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    改进的相似像元优选方法在厚云去除算法中的应用

    Application of an improved method for selecting optimal similar pixels to thick cloud removal algorithms

    • 摘要: 云污染干扰了遥感卫星对地面信息的获取,去云算法的开发有效解决了由厚云遮挡所引起的遥感数据缺失问题。然而,现有算法通过计算中心像元与邻近时空像元之间的差异,定义相似像元,在地表突变的情况下,这种选取方法面临逻辑问题。因此提出一种顾及时间变化的相似像元选取方法以增强现有去云算法。利用K-means聚类提取时序清洁像元的时间变化聚类中心,进而利用这些聚类中心对清洁时期影像进行监督分类,推算云下时序变化模式,认为与中心像元变化模式相同的像元具有相同的时序变化,进而与现有算法寻找的相似像元求交集得到优选的相似像元,按照原有去云算法完成去云。选择豫北农区作为试验区,以改进的相似像元插值法(modified neighborhood similar pixel interpolator, MNSPI)和加权线性回归(weighted linear regression, WLR)2个去云算法为例,结果表明: ①优选算法可以从快速变化的农事情景中准确提取相似像元; ②基于优选结果的去云算法在光谱差异以及结构相似度都优于原算法,改进之后的MNSPI算法平均均方根误差(root mean squared error,RMSE)从0.023 4下降到0.015 2,结构相似性指数(structural similarity index measure,SSIM)从0.602 3提升为0.716 6; 改进后的WLR算法平均RMSE从0.037 5下降到0.016 8,SSIM从0.589 7提升到0.646 1; ③实际影像的应用表明算法可以准确恢复云下信息,视觉效果良好。研究可以为快速变化地表的遥感图像去云提供科学依据,为高质量遥感时序监测提供数据支撑。

       

      Abstract: Cloud cover interferes with the ground information acquisition by remote sensing satellites, while remote sensing data missing caused by thick cloud cover can be effectively addressed using developed cloud removal algorithms. However, existing algorithms, which select similar pixels by computing differences between the central pixel and its spatiotemporal neighbors, suffer from logical defects when applied to the land surface with abrupt changes. Therefore, this study proposed an improved method for selecting similar pixels while accounting for temporal variations, aiming to enhance the performance of existing cloud removal algorithms. The improved method is detailed as follows: time-varying cluster centers of time-series clean pixels are extracted using the K-means clustering method; these cluster centers are used for supervised classification of cloud-free remote sensing images, followed by the inference of the temporal variation patterns beneath clouds; since it is considered that pixels with variation patterns consistent with those of the central pixel share the identical temporal variations, the intersections of these pixels and similar pixels identified using existing algorithms are the optimal similar pixels; finally, cloud removal is performed using the original algorithm based on the determined optimal similar pixels. Using the agricultural region in north Henan as a case study, the proposed method for selecting the optimal similar pixels was applied to two cloud removal algorithms: modified neighborhood similar pixel interpolator (MNSPI) and weighted linear regression (WLR). The results indicate that the improved method can accurately extract similar pixels from rapidly changing agricultural scenarios. Both cloud removal algorithms based on the selected optimal similar pixels outperformed their original algorithms in terms of both spectral differences and structural similarity. Specifically, the improved MNSPI algorithm reduced the average root mean squared error (RMSE) from 0.023 4 to 0.015 2 and increased the structural similarity index measure (SSIM) from 0.602 3 to 0.716 6 compared to its original algorithm. Meanwhile, the improved WLR algorithm decreased the average RMSE from 0.037 5 to 0.016 8 and increased the SSIM from 0.589 7 to 0.646 1 compared to its original algorithm. Applications to real remote sensing images demonstrate that both algorithms based on the proposed method can accurately reconstruct information beneath clouds, achieving encouraging visual effects. The results of this study provide a scientific basis for removing clouds in remote sensing images of rapidly changing land surfaces, thereby offering data support for high-quality time-series remote sensing monitoring.

       

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