©2023 International Society of Arboriculture Arboriculture & Urban Forestry 49(2): March 2023 87 conjunction with other tree assessment methods, such as microdrilling or tomography, by providing accu- rate information on the external dimensions and form of the feature. Where tree assessment leads to recommendations for remedial tree surgery, digital pruning of these pho- togrammetric models is possible. More accurate spec- ifications and more predictable outcomes for high value trees could be achieved by assessing the effects of pruning on a tree’s pruned form, and the extent of proposed pruning can be gauged easily using the model prior to the pruning of the tree. For example, proposed crown reduction work could be communi- cated very accurately in a visual form to arborists, municipal tree officers, and clients by sharing such digitally pruned models, enabling a clear conception for all involved about the proposal and potentially increas- ing the precision of the actual pruning carried out. We conclude that, where the FOIs can be viewed from multiple points by the camera of a UAV, and sub- ject to local UAV flight restrictions, the use of UAV obtained photogrammetry can provide an immediate and cost-effective method of visually inspecting the aerial FOI of trees and obtaining measurements that have comparable levels of accuracy to conventional measuring equipment. LITERATURE CITED Anderson K, Gaston KJ. 2013. Lightweight unmanned aerial vehicles will revolutionize spatial ecology. Frontiers in Ecology and the Environment. 11(3):138-146. https://doi.org/10.1890/ 120150 Buffi G, Manciola P, de Lorenzis L, Gambi A. 2018. Influence of construction joints in arch-gravity dams modelling: The case of ridracoli. In: Icold C, editor. Twenty-sixth international congress on large dams. Vienna (Austria): CRC Press. p. 1047-1062. https://doi.org/10.1201/9780429465086 Cannon CH, Scher CL, Gao A, Khan T, Kua CS. 2018. Building a tree observatory. Acta Horticulturae. 1222:85-92. https:// doi.org/10.17660/ActaHortic.2018.1222.12 Côté JF, Widlowski JL, Fournier RA, Verstraete MM. 2009. The structural and radiative consistency of three-dimensional tree reconstructions from terrestrial lidar. Remote Sensing of Environment. 113(5):1067-1081. https://doi.org/10.1016/j .rse.2009.01.017 Cox S, Melarange P. 2017. Aerial inspections: Guide to good practice. Stroud (United Kingdom): Arboricultural Association. AA Guidance Note 11. 43 p. http://www.treesource.co.uk/ guidance-note-11-aerial-inspections-a-guide-to-good -practice~4723 Dunster JA, Smiley ET, Matheny N, Lilly S. 2017. Tree risk assessment manual. 2nd Ed. Champaign (IL, USA): Interna- tional Society of Arboriculture. 194 p. number of models under far more tightly controlled conditions. Despite low replication in the field trial, key statis- tical differences and obvious visual differences in the photogrammetric models were evident. However, extended trials with a higher number of replicates are likely to provide further insights into techniques and flight paths that reliably capture FOIs in sufficient detail for tree assessment purposes. For example, from the insight that continuity of overlapping images is needed for good rendering of a visual model with the equipment used in this study, the cluster flight strat- egy could be revised to achieve more complete mod- els. We also recommend that future studies undertake the surveying of trees in urbanised areas, where addi- tional obstructions such as buildings and aboveground utilities may require changes to flight strategies. CONCLUSION This study demonstrates that it is possible to produce digital 3D models of aerial FOIs of individual trees which can be inspected and measured using low-cost equipment and software. This method of data collection and processing has practical application in arboricul- ture, effectively extending a ground-based tree assess- ment to parts of the tree otherwise only accessible to view using rope and harness or mobile elevated work platforms. In addition, this technology automatically produces a record of inspection that can be repeatedly interrogated and compared with further models of the same FOIs over time. Models of an FOI can be inspected three- dimensionally by practitioners within the software, allowing for a greater visual interrogation of the con- text and significance of a given feature than would be possible from still images or videos (Koh and Wich 2012). In addition, the ability to take measurements from the models means collecting accurate dimen- sions is simple, repeatable, and can be undertaken by the inspector rather than by a climbing arborist under instruction from the ground (Cannon et al. 2018; Krause et al. 2019). The external measurements taken from the models can be used in biomechanical assess- ments of trees; for example, the slenderness ratios of branches when considering the likelihood of their fail- ure. The form of branches and branch junctions can have their orientation recorded and used to assess the effect of loading from different directions. The use of this method can be expected to improve the analysis of the residual strength of tree sections when used in
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