基于不同空间分辨率无人机多光谱遥感影像的小麦倒伏区域识别方法

Wheat Lodging Area Recognition Method Based on Different Resolution UAV Multispectral Remote Sensing Images

[目的/意义]快速准确评估作物倒伏灾情状况,需及时获取倒伏发生位置及面积等信息。目前基于无人机遥感识别作物倒伏缺乏相应的技术标准,不利于规范无人机数据获取流程和提出问题解决方案。本研究旨在探讨不同空间分辨率无人机遥感影像及特征优化方法对小麦倒伏区域识别精度的影响。[方法]在小麦倒伏后设置3个飞行高度(30、60和90 m),获取不同空间分辨率(1.05、2.09和3.26 cm)的数字正射影像图(Digital Orthophoto Map,DOM)和数字表面模型(Digital Surface Model,DSM),从不同空间分辨率影像中分别提取5个光谱特征、2个高度特征、5个植被指数以及40个纹理特征构建全特征集,并选择3种特征选择方法(ReliefF算法、RFRFE算法、Boruta-Shap算法)筛选构建特征子集,进而利用3种面向对象监督分类方法——支持向量机(Support Vector Machine,SVM)、随机森林(Random Forest,RF)和K最近邻(K Nearest Neighbor,KNN)构建小麦倒伏分类模型,明确适宜的分类策略,确立倒伏分类技术路径。[结果和讨论]结果表明,SVM的分类效果整体优于RF和KNN,当影像空间分辨率在1.05~3.26 cm范围内变化时,全特征集和3种优化特征子集均以1.05 cm分辨率的分类精度最高,优于2.09和3.26 cm。比较发现,Boruta-Shap特征优化方法既能实现降维和提高分类精度的目标,又能适应空间分辨率的变化,当影像分辨率为3.26 cm时,总体分类精度相较1.05和2.09 cm分别降低了1.81%和0.75%;当影像分辨率为2.09 cm时,总体分类精度相较1.05 cm降低了1.06%,表现为不同飞行高度下的分类精度相对差异较小,90 m总体分类精度可达到95.6%,Kappa系数达到0.914,满足了对分类精度的需求。[结论]通过选择适宜的特征选择方法,不仅可以兼顾分类精度,还能有效缩小影像空间分辨率变化引起的倒伏分类差异,有助于提升飞行高度,扩大小麦倒伏监测面积,降低作业成本,为确立作物倒伏信息获取策略及小麦灾情评估提供参考及支持。

[Objective] To quickly and accurately assess the situation of crop lodging disasters,it is necessary to promptly obtain information such as the location and area of the lodging occurrences.Currently,there are no corresponding technical standards for identifying crop lodging based on UAV remote sensing,which is not conducive to standardizing the process of obtaining UAV data and proposing solutions to problems.This study aims to explore the impact of different spatial resolution remote sensing images and feature optimization methods on the accuracy of identifying wheat lodging areas.[Methods] Digital orthophoto images(DOM) and digital surface models(DSM) were collected by UAVs with high-resolution sensors at different flight altitudes after wheat lodging.The spatial resolutions of these image data were 1.05,2.09,and 3.26 cm.A full feature set was constructed by extracting 5 spectral features,2 height features,5 vegetation indices,and 40 texture features from the pre-processed data.Then three feature selection methods,ReliefF algorithm,RF-RFE algorithm,and Boruta-Shap algorithm,were used to construct an optimized subset of features at different flight altitudes to select the best feature selection method.The ReliefF algorithm retains features with weights greater than 0.2 by setting a threshold of 0.2;the RF-RFE algorithm quantitatively evaluated the importance of each feature and introduces variables in descending order of importance to determine classification accuracy;the Boruta-Shap algorithm performed feature subset screening on the full feature set and labels a feature as green when its importance score was higher than that of the shaded feature,defining it as an important variable for model construction.Based on the above-mentioned feature subset,an object-oriented classification model on remote sensing images was conducted using eCognition9.0 software.Firstly,after several experiments,the feature parameters for multi-scale segmentation in the object-oriented classification were determined,namely a segmentation scale of 1,a shape factor of 0.1,and a tightness of 0.5.Three object-oriented supervised classification algorithms,support vector machine(SVM),random forest(RF),and K nearest neighbor(KNN),were selected to construct wheat lodging classification models.The Overall classification accuracy and Kappa coefficient were used to evaluate the accuracy of wheat lodging identification.By constructing a wheat lodging classification model,the appropriate classification strategy was clarified and a technical path for lodging classification was established.This technical path can be used for wheat lodging monitoring,providing a scientific basis for agricultural production and improving agricultural production efficiency.[Results and Discussions] The results showed that increasing the altitude of the UAV to 90 m significantly improved flight efficiency of wheat lodging areas.In comparison to flying at 30 m for the same monitoring range,data acquisition time was reduced to approximately 1/6th,and the number of photos needed decreased from 62 to 6.In terms of classification accuracy,the overall classification effect of SVM is better than that of RF and KNN.Additionally,when the image spatial resolution varied from 1.05 to 3.26 cm,the full feature set and all three optimized feature subsets had the highest classification accuracy at a resolution of 1.05 cm,which was better than at resolutions of 2.09 and 3.26 cm.As the image spatial resolution decreased,the overall classification effect gradually deteriorated and the positioning accuracy decreased,resulting in poor spatial consistency of the classification results.Further research has found that the Boruta-Shap feature selection method can reduce data dimensionality and improve computational speed while maintaining high classification accuracy.Among the three tested spatial resolution conditions(1.05,2.09,and 3.26 cm),the combination of SVM and Boruta-Shap algorithms demonstrated the highest overall classification accuracy.Specifically,the accuracy rates were 95.6%,94.6%,and 93.9% for the respective spatial resolutions.These results highlighted the superior performance of this combination in accurately classifying the data and adapt to changes in spatial resolution.When the image resolution was 3.26 cm,the overall classification accuracy decreased by 1.81% and 0.75% compared to 1.05 and 2.09 cm;when the image resolution was 2.09 cm,the overall classification accuracy decreased by 1.06% compared to 1.05 cm,showing a relatively small difference in classification accuracy under different flight altitudes.The overall classification accuracy at an altitude of 90 m reached 95.6%,with Kappa coefficient of 0.914,meeting the requirements for classification accuracy.[Conclusions] The study shows that the object-oriented SVM classifier and the Boruta-Shap feature optimization algorithm have strong application extension advantages in identifying lodging areas in remote sensing images at multiple flight altitudes.These methods can achieve high-precision crop lodging area identification and reduce the influence of image spatial resolution on model stability.This helps to increase flight altitude,expand the monitoring range,improve UAV operation efficiency,and reduce flight costs.In practical applications,it is possible to strike a balance between classification accuracy and efficiency based on specific requirements and the actual scenario,thus providing guidance and support for the development of strategies for acquiring crop lodging information and evaluating wheat disasters.