Abstract Field measurements of leaf area density (LAD) and leaf area index (LAI) for individual trees have increased in importance and relevance with the advent of high spatial resolution remote sensing for the urban landscape. However, indirect field measurements of LAD/LAI for widely dispersed individual trees have not been comprehensively evaluated. The present study compares the accuracy of three indirect LAD/LAI estimation methods, including single-return terrestrial laser scanning (TLS), LAI-2200, and digital hemispherical photography (DHP) on urban trees. To this end, field measurements were inter-compared and physically modeled by discrete anisotropic radiation transfer (DART). The inter-comparisons of field data revealed substantial inconsistencies between DHP and the other two approaches. For the physical modeling, reference LAI was obtained from realistic 3-D tree objects in DART, and the LAD/LAI was derived from simulated TLS, LAI-2200, and DHP acquisitions and was evaluated against the references. The physical modeling results showed that TLS could reasonably estimate LAD/LAI for LAD 3 and LAI > 6). The LAD/LAI accuracy was sharply reduced as voxel size increased. In addition, the trunks caused overestimation for both of LAD and LAI, while branches caused LAD underestimation and LAI overestimation. Further research is needed to compensate for the effects of occlusions and clumping in the estimates of LAD/LAI for dense-foliated trees using TLS. LAI-2200 grossly underestimated LAD for all cases, while it accurately estimated LAI for LAI 5. The estimation accuracy of LAI-2200 declined markedly with increasing uncertainty in crown shape. The 90° view cap had higher accuracy than the 180° or 270° view caps using four or all five LAI-2200 rings. Additional sensors or algorithms for crown shape measurement should be developed for LAI-2200 to reduce its reliance on other data sources. DHP is not recommended for individual trees as the LAI estimations were biased from the reference values, and improvements in reliability will depend on new algorithms to account for differences in path length. These results serve as a benchmark for evaluating the accuracy of in situ LAD/LAI measurement techniques and for optimizing the configurations required for each indirect measurement method applied to individual trees.

中文翻译：

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估算单株叶面积密度和叶面积指数的三种间接方法的评价研究

摘要 随着城市景观高空间分辨率遥感的出现，单株叶面积密度 (LAD) 和叶面积指数 (LAI) 的现场测量变得越来越重要和相关。然而，广泛分布的单株树的 LAD/LAI 的间接现场测量尚未得到全面评估。本研究比较了三种间接 LAD/LAI 估计方法的准确性，包括单回地面激光扫描 (TLS)、LAI-2200 和城市树木上的数字半球摄影 (DHP)。为此，通过离散各向异性辐射转移 (DART) 对现场测量进行了相互比较和物理建模。现场数据的相互比较揭示了 DHP 与其他两种方法之间的重大不一致。对于物理建模，参考 LAI 是从 DART 中真实的 3-D 树对象获得的，LAD/LAI 是从模拟的 TLS、LAI-2200 和 DHP 采集中获得的，并根据参考进行评估。物理建模结果表明，对于 LAD 3 和 LAI > 6)，TLS 可以合理地估计 LAD/LAI。随着体素尺寸的增加，LAD/LAI 的准确度急剧下降。此外，主干导致 LAD 和 LAI 的高估，而分支导致 LAD 低估和 LAI 高估。需要进一步的研究来补偿使用 TLS 的密集叶树的 LAD/LAI 估计中的遮挡和结块的影响。LAI-2200 严重低估了所有情况下的 LAD，而它准确地估计了 LAI 5 的 LAI。随着冠形不确定性的增加，LAI-2200 的估计精度显着下降。90° 观察帽比使用四个或所有五个 LAI-2200 环的 180° 或 270° 观察帽具有更高的精度。应为 LAI-2200 开发用于牙冠形状测量的附加传感器或算法，以减少其对其他数据源的依赖。不建议将 DHP 用于单个树木，因为 LAI 估计与参考值有偏差，可靠性的改进将取决于新算法来解决路径长度的差异。这些结果可作为评估原位 LAD/LAI 测量技术准确性和优化应用于单个树木的每种间接测量方法所需配置的基准。应为 LAI-2200 开发用于牙冠形状测量的附加传感器或算法，以减少其对其他数据源的依赖。不建议将 DHP 用于单个树木，因为 LAI 估计与参考值有偏差，可靠性的改进将取决于新算法来解决路径长度的差异。这些结果可作为评估原位 LAD/LAI 测量技术准确性和优化应用于单个树木的每种间接测量方法所需配置的基准。应为 LAI-2200 开发用于牙冠形状测量的附加传感器或算法，以减少其对其他数据源的依赖。不建议将 DHP 用于单个树木，因为 LAI 估计与参考值有偏差，可靠性的改进将取决于新算法来解决路径长度的差异。这些结果可作为评估原位 LAD/LAI 测量技术准确性和优化应用于单个树木的每种间接测量方法所需配置的基准。