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基于地块尺度的南方山区耕地非粮化遥感制图方法

Remote sensing mapping for non-grain cultivated land at plot scale in southern mountainous areas

  • 摘要: 对耕地非粮化进行科学有效地精准识别与测度对于耕地保护和农业土地资源的合理配置意义重大。该研究旨在利用深度学习技术的强大表达能力和海量遥感数据的高频地物观测优势,探索自动化地块边界提取与耕地非粮化类型的遥感制图方法。以福建省建阳区为典型案例,首先使用三调数据裁剪出耕地的影像图,随后采用边缘检测模型—richer convolutional features(RCF)深度学习网络提取耕地地块边缘,获取现势耕地范围;然后采取先提取葡萄大棚后识别其他非粮化类型的策略,基于GEE(Google Earth Engine)云计算平台获取Sentinel-2号卫星的月度影像集,以地块单元作为空间约束,计算地块的纹理特征、多时相光谱特征,同时采用Savitzky-Golay(S-G)滤波重建物候特征;最后,经过特征优选,采用随机森林算法进行作物分类,并叠加第三次全国国土调查数据获取地块尺度非粮化类型的空间分布。结果表明:1)共提取耕地地块约28.3万个,IoU值达到0.69,模型推理结果与实际地块形态较为吻合;2)S-G滤波能够有效捕捉作物所蕴含的季相节律特征及季节变化趋势,处理后的绿色归一化植被指数(green normalized difference vegetation index, GNDVI)时序曲线能够更真实地反映作物的生长状态;3)分类特征重要性分析显示,Sentinel-2影像的红边特征主要是红边位置指数和红边拐点指数、冬春季节的光谱特征及生长季峰值对分类贡献显著;4)作物分类的总体精度为81.6%,Kappa系数为0.73,提取的主要耕地非粮化类型主要为莲藕、葡萄大棚、柑橘园、茶园和耕地撂荒5种类型,分别占非粮化总面积的27.20%、24.28%、20.00%、11.56%、6.17%。该研究成果可为地块尺度的耕地非粮化遥感监测提供参考,也为南方山区作物识别提供借鉴。

     

    Abstract: An accurate identification and measurement of the cultivated land "non-grain" are crucial to ensure the national food security, farmland resources, and agricultural land use. However, the existing research on the non-grain cannot fully meet the requirement of the spatial distribution on the specific non-grain types. It is often required for the high-frequency monitoring and survey, particularly in the mountainous areas, such as the southern China. The cultivated land plots are also fragmented with the complex planting structure. There is an urgent need for monitoring at the plot-scale. In this study, an integrated framework was proposed to extract the automatic field boundary, and then classify the fine-scale non-grain cropland. The deep learning was also combined with the temporal richness of the satellite remote sensing data. The time series were applied to identify the non-grain types in Jianyang District, Nanping City, Fujian Province, China. Firstly, the cropland imagery was clipped using data from the Third National Land Resource Survey. The Richer Convolutional Features (RCF) deep learning model was applied to extract the field boundaries, and then delineate the current cropland areas. A sequential strategy was adopted to first identify the grape greenhouses, followed by the rest non-grain types. Monthly Sentinel-2 imagery was obtained via the Google Earth Engine (GEE). The plot units were utilized as the spatial constraints to extract the texture and multi-temporal spectral features. Phenological curves were reconstructed using the Savitzky-Golay (S-G) filter. Finally, the Random Forest algorithm was used for the crop classification after feature selection. The classified data was overlaid with the Third National Land Resource Survey, in order to map the spatial distribution of the non-grain cropland types at the plot scale. The results demonstrated that: 1) The improved RCF model was achieved in the high accuracy to extract the plot boundaries. The RCF model was effectively identified the plot boundaries, with a high intersection-over-union (IoU) value. The inference of the model was closely matched the actual plot shapes. 2) The Savitzky-Golay (S-G) filter was significantly improved to represent the crop phenology. The S-G filter was effectively captured the seasonal features and variation trends in crops. The time series curves of the vegetation index were more accurately reflected the growth status of the crops. 3) Feature importance analysis revealed that the specific spectral features were dominated the classification accuracy. A great contribution to the classification was from the red-edge features from the Sentinel-2 imagery, including the Red-Edge Position Index (S2REP) and Red-Edge Inflection Point Index (REIP), along with the spectral features in the winter-spring season and peak values during the growing season. 4) The classification model was achieved in the reliable performance. The overall accuracy of the crop classification was 81.6%, with a Kappa coefficient of 0.73. The non-grain cropland types were identified, including the lotus, grape greenhouse, citrus, tea garden and abandoned farmland. Among them, the lotus covered the largest proportion of the non-grain area. The abandoned farmland was also represented the smallest. A precise method was utilized to monitor the cultivated land non-grain using deep learning, remote sensing, and cloud computing. The finding can provide some insights into the land-use dynamics in the southern mountainous regions. A practical tool can serve as the sustainable farmland and the large-scale agricultural monitoring.

     

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