ABSTRACT The Global Land Surface Satellite (GLASS) leaf area index (LAI) product is one of the most widely used global LAI products in the scientific community. The latest (version 5) GLASS LAI product has been generated from Moderate Resolution Imaging Spectroradiometer (MODIS) surface reflectance data. The purpose of this paper is to evaluate the quality of the version 5 GLASS LAI product. The GLASS LAI product was compared with the latest MODIS LAI product (MCD15A2 H, Collection 6) and the second version of Geoland2 (GEOV2) LAI product to evaluate their temporal and spatial discrepancies. A direct validation was conducted to compare these LAI products to the LAI values derived from the high-resolution reference maps from the Validation of Land European Remote Sensing Instruments (VALERI) and Implementing Multi-Scale Agricultural Indicators Exploiting Sentinels (IMAGINES) sites. The results show that the GLASS and GEOV2 LAI products have great spatial integrity. However, the MODIS LAI product contains many missing pixels in tropical areas. These LAI products follow fairly consistent seasonal characteristics. The spatial discrepancies of these LAI products mainly exist in forest areas, especially evergreen broadleaf forests where the GLASS LAI values are generally lower than the GEOV2 LAI values by approximately 1.0 LAI units and lower than the MODIS LAI values by 0.5 to 1.0 LAI units. The spatial distribution of these LAI products has slight discrepancies in savannahs, broadleaf crops, grasses/cereal crops and shrubs. The GLASS and GEOV2 LAI products capture a complete and reasonable temporal profile, contrasting with the MODIS LAI product, which shows dramatic fluctuations, particularly during the growing seasons. These LAI products show similar temporal trajectories and interannual variations for all biome types except evergreen broadleaf forests. The direct validation shows that the accuracy of the GLASS LAI product is better than the accuracy of the MODIS and GEOV2 LAI products. The coefficient of determination (R2) of the GLASS, MODIS and GEOV2 LAI products versus the LAI values derived from the high-resolution reference maps are 0.68, 0.47 and 0.55, respectively, and the root mean square error (RMSE) of these products are 0.86, 1.22 and 1.21, respectively.

中文翻译：

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MODIS数据衍生的5.0版全球地表卫星（GLASS）叶面积指数产品评估

摘要 全球陆地表面卫星 (GLASS) 叶面积指数 (LAI) 产品是科学界使用最广泛的全球 LAI 产品之一。最新的（第 5 版）GLASS LAI 产品是根据中分辨率成像光谱仪 (MODIS) 表面反射数据生成的。本文的目的是评估第 5 版 GLASS LAI 产品的质量。将 GLASS LAI 产品与最新的 MODIS LAI 产品（MCD15A2 H，Collection 6）和第二版 Geoland2 (GEOV2) LAI 产品进行比较，以评估它们的时空差异。进行了直接验证，将这些 LAI 产品与来自欧洲陆地遥感仪器验证 (VALERI) 和实施多尺度农业指标利用哨兵 (IMAGINES) 站点的高分辨率参考地图的 LAI 值进行比较。结果表明，GLASS 和 GEOV2 LAI 产品具有很好的空间完整性。然而，MODIS LAI 产品在热带地区包含许多缺失的像素。这些 LAI 产品遵循相当一致的季节性特征。这些 LAI 产品的空间差异主要存在于林区，尤其是常绿阔叶林，其 GLASS LAI 值一般比 GEOV2 LAI 值低约 1.0 LAI 单位，比 MODIS LAI 值低 0.5 至 1.0 LAI 单位。这些 LAI 产品的空间分布在热带草原、阔叶作物、禾本科/谷类作物和灌木中略有差异。GLASS 和 GEOV2 LAI 产品捕获了完整且合理的时间剖面，与 MODIS LAI 产品形成对比，后者显示出剧烈的波动，尤其是在生长季节。除了常绿阔叶林外，这些 LAI 产品对所有生物群落类型都显示出相似的时间轨迹和年际变化。直接验证表明，GLASS LAI 产品的精度优于 MODIS 和 GEOV2 LAI 产品的精度。GLASS、MODIS 和 GEOV2 LAI 产品相对于高分辨率参考地图得出的 LAI 值的决定系数 (R2) 分别为 0.68、0.47 和 0.55，