Estimation and analysis of the green-tide drift velocity using ship-borne UAV
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摘要: 无人机遥感具有应用灵活、不受云层干扰以及时空分辨率高的显著优势。为探索无人机在海洋灾害监测中的应用,本文以科考船为起降平台,首次基于无人机获取的双时相绿潮正射影像,开展了黄海绿潮漂移速度的估算研究。同时对比了卫星影像提取的速度结果,并探讨了风与潮流对海上绿潮漂移的驱动。研究发现:(1)可见光波段的漂浮藻类指数能高精度地提取无人机可见光影像中的绿潮(kappa 系数=0.95);(2)无人机遥感估算3个站位的绿潮漂移速率为0.26~0.44 m/s,漂移方向在1天之内变化明显;(3)绿潮短时间内的漂移受到风与潮流的共同影响,漂移方向与M2分潮的潮流方向基本一致,位于风向右侧1°~62°。基于船载的无人机航测,能高精度地估算绿潮漂移速度,为精细化的绿潮灾害预警与防控提供技术支撑。Abstract: Unmanned aerial vehicle (UAV) remote sensing has distinct advantages of flexible use, no cloud interference, and high spatial-temporal resolution. Aim to explore UAV’s utilization potential in marine disaster monitoring, research ship was used as the UAV landing pad, and for the first time, based on the bi-temporal orthophotos acquired by the ship-borne UAV, the drift velocity of green-tide in the Yellow Sea was estimated. In addition, the velocity result extracted from satellite images was compared, and the influences of wind and tidal currents on green-tide drift were analyzed. The results show that: (1) the red-green-blue floating algae index (RGB-FAI) can extract green-tide patches from UAV-based RGB orthophotos with a high-accuracy (kappa coefficient=0.95); (2) the green-tidal speed of three sites estimated by UAV remote sensing are 0.26−0.44 m/s, and the drift direction changed significantly throughout the day; (3) the short-term drift of green-tide is forced by the wind and tidal current. The drift direction of the green-tide is basically consistent with the tidal current of M2, at 1°−62° to the right of wind direction. The ability to estimate green-tidal velocity accurately from the ship-borne UAV images is expected to provide technical support for the precise prediction, warning and control of green-tide disaster.
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Key words:
- UAV remote sensing /
- Yellow Sea /
- green tides /
- drift velocity /
- RGB-FAI
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图 7 绿潮、风、潮流的移动矢量图
a、c、e分别为S1、S2、S3站点风与绿潮漂移的矢量图,红色箭头为风矢量,绿色箭头为绿潮漂移矢量;b、d分别为南黄海6月表层低潮时、落潮中间时的流场分布(改绘自文献[29]),红点标识了3个站点的位置
Fig. 7 Motion vector of green-tide, wind and tidal current
a, c, e are vector diagrams of wind and green-tide drift at sites S1, S2 and S3, respectively, with red arrow as the wind vector and green arrow as the green-tide drifting vector; b, d are the tidal current field of the low tide and the middle of ebb in surface of the southern Yellow Sea in June (modified from reference [29]), where the red dots indicate the positions of three sites
表 1 绿潮提取误差矩阵表
Tab. 1 Error matrix of green-tide extraction
海水 绿藻 行总和 用户精度/% 海水 692 17 709 97.60 绿藻 4 286 290 98.62 列总和 696 303 999 生产者精度/% 99.43 94.39 表 2 绿潮斑块漂移速度与风速的统计
Tab. 2 Statistics of green-tide drifting velocity and the corresponding wind velocity
站点 站点坐标 航拍日期 航拍时段 漂移速率/(m·s−1) 漂移方向*/(°) 有效风时段 风速均值/(m·s−1) 风向均值*/(°) S1 36°N, 121°E 2019年6月19日 13:04−13:12 0.257 17.1 13:03−13:13 3.17 139.7 S2 36°N, 121.5°E 2019年6月19日 09:36−10:06 0.439 69.0 09:29−10:01 3.23 186.9 S3 35°N, 121°E 2019年6月16日 07:55−08:03 0.256 136.5 07:48−07:58 1.75 315.7 注:*方向以正北向为0°,顺时针为正。 表 3 基于UAV和GOCI影像的绿潮漂移速度提取结果的对比
Tab. 3 Comparison of green-tide drifting velocity derived from UAV and GOCI images
无人机遥感 GOCI卫星遥感 偏差* 影像分辨率/m 0.14 500 漂移方向(北偏东)/(°) 69.0 58.4 2.9% 漂移速率/(m·s−1) 0.439 0.547 0.108 注:*方向偏差=∣测定值−基准值∣/360°×100%;速率偏差=∣测定值−基准值∣。 -
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