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Algorithm for cloud shadow identification on complex terrains

  • role： First author第一作者
  • Affiliation：

    State Key Laboratory of Remote Sensing Science, Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing 100101, China

    Ecological and Agricultural Meteorology Center of Inner Mongolia Autonomous Region, Hohhot 010051, China

  • Email：983229508@qq.com
  • Introduction：李彬(1989—　)，男，助理工程师，研究方向为地表辐射平衡方面的研究。E-mail：983229508@qq.com
LI Bin12,  
  • role： Corresponding author通信作者
  • Affiliation：

    State Key Laboratory of Remote Sensing Science, Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing 100101, China

  • Email：xin_xzh@163.com
  • Introduction：辛晓洲(1976—　)，男，副研究员，研究方向为地表辐射与能量平衡遥感估算理论与方法研究。E-mail：xin_xzh@163.com
XIN Xiaozhou1*,  
  • Affiliation：

    State Key Laboratory of Remote Sensing Science, Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing 100101, China

ZHANG Hailong1,  
  • Affiliation：

    School of Applied Meteorology, Nanjing University of Information Science & Technology, Nanjing 210044, China

HU Jichao3

Resümee

Many methods for cloud shadow detection in remote sensing have been proposed, such as utilizing the threshold and time series. Adopting the spectrum threshold can encounter many difficulties, such as the selected threshold having significant uncertainty and can be influenced by other factors. The time series method requires much time, and its process is complicated. The geometric method is particularly more accurate and practical. However, few studies have investigated the geometry method to calculate the cloud shadows on complex terrains. Therefore, this paper reports on a method that identifies cloud shadows on complex topography based on the principle of geometrical optics.When a straight line crosses a plane, it intersects with the plane at a point. However, when a straight line crosses a curved surface, it can intersect with the surface at many points. We can abstract the light through the cloud pixels as a straight line, whereas the complex terrain can be treated as a curved surface. When the light arrives on the surface by a cloud, it inevitably intersects with the surface at a certain point. This point is the shadow position casted by the cloud. We combined the previous formula to calculate the cloud shadows on the horizontal surface with DEM, which is based on the above principle of the proposed algorithm.A series of simulations and experiments determined that the proposed algorithm could properly describe the cloud shadows on the complex surface. The actual calculated and visual interpretation results had high consistency in the shadow position, and these had better fitting with the coverage by comprehensive interpretation. Theγ2 value was 0.78, whereas the RMSE was 3.49, which were much better than the results that did not consider the terrain effect. Our algorithm had a clearer advantage in terms of comprehensively identifying cloud shadows compared with the results of the threshold method based on images.An algorithm was proposed in this study based on geometry method to calculate the cloud shadows on complex terrain and overcome the existing defects of the geometry method in identifying cloud shadows. This algorithm could obtain the shadow positions of corresponding cloud pixels on a complex terrain, and the distortion of cloud shadows on complex surfaces could be properly described. However, the accuracy of the calculated results could be influenced by many factors, such as the accuracy of cloud detection and thin clouds that cannot be easily detected. However, these clouds can also cast shadows on the ground. Cloud height is an important parameter in the calculation process that largely influences the result, but the acquisition of accurate cloud heights has a certain difficulty level in remote sensing. The DEM and visible band data also have a matching problem.

Schlüsselwort

cloud shadow;complex terrains;DEM;geometric effelt

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