures. All the flights were undertaken during similar flight con- ditions: less than 10-mph (4.5 m/s) winds to reduce movement of the elevated branches, and with overcast skies to provide a more even light with fewer shadows. Figure 1. Photograph of the commercially available hobby drone, a DJI Mini 2® (DJI, Shenzhen, China) UAV, which was used for this study.
March 2023
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