Effects of Photographic Distance on Tree Crown Attributes Calculated Using UrbanCrowns Image Analysis Software

UrbanCrowns is a software program developed by the USDA Forest Service that computes crown attributes using a side-view digital photograph and a few basic field measurements. From an operational standpoint, it is not known how well the software performs under varying photographic conditions for trees of diverse size, which could impact measurement reproducibility and therefore software utility. Researchers evaluated the robustness of crown dimension computations made with UrbanCrowns for open-grown sugar maples (Acer saccharum) across a range of sizes from recently transplanted to full maturity. It was found that computations of both crown volume and density were highly repeatable across varying photographic distances. For the majority of tree size classes, crown volume and density varied less than 5% on average over distances ranging from 1.5× to 3.0× tree height; however, crown volume errors of 5%–10% were common for larger trees (>46 cm trunk diameter). UrbanCrowns calculations of crown volume showed strong agreement with calculations derived from equations for geometric solids, both in terms of precision (R = 0.9783) and accuracy (B 1 = 1.0033). These findings suggest that UrbanCrowns has potential as an objective, reliable method for measuring tree crown attributes that are commonly assessed during urban forest inventories.