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基于无人机植被指数与纹理-颜色特征融合的冬小麦冻害评价

Evaluating freeze damage in winter wheat using vegetation index and texture-color features of unmanned aerial vehicle

  • 摘要: 为提升冬小麦冻害评价的精度与效率,该研究提出一种基于无人机植被指数与纹理-颜色特征融合的冻害等级分类方法。针对传统田间表型鉴定法主观性强、实验室模拟与田间数据偏差显著、生理生化检测周期长等问题,通过融合植被指数、纹理特征及颜色特征,结合机器学习算法构建冻害评价模型。研究利用无人机采集多光谱与可见光影像数据,提取归一化植被指数(normalized difference vegetation index,NDVI)、比值植被指数(ratio vegetation index, RVI)、超蓝指数(excess blue index,ExB)等16个植被指数,基于灰度共生矩阵(gray-level co-occurrence matrix,GLCM)解析方差、均值等纹理特征,并计算颜色特征;采用皮尔逊相关系数法(pearson correlation coefficient,PCC)和最小冗余特征选择算法(minimum redundancy feature selection algorithm,mRMR)筛选特征,最终利用随机森林(random forest,RF)、支持向量机(support vector machine,SVM)、反向传播神经网络(back-propagation neural network,BPNN)、分布式梯度增强(extreme gradient boosting,XGBoost)等算法构建分类模型。结果表明:多源特征融合策略显著提升模型性能,分布式梯度增强XGBoost在植被指数、纹理及颜色特征融合条件下综合表现最优,准确率达0.73,召回率、精确率和F1分数分别为0.68、0.71和0.69;纹理特征与人工调查冻害等级相关性最高,证实其对冠层结构损伤的敏感捕捉能力。该研究验证了多源遥感数据融合在冻害评价中的有效性,为冬小麦冻害精准监测与防灾决策提供了可靠的技术手段,助力农业智能化发展。

     

    Abstract: High accuracy and efficiency can often be required to assess the freeze damage of the winter wheat. However, the traditional field phenotypic identification can be confined to strong subjectivity, serious deviations between laboratory simulations and field data, and the lengthy cycle of physiological and biochemical detection. In this study, a freeze damage classification was proposed to fuse the vegetation indices, texture, and color features. A freeze damage model was also constructed to combine with the machine learning. The unmanned aerial vehicles (UAVs) were utilized to collect the multispectral and visible light imagery. Specifically, the UAVs were equipped with multispectral sensors and high-resolution visible light cameras. The optimal weather (clear skies, stable and light) was employed to ensure the data quality. A total of 16 vegetation indices were extracted, including the Normalized Difference Vegetation Index (NDVI), Ratio Vegetation Index (RVI), Excess Blue Index (ExB), and Soil Adjusted Vegetation Index (SAVI). The spectral reflectance was also selected from the specific wavelength bands in order to effectively represent the vegetation’s photosynthetic capacity and canopy health status. Furthermore, the Gray-Level Co-Occurrence Matrix (GLCM) was employed in texture feature analysis. Some parameters were calculated to quantify the spatial distribution of the pixel gray levels in the imagery, such as the mean, variance, contrast, and homogeneity from the GLCM. These texture features were then captured for the subtle structural variations in the wheat canopy caused by freeze damage. Additionally, the color features were derived from the RGB color space of the visible light images. Statistical parameters were computed to represent the visual color of the winter wheat under freeze damage, such as the color mean, standard deviation, and color moment. Feature screening was conducted using the Pearson correlation coefficient (PCC) method and the minimum redundancy feature selection algorithm (mRMR). Firstly, the Pearson correlation analysis was used to measure the linear correlation between each feature and freeze damage levels, thus eliminating the features with a low correlation. Subsequently, the minimum redundancy feature selection was optimized the feature set, in order to balance the feature relevance and redundancy. The final features were selected for the informative and non-repetitive ones. Four classification models were then constructed: Random Forest (RF), Support Vector Machine (SVM), Backpropagation Neural Network (BPNN), and eXtreme Gradient Boosting (XGBoost). Each model was also trained with tunable parameters. For example, the XGBoost model was utilized to adjust the parameters, such as the learning rate, the number of estimators, and the maximum depth after cross-validation. The experimental results demonstrated that the multi-source feature fusion significantly improved the performance of the model. Among them, eXtreme Gradient Boosting (XGBoost) showed the optimal performance under the fusion of vegetation indices, texture, and color features. The accuracy, recall, precision, and F1-score reached 0.725, 0.683, 0.705, and 0.688, respectively. Further analysis revealed that the texture features shared the highest correlation with the manually surveyed levels of the freeze damage. The structural damages of the canopy were also sensitive to capture, as the freeze damage often disrupted the orderly arrangement of the wheat leaves and stems. The texture features were effectively detected after capture. The multi-source remote sensing data fusion was validated to assess the freeze damage. The spectral, texture, and color information were integrated to precisely monitor the freeze damage of the winter wheat for disaster prevention decision-making. Remote sensing and machine learning were also applied to assess agricultural disasters in smart agriculture. The finding can serve as a practical reference for crop stress monitoring in the disaster early warning of intelligent agriculture.

     

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