Visual Estimation Accuracy of Tree Part Diameter and Fall Distance

When professionals assess tree risk, they must consider the potential consequences associated with a branch or whole tree striking a person, vehicle, or structure. This process requires an assessor to determine the diameter and fall distance of a tree part and then gauge the likely damage to a target if failure occurred. The ability to accurately estimate diameter and fall distances is important, as direct measurements are not always possible. In this study, we examined whether differences exist between visual estimations and direction measurements of tree part diameters and fall distances among 106 arborists of differing experience levels. Our findings suggest arborists' estimations were reasonably accurate in comparison to direct measurements. International Society of Arboriculture Certified Arborists and experienced arborists were more accurate in diameter estimations than arborists lacking assessment experience. In contrast, nonexperienced arborists were closer in their fall distance estimations than arborists with risk assessment experience. Study Implications: Tree risk assessment is a human endeavor that can be influenced by an individual's risk perceptions, risk tolerance, and personal bias. Training, best management practices (BMPs), and industry credentials all strive to make the tree risk assessment process more consistent among different assessors. Despite this, variability still exists among the different components considered during a risk assessment. In particular, the consequences of failure ratings (i.e., qualitative assessments of a tree's potential to cause injury, damage, disruption, or death), have been identified as significant source of interassessor variability. In this brief communication, we evaluated how accurate risk assessors with different levels of experience and training are at estimating tree part diameters and fall distances. Limiting excess variability in this risk assessment input will ultimately help reduce differences in the assessor's final risk ratings.