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Published： 07 May 2023 ，
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佟一冬，焦子锑，尹思阳，张小宁，崔磊，谢蕊，郭静，李思杰，朱子栋.2023.数字半球摄影法获取植被表观聚集指数（ACI）采样尺度效应研究.遥感学报，27（5）： 1254-1268
Tong Y D，Jiao Z T，Yin S Y，Zhang X N，Cui L，Xie R，Guo J，Li S J and Zhu Z D. 2023. Sampling scale effects of vegetation apparent clumping index obtained by digital hemispherical photography. National Remote Sensing Bulletin， 27（5）：1254-1268
植被聚集指数CI（Clumping Index）表征植被冠层叶片的聚集程度，是影响冠层内辐射传输的重要植被结构参数，对更好地理解冠层的辐射传输具有重要意义。数字半球摄影法DHP（Digital Hemispherical Photography）与有限长度平均方法的结合被广泛应用于地面CI估算，研究表明，有限长度平均法应用于DHP图像时所划分的采样单元大小与CI测量结果密切相关，即CI随图像采样尺度的变化会表现出有意义的变化，植被表观聚集指数ACI（Apparent Clumping Index ）故应运而生，ACI的提出有助于加深对植被聚集效应的理解，拓展植被CI的地面间接测量，同时也对CI星载遥感产品的验证提出了新的挑战。目前，应用有限长度平均法估算ACI时，DHP图像中划分的采样单元的大小对ACI估算的影响规律，及其随不同植被类型的变化规律有待进一步探索。针对上述问题，本研究基于DHP图像，分别以一系列逐级增大的角度采样尺度对观测天顶角和方位角进行划分，共得到30种采样方法、17级角度采样分辨率，并将其分别应用于林、灌、草和农作物四种地类，探究了ACI在不同植被类型下随17级角度采样分辨率变化而变化的规律；同时，探讨了ACI的角度采样尺度效应与植被在空间中的分布形态，以及农作物生长阶段之间的密切联系。结果表明，在四种地类下，随17级角度采样分辨率的降低ACI从约0.5变化至0.8，平均增幅分别达到26%、29%、14%和35%；在大豆作物的不同生长阶段，随17级角度采样分辨率由高到低的变化，ACI的增长存在一定差异，增幅最大可达60%。本研究针对CI地面测量的不一致性问题展开研究，揭示了ACI角度采样尺度效应的方向依赖性规律，总结了不同植被类型下角度采样尺度对ACI估算结果影响的统一性和差异性，有望为CI遥感产品验证的不确定性分析提供地面测量数据变化的精细分析支撑。
The Clumping Index (CI) represents the degree of leaf aggregation in a canopy. It is an important vegetation structural parameter that affects radiative transport and is crucial to understanding radiative transport in canopies. The apparent clumping index (ACI) was proposed to deepen the understanding of the clumping effect and expand the field indirect measurement of CI. Studies have shown that ACI is closely related to the angular sampling scale. Even ACI measured by the same instrument changes considerably with the sampling scale
which presents a new challenge to the verification of CI products. The combination of digital hemispherical photography (DHP) and logarithmic average methods is widely used in field ACI estimation
and one of the key steps of this combination is the determination of the angular sampling resolution. At present
the influence of angular sampling resolution on ACI estimation and its variation with different vegetation types need to be further explored.
When using the logarithmic average method based on DHP measurement data
the angular sampling resolution is jointly determined by two factors: view zenith angle sampling scale (∆
θ
) and view azimuth angle sampling scale (∆
ϕ
). On the basis of DHP images
this study divided view zenith and view azimuth angles by using a series of progressively increasing angular sampling scales. A total of 30 sampling methods and 17 levels of angular sampling resolutions were obtained and applied to forest
shrub
grass
and crops. The variation in ACI under different vegetation types was investigated with the decrease in the resolution of the 17 levels of sampling. At the same time
the relationship among the angular sampling scale effect of ACI
the spatial distribution of vegetation
and the growth stage of crops was discussed.
Results showed that ACI varied from 0.5 to 0.8 in the four vegetation types with the decrease in the 17-level angular sampling resolution and exhibited an average increase of 26%
29%
14%
and 35%. At the different growth stages of soybean crops
ACI increased (up to 60%) with the decrease in the 17-level angular sampling resolution. In general
ACI was directionally dependent on the change in the angular sampling scale
and this characteristic was affected by the spatial distribution of the observed objects. When the observed objects were randomly distributed (forest
shrub
and grass)
ACI generally showed a linear growth trend with the increase in VZA and VAA sampling scales; when the observed object had a regular distribution (soybean crop)
ACI was still affected by VZA and VAA sampling scales
and the influence of the VZA sampling scale on ACI was highly significant.
The results of this study showed that ACI increased with the increase in the angular sampling scale
that is
with the increase in the angular sampling scale
vegetation aggregation weakened and gradually exhibited a random distribution. This study further explored the directional dependence of ACI on the change in the angular sampling scale for different vegetation types. How to select the optimal angular sampling scale for different vegetation types is expected be the focus of future research. A detailed analysis of ACI field measurement and an in-depth discussion of the possible uncertainties in the measurement process are prerequisites for performing an authenticity inspection of CI products. They are also essential for improving the accuracy of CI estimation and CI product verification.
植被聚集指数（CI）植被表观聚集指数（ACI）数字半球摄影法（DHP）角度采样尺度尺度效应方向性
Clumping Index (CI)Apparent Clumping Index (ACI)Digital Hemispherical Photography (DHP)angular sampling scalescale-effectdirectionality
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