174 Patterson et al.: Effects of Photographic Distance on Tree Crown Attributes work 2.0. Required field measurements include photographic angles to the top and base of the tree and horizontal distance from the photographer’s position to the lower trunk of the tree. Because UrbanCrowns is public domain software and requires no highly specialized equipment or field skills, the program has tremendous potential for improving the efficiency, accuracy, and precision of tree crown measurements. However, UrbanCrowns has received limited empirical testing. From a strictly operation- al standpoint, it is not known how well the software performs under varying photographic conditions for trees of diverse size, which could impact the program’s practicality and measurement reproducibility. To address these questions, a field study of Ur- banCrowns software was undertaken to evaluate the effect of photographic distance on crown dimension estimates for open- grown sugar maple (Acer saccharum) of various size classes. METHODS The field study was conducted on the Virginia Tech campus in Blacksburg, Virginia, U.S. (latitude: 37.23°; longitude: -80.42°) over a two-week period in late summer 2009. Using the cam- pus tree inventory, fifty open-grown sugar maples were pur- posefully selected to obtain ten trees each in five 15 cm trunk diameter classes (0–15, 16–30, 31–45, 46–60, >60 cm). Trees in poor health or having severe crown damage were excluded from the sample. Sample trees were visited in the field to obtain digital photographs and collect physical measures. First, the sur- roundings of the tree were assessed to identify a photographic position that would permit the entire tree to be photographed while also minimizing foreground and background obstruc- tions and sunlight glare. Using a measuring tape, the maximum crown diameter, both perpendicular and parallel to the photo- graphic position was measured along with the trunk diameter at 1.37 m above ground line. Four photographs were taken of each tree at horizontal distances equaling 1.5×, 2×, 2.5×, and 3× the tree’s total height, which (along with angle to the top and base of the tree) was measured using a laser rangefinder (Tru- Pulse 200, Laser Technology Inc., Centennial, Colorado, U.S.). Photographs were taken with a 3.1 megapixel compact digital camera (Sony Cyber-shot DSC-W5, Sony, Tokyo, Japan) set to 7.9 mm focal length and f/5.6 aperture and held plumb 1.67 m above ground line. All four photographs of a single tree were taken at the same azimuth, which was measured with a compass. Tree photographs were uploaded to a desktop computer and imported to the UrbanCrowns software application. For each im- age, several manual inputs were made to the application to per- mit crown analysis by the software. Field values for the camera azimuth to tree, horizontal distance to tree, angle to crown top, and angle to trunk base were first entered. Next, three reference lines were manually digitized on the tree image (Figure 1). The first line (yellow) was drawn from the base to the top of the tree, following any lean in the stem. This line input, combined with the view angles and horizontal distance measurements from the field, scaled the photograph (determined the actual area repre- sented by each pixel). The second line (pink) was drawn around the portion of the crown unobstructed by vegetation or struc- tures present in the foreground and background. This region was used by the software to calculate crown transparency and den- sity. The third line (blue) was drawn around the entire crown to roughly delineate the area for crown volume calculation. Both the crown density and crown volume reference lines were then ©2011 International Society of Arboriculture Figure 1. Screenshot from UrbanCrowns image analysis software showing reference lines digitized by the operator during the first stage of analysis: tree height (yellow), crown volume (blue), and crown density (pink). Shown here is a sugar maple (Acer sac- charum), measuring 48 cm trunk diameter and 12 m tall, photo- graphed at a distance of 24 m. Figure 2. Screenshot from UrbanCrowns image analysis soft- ware showing computer-processed reference lines (“wrapping” of crown) made during the second stage of analysis: tree height (yellow), crown volume (red), and crown density (pink). Shown here is a sugar maple (Acer saccharum), measuring 48 cm trunk diameter and 12 m tall, photographed at a distance of 24 m.
July 2011
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