Refined remote sensing classification of Yancheng coastal wetland considering tide-level changes and vegetation phenological characteristics on the GEE platform
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摘要: 滨海湿地具有重要的经济价值与生态价值，快速准确地监测其现状对滨海湿地资源的保护和管理具有重要意义。由于潮汐动态变化、植被光谱相似性以及云覆盖等因素的影响，滨海湿地的遥感监测具有较大挑战。本文提出了一个综合考虑潮位变化及植被物候特征的滨海湿地遥感分类方法，基于GEE（Google Earth Engine）平台，首先引入Fmask（Function of mask）算法进行云检测与去云处理，然后利用S-G（Savitzky-Golay）滤波算法重构NDVI（Normalized Difference Vegetation Index）时间序列数据，提取植被物候特征参数，采用随机森林算法实现互花米草（Spartina alterniflora）、芦苇（Phragmites australis）、碱蓬（Suaeda salsa）与茅草（Imperata cylindrica）4种湿地植被类型的提取；最后利用最大光谱指数合成算法 (Maximum Spectral Index Composite, MSIC) 生成最高与最低潮位合成影像，结合大津算法（Otsu）提取光滩与海水，实现滨海湿地的精细化遥感分类。研究结果表明，生长季开始时间、生长季结束时间、生长季时长、基准值、振幅、小季节积分是区分滨海湿地植被的重要植被物候特征参数。利用本方法对盐城滨海湿地进行分类，湿地总体分类精度达96.50%，Kappa系数为0.957 1，湿地植被中互花米草的使用者精度最高，为96.59%；其次是芦苇与碱蓬；茅草最低，为93.55%。与面向对象分类相比，本方法不仅能够提取完整的光滩范围，而且将总体精度提高了10.25%，体现出植被物候特征在滨海湿地动态变化遥感监测中的应用潜力。Abstract: Coastal wetlands have important economic and ecological value. Rapid and accurate monitoring of the status of coastal wetlands is of great significance for the protection and management of coastal wetland resources. Due to factors such as the variability of the tide-level changes, similarity of vegetation spectra, and frequent cloud cover, remote sensing monitoring of coastal wetlands faced certain challenges. In this paper, we proposed a multi-technology coupled remote sensing classification method of coastal wetlands that considers tide-level changes and vegetation phenological characteristics. Based on the Google Earth Engine (GEE) platform, the Fmask (Function of mask) algorithm was first performed for cloud testing and cloud removal processing. Then, the S-G (Savitzky-Golay) filtering algorithm was used to reconstruct NDVI time series data and extract vegetation phenological characteristic parameters. In this phase, the random forest algorithm was applied for the classification of four vegetation types namely Phragmites australi, Suaeda salsa, Spartina alterniflora, and Imperata cylindrical. Finally, the Maximum Spectral Index Composite (MSIC) algorithm was used to generate composite images of the highest and lowest tide levels. The tidal flats and seawater were precisely extracted using the Otsu algorithm based on these two composite images. Combining these feature types, the refined remote sensing classification of coastal wetlands was ideally obtained. The results showed that start-of-season time, end-of-season time, length of season, base value, amplitude, and small seasonal integral were the six key vegetation phenological characteristic parameters for distinguishing different types of coastal wetland vegetation. Applying this method to classify coastal wetlands on the Yancheng coast, the overall classification accuracy was 96.50%, and the Kappa coefficient reached 0.957 1. Among the wetland vegetation, the highest user accuracy was 96.59% for Spartina alterniflora, followed by P. australi and Suaeda salsa, and the lowest was 93.55% for Imperata cylindrical. Compared with object-oriented methods, our method can extract the complete range of tidal flats, and the overall accuracy is improved by 10.25%, reflecting the potential application of vegetation phenological characteristics in remote sensing monitoring of dynamic changes in coastal wetlands.
