328 measurement of the tree height in meters; therefore, the percent error of the measurements may still be within an acceptable region. Based on the lower predictability of the stick method and its reduced accuracy as found in the lin- ear regression (Figure 4) and relationship to the line y = x (Tables 3 and 4), it is more challenging to create an evaluative model that can help correct errors in tree height measurements, especially with greater variation about the line y = x. From the viewpoint of a practitioner, knowing that a measurement device tends to under- or overestimate allows for on-the-spot corrections, especially within a felling situation where one might choose to give themselves an “extra bit of room.” Because of the crossover between underesti- mation and overestimation, it is challenging to have a general rule of thumb for field use. Future research examining the accuracy and preci- sion of common arborist and urban forester mensura- tion tools should seek to average the measured values across multiple measurements conducted by different measurers. This would help control for error by the individual measurer and introduce additional statisti- cal tests for inter-coder reliability and the extent of variation between the 3 measures. Measurements by Tree Type All 3 methods were more accurate (i.e., reduced RMSE and higher Pearson correlations) when measuring conifers. The average RMSE difference was 0.2321 between conifer and broadleaf measurements. This might be attributed to the clearer identification of the top of the tree for conifers as opposed to broadleaf trees, which helps the measurer to identify the treetop more readily. This finding is consistent with the lim- itations described in the evaluation of smartphone apps by Vastaranta et al. (2015), who surmised that inaccuracies in tree height measurement may arise as a result of misidentification of the true highest branch or foliage of the tree. Decurrent or fastigiate trees with sparse foliage spread across many upright branches may create difficulty in determining the most suitable measuring point for tree height. Because this study used the same measuring point consistently, another hypothesis is that there is less movement of the apical point of a conifer versus a broadleaf tree, introducing a more static measurement point. Further research on accuracy and precision of measurement tools may choose to examine methods ©2022 International Society of Arboriculture Implications for Arborists and Urban Foresters Although the duration of each method per tree was not measured in this study (although it would certainly be a worthy study in the future), anecdotally, the rangefinder was substantially faster, measuring trees in less than a quarter of the time used for the clinom- eter and stick methods. In addition to the shorter time required to take a measurement and the greater accu- racy, the benefit of the rangefinder is that it reduces the influence of human error in calculations, as the calculations are performed by the unit itself. The accuracy of the rangefinder was highest, and, as a result, should be considered a best practice for tree height measurement during inventories. Because the rangefinder involves no calculations on the part of the user, it may become especially pertinent for urban forest inventories completed with the help of volunteers/citizen scientists, a practice that is becom- ing increasingly common (Harrison et al. 2020). Using more user-friendly technology may decrease error and will make it easier to teach volunteers the tree mensuration methods. The stick method had greater error than the other 2 methods in all cases, even under the application of improved practices. While the recorded measure- ments for inventory data analysis benefit from the least error (i.e., rangefinder), the RMSE findings of the stick method may still be within tolerable error for height measurements in a tree felling context. Thus, despite the greater error, the stick method may remain a suitable method for felling purposes in determining the extent of the felling radius. Future research on tree height measurements may look at measurer reliability across different methods, especially with different levels of experience and/or training. Additionally, expanding this study into an area with higher trees will better demonstrate the pre- cision and accuracy of these methods among higher tree heights. Opportunities and Limitations of the Methods The results examine the reliability of the methods which includes human error. As is true for all 3 methods, Martin: Accuracy and Precision of Tree Height Measurement Tools using more specific qualitative growth form descrip- tions, which may be beneficial in modeling measure- ment differences between tree genera.
November 2022
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