基于大疆精灵4无人机多光谱影像的西沙植被监测

西沙群岛位于热带，常年多云，在光学卫星数据获取时易受天气影响导致缺失，使得地表动态监测困难。为解决这一问题，探讨无人机低空平台对西沙群岛植被的监测能力，选取大疆精灵4多光谱无人机，通过5个多光谱波段提取4项植被指数，包括归一化差值植被指数（NDVI）、叶绿素指数（GCI）、绿色归一化植被指数（GNDVI）以及归一化绿红差值指数（NGRDI），评估了2020年5月西沙群岛北岛的植被生长状况，并结合关键气象参数以及Worldview2卫星光学影像对比分析了2020年5月和2018年5月北岛植被生长变化及其潜在归因。研究结果表明：2020年5月北岛平均NDVI、GCI、GNDVI和NGRDI别为0.30、0.84、0.26和0.05，反映出植被覆盖度较低，可能存在枯黄现象，与地面监测结果一致；2020年人工管理植被区和自然生长植被区各项指数差异由2018年的-23%—15%增加到15%—40%，表明2020年自然生长植被长势显著差于人工管理植被，反映出较强的环境胁迫；气象数据显示2020年4月—5月该地区日平均温度较常年同期升高、累计降水量减少、平均风速增大同时增加了土壤水分亏缺，可能是引起植被生长状况变差的主要原因。综上所述，大疆精灵4无人机可定量反映热带岛屿植被生长状况，可为其生态环境监测提供有效途径。

Xisha Vegetation Monitoring based on UAV Multispectral Images Obtained with the DJI Phantom 4 Platform

Xisha Islands locate in the tropical zone which frequently suffers from cloud cover. Optical systems are vulnerable to bad weather which results in data gaps or low data quality， resulting in difficulties in tropical surface monitoring. To solve this problem， a study on analyzing Xisha vegetation was conducted based on a low-altitude platform. Multi-spectral images were obtained via the DJI Phantom 4 UAV and four vegetation indices from five spectral bands were derived， including the Normalized Difference Vegetation Index （NDVI）， Grassland Chlorophyll Index （GCI）， Green Normalized Difference Vegetation Index （GNDVI）， and Normalized Green-red Difference Index （NGRDI） to analyze the vegetation growth in North island during May 2020. Combined with key meteorological parameters and Worldview2 optical images， the vegetation growth changes between 2020 and 2018 as well as their potential attribution were analyzed. Results showed that the average NDVI， GCI， GNDVI and NGRDI in North Island were 0.30， 0.84， 0.26 and 0.05 in May 2020， reflecting a low vegetation coverage and health status， which was consistent with the ground monitoring results. In 2020， the index difference between artificially managed and natural vegetated region increased from -23%—15% in 2018 to 15%—40%， indicating that the growth of natural vegetation is significantly worse than that of artificially managed vegetation in 2020 which demonstrates strong environmental stress. Meteorological data in this region showed that from April to May 2020， the average daily temperature and wind speed increased while the cumulative precipitation decreased compared with the same period of previous years， leading to increased soil water deficit. This may be the main reason for the deterioration of vegetation growth. These results demonstrated that DJI Phantom 4 images could effectively and quantitatively reflect the vegetation growth which will greatly support the ecological environmental monitoring over tropical islands.