Assessment of FY-3D MERSI/NDVI global product
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摘要: MERSI/NDVI是风云三号D星的一个关键业务产品,深入了解其质量状况对推广产品应用、改进产品算法都具有重要意义。文中针对业务化运行后的全球MERSI/NDVI产品(2019年5月至2020年12月),以同期Terra MODIS/NDVI产品(MOD13A2)为参考,通过空间格局和时间序列的对比、APU(准确度Accuracy,精密度Precision,不确定性Uncertainty)指标计算以及回归分析等手段,评估MERSI/NDVI数据质量和可用性。结果显示,MERSI/NDVI和MODIS/NDVI在空间分布和时序特征方面具有较高一致性,但MERSI/NDVI有对高值低估、低值高估的倾向,故动态范围略窄;在全球平均水平上,MERSI/NDVI比MODIS/NDVI系统性偏低0.02—0,P和U值为0.06—0.08,MERSI/NDVI与MODIS/NDVI的差别由小到大的顺序大致为:裸土荒漠、稀疏灌丛和草地、密闭灌丛与农田、除常绿阔叶林以外的森林、常绿阔叶林;以MODIS/NDVI为自变量、MERSI/NDVI为因变量的线性回归模型精度较高(R2:0.91—0.95,RMSE:0.048—0.068),回归系数具有一定的时间变化(斜率:0.87—0.94,截距:0.02—0.04)。本研究是首次对风云三号D星MERSI/NDVI产品开展近乎全样本的对比检验,证明该产品基本可以替代MODIS/NDVI在全球开展物候信息提取、植被长势监测等应用。Abstract: FY-3D MERSI/NDVI is a critical operational product used in many studies (ecosystem monitoring, climate change, agriculture drought, etc.), and it is essential to obtain a comprehensive assessment of this product's quality. In this paper, the first assessment results of global MERSI/NDVI for the period from May 2019 to December 2020 are reported using Terra MODIS/NDVI as the reference. Quantitative measures of APU (Accuracy, Precision and Uncertainty) are calculated and the variations associated with different factors (seasons, land cover types and NDVI values) are analyzed. Results indicate that generally the two products share a high similarity concerning the spatial pattern and temporal profiles features. The dynamic range of MERSI/NDVI is slightly narrower because it overestimates NDVI for barren land and underestimates NDVI for dense vegetation. Sensitivity analysis indicates that the overestimation is mainly attributed to overestimation of NIR reflectance, whereas the underestimation is mainly attributed to overestimation of red reflectance. Phonological features conveyed by the two NDVI products are consistent, but there are slightly noisier fluctuations in MERSI/NDVI time series probably caused by cloud contamination during growing season. Over the 20 months period checked in this study, the global mean of the Accuracy value ranges within −0.02—0, and the global mean of the Precision and Uncertainty values generally range within 0.06—0.08. With respect to the spatial pattern, APU values are the highest in forests, moderate in grassland/shrubland, and lowest in desert. Linear regression models with MODIS/NDVI as independent variables and MERSI/NDVI as dependent variables achieve high accuracies (R2: 0.91—0.95, RMSE: 0.048—0.068), confirming that it is feasible to build a compatible long-term NDVI dataset using both products. This study is the first cross-sensor comparison study using almost all of the global MERSI/NDVI data available since the operational application of the FY-3D satellite. Overall, the MERSI/NDVI data are of very high quality and can be effectively used for deriving vegetation phonological and greenness information. Its performance on the global scale will be monitored continually.
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Key words:
- FY-3D /
- MERSI /
- NDVI /
- Quality evaluation
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图 8 APU随土地覆盖类型的变化特征 (1:常绿针叶林,2:常绿阔叶林,3:落叶针叶林,4:落叶阔叶林,5:混交林,6:密闭灌丛,7:开放灌丛,8:木质稀树草原,9:稀树草原,10:草地,12:农田,14:农田和自然交错,16:荒漠/裸地)
Figure 8. Averaged APU for different land cover types (1:Evergreen,needleleaf forest,2:Evergreen broadleaf forest,3:Deciduous needleleaf forest, 4:Deciduous broadleaf forest,5:Mixed forests, 6:Closed shrublands,7:Open shrublands,8:Woody savannas, 9:Savannas, 10: Ggrasslands, 12:Croplands,14:Cropland/Natural vegetation mosaic, 16:Barren or sparsely vegetated)
图 13 不同覆盖度条件下的植被光谱反射率 (Lu,et al,2021)
Figure 13. Influence of canopy green vegetation fraction on spectral reflectance (Lu,et al,2021)
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