©2023 International Society of Arboriculture 76 Roberts and Slater: Miniature UAVs and Photogrammetry ground-based tree inspection identifies a defect or structural feature of interest (FOI) higher up in a tree, a climbing arborist may be brought in to climb the tree and inspect these FOIs to inform tree management recommendations. Specialist aerial inspections can provide invaluable information otherwise unavailable to a tree surveyor. However, such aerial inspections involve specialist training, equipment, and insurance, and can add significant expense and time to a tree assessment. Although tree structure can be modelled success- fully with terrestrial LiDAR (Côté et al. 2009), this does not allow for a view downwards onto aerial fea- tures of trees. Aerial photography can provide addi- tional visual information to a tree assessment, such as damage to the upper sides of branches and identifica- tion of pests and pathogens: e.g., branch decay caused by massaria disease of plane (Splanchnonema platani), which is often only visible on the upper side of hori- zontal branches near their point of attachment to the tree’s main stem (Schmitt et al. 2014). However, it can be difficult to extract quantifiable information on the dimensions of features from images or video alone. The dimensions of the features of a tree are important to a tree surveyor, to assess the scale of a structural fault or other feature. For example, the length of branches, diameter of wounds, angle and form of branch junctions, and the size of any dead- wood in the crown of a tree are all valuable metrics for assessing the significance of these features and for providing suitable recommendations for their manage- ment (Cox and Melarange 2017; Slater 2022). Photogrammetry, the production of digital 3D models from overlapping images, can produce high-definition, scaled 3D models from which detailed observations and measurements can be made (Ganz et al. 2019). Photogrammetry is a means of increasing the useable data from the information collected during an aerial inspection by a UAV. Photogrammetry works by obtaining images of the object from multiple positions. The photogrammetry software uses data from those images, such as posi- tion and angle of the camera for each image, and the camera characteristics, such as focal length, to create point clouds. From the point cloud a 3D mesh is pro- duced and information from the input images is over- laid to produce a final digital 3D model. The quality and accuracy of the 3D model is dependent on the quality and accuracy of the data used to build it. UAVs and photogrammetry software are regularly used to assess solid, static structures such as build- ings, dams, and bridges. The digital 3D models which they produce can be easily shared with people for inspection, to communicate ideas, and for planning pur- poses. However, the sometimes complex form and flexibility of tree stems and branches often makes the capturing of quality imagery of these structures by UAVs more challenging. Structures which sway in windy conditions can make capturing sharp, well- exposed images more difficult. In addition, every tree has a different form and grows in different surround- ings. Some methods of capturing images with a UAV may be more achievable than others in certain scenar- ios; for example, an automated orbital flight around a tree may not be possible if the subject tree is growing close to other trees in a woodland setting or close to tall buildings in an urban environment. Current Research Gaps The value and accuracy of UAV surveys is recognised in associated literature (e.g., Koh and Wich 2012; Seo et al. 2018; Nitoslawski et al. 2021). Several studies have investigated the use of UAVs and photogram- metry as a method of obtaining accurate measurements from trees for varying purposes, such as understand- ing tree growth and behaviour (e.g., Scher et al. 2019; Moreira et al. 2021) and forest mensuration (e.g., Krause et al. 2019; Ramalho de Oliveira et al. 2021). Many of these studies focus on landscape scale pho- togrammetric models or models of single trees from which core measurement data can be obtained, such as diameter at breast height (DBH) and overall tree height. For example, Krause et al. (2019) have shown the successful use of photogrammetry to accurately measure tree heights in forest stands, whilst Nezami et al. (2020) showed how UAV technology could identify different tree species within forests through spectral analysis and the use of artificial intelligence (AI). However, such studies have typically involved the use of high-cost systems, often bespoke systems, and most have not focused upon individual assess- ments of open-grown or urban trees. To take existing UAV technology and make it into a viable tool for an arborist carrying out a visual inspection of the aerial parts of a single tree, the use of low-cost equipment that provides sufficiently informative visual models is needed. In addition, it is important to establish the best way a UAV can be
March 2023
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