Light detection and ranging (LiDAR) provides a state-of-the-art technique for measuring forest canopy height. Nevertheless, it may miss some forests due to its spatial separation of individual spots. A number of efforts have been made to overcome the limitation of global LiDAR datasets to generate wall-to-wall canopy height products, among which a global satellite product produced by Simard et al. (2011) (henceforth, the Simard-map) has been the most widely applied. However, the accuracy of the Simard-map is uncertain in boreal forests, which play important roles in the terrestrial carbon cycle and are encountering more extensive climate changes than the global average. In this letter, we evaluated the Simard-map in boreal forests through a literature review of field canopy height. Our comparison shows that the Simard-map yielded a significant correlation with the field canopy height (R2 = 0.68 and P < 0.001). However, remarkable biases were observed with the root mean square error (RMSE), regression slope, and intercept of 6.88 m, 0.448, and 10.429, respectively. Interestingly, we found that the evaluation results showed an identical trend with a validation of moderate-resolution imaging spectroradiometer (MODIS) tree-cover product (MOD44B) in boreal forests, which was used as a crucial input data set for generating the Simard-map. That is, both the Simard-map and MOD44B yielded an overestimation (underestimation) in the lower (upper) tails of the scatterplots between the field and satellite data sets. This indicates that the MOD44B product is the likely source of error for the estimation biases of the Simard-map. Finally, a field calibration was performed to improve the Simard-map in boreal forests by compensating for the estimation biases and discarding non-forest areas, which provided a more reliable canopy height product for future applications.

中文翻译：

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Simard等人的评估。2011年北方森林全球树冠高度图

光检测和测距（LiDAR）提供了用于测量森林冠层高度的最新技术。但是，由于各个点的空间分隔，它可能会错过一些森林。为了克服全球LiDAR数据集生成墙到墙冠层高度产品的局限性，已经做出了许多努力，其中包括Simard等人生产的全球卫星产品。（2011年）（此后称为Simard地图）应用最为广泛。但是，Simard-map的准确性在北方森林中尚不确定，北方森林在陆地碳循环中起着重要作用，并且遇到的气候变化要比全球平均水平更为广泛。在这封信中，我们通过对田间冠层高度的文献综述，评估了北方森林中的Simard地图。我们的比较表明Simard贴图与田间冠层高度显着相关（R2 = 0.68和P <0.001）。但是，在均方根误差（RMSE），回归斜率和截距分别为6.88 m，0.448和10.429处观察到明显的偏差。有趣的是，我们发现评估结果显示出相同的趋势，并验证了北方森林中中等分辨率成像光谱仪（MODIS）树木覆盖产品（MOD44B），该数据被用作生成Simard图的关键输入数据集。也就是说，Simard-map和MOD44B都在场数据集和卫星数据集之间散点图的下（上）尾部产生了高估（低估）。这表明MOD44B乘积可能是Simard映射估计偏差的误差源。