Arboriculture & Urban Forestry 37(4): July 2011 Arboriculture & Urban Forestry 2011. 37(4): 173–179 173 Effects of Photographic Distance on Tree Crown Attributes Calculated Using UrbanCrowns Image Analysis Software Mason F. Patterson, P. Eric Wiseman, Matthew F. Winn, Sang-mook Lee, and Philip A. Araman Abstract. 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 di- mension 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 cal- culations derived from equations for geometric solids, both in terms of precision (R2 banCrowns has potential as an objective, reliable method for measuring tree crown attributes that are commonly assessed during urban forest inventories. Key Words. Crown Density; Crown Transparency; Crown Volume; Leaf Area; Tree Inventory; Tree Measurement; Tree Monitoring. = 0.9783) and accuracy (B1 = 1.0033). These findings suggest that Ur- Measuring tree crown attributes such as volume and density is a common procedure during the assessment and management of landscape trees. These measurements are relevant to numer- ous endeavors, such as monitoring tree health and vitality (Bus- sotti et al. 1995; Solberg 1999), evaluating tree responses to management (Kjelgren 1995; Bühler et al. 2007), and modeling tree ecophysiological functions (Donat and Ruck 1999; Bucco- lieri et al. 2009). Measuring crown attributes typically entails conventional physical measures or subjective visual ratings. Depending on the circumstances, these techniques can be time- consuming, imprecise, and inaccurate, making their application impractical or unreliable – particularly for longitudinal stud- ies (Innes 1988; Ghosh et al. 1995; Solberg and Strand 1999). Numerous methods have been used to measure tree crown di- mensions. Crown height and width can be measured directly in the field using conventional tools such as a measuring tape, cli- nometer, or laser hypsometer. From these measures, crown vol- ume can be readily calculated using the formula for a geometric solid that approximates the shape of the crown. Various geomet- ric solids, ranging from a simple sphere to a complex polyhe- dron have been used to model tree crown volume (Mawson et al. 1976; Brown 1978; McPherson and Rowntree 1988; Cluzeau et al. 1995; Wilkinson 1995; Baldwin and Peterson 1997; Wright et al. 2006). Crown volume estimates based on geometric solids and field measurements have been found to correlate well with leaf area (Tucker et al. 1993; Nowak 1996; Karlik and Winer 1999), which is a key parameter for modeling urban tree func- tions such as air pollutant interception (Nowak 1994) and rainfall storage (Xiao et al. 2000). Despite the practicality of geometric crown calculations, accuracy can be compromised when using simple geometric solids due to assumptions that the crown is symmetric and has a convex surface (i.e., there are no indenta- tions), which is often not the case for urban trees subjected to prevailing winds, storm damage, pests, and pruning. Although complex geometric solids can improve accuracy by account- ing for undulating crown surfaces (Tucker et al. 1993; Cluzeau et al. 1995), these models require additional tree measurements and intricate equations, which reduces measurement practicality. Imaging technologies have also been employed to measure tree crown attributes. Ground-based photography has been used to reliably estimate crown height and width (Seiler and McBee 1992; Wilkinson 1994), crown silhouette area (Lindsey and Bassuk 1992), crown volume (Bignami and Rossini 1996), and crown density (Osawa and Kurachi 1997). More recently, digital imaging coupled with computer analysis has improved the preci- sion and automation of crown measurements from photographs (Brown et al. 2000; Phattaralerphong and Sinoquet 2005). Pho- tographic measurement of crown attributes has several potential advantages over conventional physical measures: quicker field acquisition, less observer bias, and greater reproducibility. How- ever, even these techniques can bear limitations, such as special- ized equipment needs or intensive image post-processing, which reduces their suitability for arboricultural research and practice. UrbanCrowns is a software program developed by the USDA Forest Service Southern Research Station that calculates sever- al tree crown attributes using a single side-view digital photo- graph and a few basic field measurements (Winn et al. 2007). The program provides estimates of tree height and length as well as crown height, diameter, ratio, volume, transparency, and den- sity. Photographs can be taken with a basic digital camera, and the software can be operated on any computer running Microsoft Windows version XP or later along with Windows .NET Frame- ©2011 International Society of Arboriculture
July 2011
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