Effects of photographic distance on tree crown attributes calculated using UrbanCrowns image analysis software

In 2007, the UrbanCrowns software tool was released to allow users to assess several crown attributes with one photograph and some simple field measurements; unlike other methods, no special equipment or computer post-processing is necessary. The objective was to improve the accuracy of crown measurements, which are an important component of many urban forest inventories and many of the programs that estimate urban forest ecosystem services. This study tested the accuracy of UrbanCrowns across a variety of tree sizes under different photographic conditions. Fifty open-grown sugar maples (Acer saccharum) (10 each in five size classes) were photographed and measured according to the UrbanCrowns procedures. For each tree, photographs were taken along one azimuth at four distances to determine if the accuracy was affected by distance to the tree. The software showed highly repeatable results for both crown volume and density. Crown volume measurements had repeatability errors of less than 5% in most cases; errors of 5-10% were found for the largest trees at the greatest distances. Crown density showed most repeatability errors of <1%.