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图 3 不同植被类型NDVI时间序列拟合曲线
Fig. 3 Time series fitting curves of different vegetation types
图 4 盐城滨海湿地植被物候特征参数对比
Fig. 4 Comparison of vegetation phenological characteristics parameters of Yancheng coastal wetland vegetation
图 5 盐城滨海湿地分类结果
a. 盐城滨海湿地分类结果图；b. 植被混生带；c. 植被与内陆水体相接地带；d. 植被与光滩相接地带
Fig. 5 Classification results of Yancheng coastal wetland
a. Classification results map of Yancheng coastal wetland; b. the mixed zone of vegetation; c. the adjacent zone of vegetation and inland water; d. the adjacent zone of vegetation and tidal flats
图 6 面向对象方法分类结果
a. 盐城滨海湿地分类结果图；b. 植被混生带；c. 植被与内陆水体相接地带；d. 植被与光滩相接地带
Fig. 6 Classification results of the object-oriented methods
a. Classification results map of Yancheng coastal wetland; b. the mixed zone of vegetation; c. the adjacent zone of vegetation and inland water; d. the adjacent zone of vegetation and tidal flats
图 7 不同时间序列数据重构方法分类精度对比
Fig. 7 Comparison of classification accuracy of different time series data reconstruction methods
图 8 判别碱蓬的3个重要物候特征参数对比
Fig. 8 Comparison of three important phenological characteristics parameters of the discriminate Suaeda salsa
图 9 盐城滨海湿地植被分类结果图
Fig. 9 Classification results map of coastal wetland vegetation in Yancheng
表 1 Fmask算法的公式与阈值
Tab. 1 Formula and threshold of Fmask algorithm
名称 公式与阈值 公式编号 基本检测 ${ \begin{gathered} {\rho _{{\mathrm{SWIR2}}}} > 0.03, {\mathrm{NDSI}} < 0.8, {\mathrm{NDVI}} < 0.8, \\ \left\{ \begin{gathered} {\mathrm{NDSI}} = ({\rho _{{\mathrm{Green}}}} - {\rho _{{\mathrm{SWIR1}}}})/({\rho _{{\mathrm{Green}}}} + {\rho _{{\mathrm{SWIR1}}}}) \\ {\mathrm{NDVI}} = ({\rho _{{\mathrm{NIR}}}} - {\rho _{{\mathrm{Red}}}})/({\rho _{{\mathrm{NIR}}}} + {\rho _{{\mathrm{Red}}}}) \\ \end{gathered} \right. \\ \end{gathered}} $ （1） 白度检测（Whiteness） ${ \begin{gathered} {\mathrm{Whiteness}} = \left| {\left( {{\rho _{{\mathrm{Red}}}} - {\mathrm{MeanVis}}} \right)/{\mathrm{MeanVis}}} \right|+\left| {\left( {{\rho _{{\mathrm{Green}}}} - {\mathrm{MeanVis}}} \right)/{\mathrm{MeanVis}}} \right| + \\ \left| {\left( {{\rho _{{\mathrm{Blue}}}} - {\mathrm{MeanVis}}} \right)/{\mathrm{MeanVis}}} \right| < 0.7,\; {\mathrm{MeanVis}} = \left( {{\rho _{{\mathrm{Red}}}} + {\rho _{{\mathrm{Green}}}} + {\rho _{{\mathrm{Blue}}}}} \right)/3 \end{gathered} }$ （2） 霾优化转换检测（HOT） $ {{\mathrm{HOT}} = {\rho _{{\mathrm{Blue}}}} - 0.5 \times {\rho _{{\mathrm{Red}}}} - 0.08 > 0 }$ （3） 比值检测 $ {{\rho _{{\mathrm{NIR}}}}/{\rho _{{\mathrm{SWIR1}}}} > 0.75 }$ （4） 卷云检测（Cir） ${ {\mathrm{Cir}} = {\rho _{{\mathrm{Cir}}}}/0.04 > 0.01 }$ （5） 水检测（Water） ${ \begin{gathered} {\mathrm{Water}} = ({\mathrm{NDVI}} < 0.01, {\rho _{{\mathrm{NIR}}}} < 0.11){\text{或}}({\mathrm{NDVI}} < 0.1, {\rho _{{\mathrm{NIR}}}} < 0.05) \\ {\text{或}}({\mathrm{GSWO}} > {O_{{\mathrm{water}}}}, {\mathrm{snow}}/{\mathrm{ice}} = {\mathrm{false}}) \\ \end{gathered} }$ （6） 陆地云概率（lCloudp） ${ \begin{gathered} {\mathrm{lCloudp}} = {\mathrm{lVar}} \times {\mathrm{lHOT}} + 0.5 \times {\mathrm{Cir}} > {\mathrm{lCloudt}}, \\ \left\{ \begin{gathered} {\mathrm{lHOT}} = \frac{{{\mathrm{HOT}} - \left( {{\mathrm{HOT_{low}}} - 0.04} \right)}}{{\left( {{\mathrm{HOT_{high}}} + 0.04} \right) - \left( {{\mathrm{HOT_{low}}}-0.04} \right)}} \\ {\mathrm{lVar}} = 1 - \max \left( {\left| {{\mathrm{NDVI}}} \right|,\left| {{\mathrm{NDSI}}} \right|,\left| {{\mathrm{Whiteness}}} \right|} \right) \\ \end{gathered} \right. \\ \end{gathered}} $ （7） 水体云概率（wCloudp） ${ {\mathrm{wCloudp}} = {\mathrm{wBright}} + 0.5 \times {\mathrm{Cir}} > {\mathrm{wCloudt}},\ {\mathrm{wBright}} = \min ({\rho _{{\mathrm{SWIR1}}}},0.11)/0.11} $ （8） 云阴影检测 ${ \left\{ \begin{gathered} {\rho _{{\mathrm{NIR}}}} < 0.25 \\ {\rho _{{\mathrm{SWIR1}}}} < 0.11 \\ \end{gathered} \right. }$ （9） 注：ρBlue、ρGreen、ρRed、ρNIR、ρSWIR1、ρSWIR2与ρcir分别为蓝、绿、红、近红外、短波与卷云波段的反射率。MeanVis为平均反射率。GSWO（Global Surface Water Occurrence）为全球地表水发生率数据，为每个像元提供水发生率，其中0%表示永久的陆地，100%表示永久的水体；snow/ice为雪、冰；Owater为水发生率阈值，取光谱识别的水体像元集的低值（17.5百分位值）再减去5%的GSWO数据误差。lHOT为陆地HOT云概率，HOTlow等于陆地绝对无云像元的HOT低值（17.5百分位值），HOThigh等于陆地绝对无云像元的HOT高值（82.5百分位值）；lCloudt为陆地云概率阈值，等于陆地绝对无云像元的lCloudp高值（82.5百分位值）再加上常数0.2。lVar为陆地光谱差异概率。wCloudt为水体云概率阈值，等于水体绝对无云像元的wCloudp高值（82.5百分位值）再加上常数0.2。wBright为水体亮度概率。 
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表 2 不同植被类型的植被物候特征判别参数与阈值
Tab. 2 Discriminative parameters and thresholds for vegetation phenological characteristics of different vegetation types
植被类型 植被物候特征判别参数与阈值 芦苇 120 < SOS < 150 碱蓬 SI < 50, BV > 0.22, AV < 0.4 互花米草 340 < EOS < 380 茅草 LOS < 130, AV > 0.55
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表 3 盐城滨海湿地分类精度评价结果
Tab. 3 Accuracy evaluation of coastal wetland classification
芦苇 碱蓬 互花米草 茅草 内陆水体 光滩 海水 使用者精度/% 芦苇 113 1 2 1 0 0 0 96.58 碱蓬 1 52 1 1 0 0 0 94.55 互花米草 1 2 85 0 0 0 0 96.59 茅草 0 0 2 29 0 0 0 93.55 内陆水体 0 0 1 0 36 0 0 97.30 光滩 0 0 0 0 0 35 0 100.00 海水 0 0 0 0 0 1 36 97.30 生产者精度/% 98.26 94.55 93.41 93.55 100.00 97.22 100.00 总体精度/% 96.50 Kappa系数 0.957 1
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表 4 不同分类方法分类精度对比
Tab. 4 Comparison of classification accuracy of different techniques
指标 面向对象方法 与本文方法相比 精度/% 芦苇 82.76 −13.82 碱蓬 88.89 −5.66 互花米草 82.02 −14.57 茅草 93.33 −0.22 内陆水体 78.57 −18.73 光滩 96.88 −3.12 海水 97.30 0.00 总体精度 86.25 −10.25 Kappa系数 0.831 6 −0.125 5
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