黑臭水体水面阴影提取的自适应阈值算法研究

doi:10.12082/dqxxkx.2020.190489

Abstract

Abstract:

The shadow on black and odor water interfere with the spectral information of the water surface and seriously affects the accuracy of water quality monitoring with high spatial resolution remote sensing data. Therefore, it is necessary to remove the shadow before evaluating river water quality. This paper tries to constructan objective and efficient shadow recognition algorithm on black and odor water to reduce the interference of adjacent object and improve the accuracy of remote sensing monitoring and evaluation of river water quality. In this study, the shadow and water pixels were sampled based on the hyperspectral remote sensing data of Unmanned Aerial Vehicle (UAV).The spatial distribution of different band combinations was analyzed by means of spectral feature spatial analysis to obtain spectral band combinations that can effectively distinguish water and water surface shadows, and the coefficients of band combinations were calibrated to obtain the best discrimination effect. By comparing the discernibility of shadow and non-shadow water by various band combinations, it was found that the ration of remote sensing reflectance Rrs(666)/Rrs(791) combining with Rrs(492) has a higher discrimination between water pixels and shadow pixels. Therefore, remote sensing reflectance at 492 nm, 666 nm and 792 nm were selected to establish the River Surface Shadow Index (RSSI). In general, the threshold of distinguishing shadow and non-shadow pixels needs to be adjusted according to different images. In this case, manually adjusting the threshold may produce errors, which are difficult to apply to other images. In order to reduce the error caused by artificial threshold calibration, the maximum category variance method (OTSU)was adopted to automatically determine the threshold of shadow recognition. According to the complexity of the riverbank object, the reflectance spectra of the shadows were classified to two types: umbra and penumbra. The magnitude difference between penumbra and umbra reflectance was similar to that between penumbra and water reflectance. Therefore, in order to highlight the difference between penumbra and water, the number of classification recognition types was set as 3. Firstly, the OTSU method was used to automatically determine the recognition threshold of umbra, penumbra and water, and then the extracted umbra and penumbra were combined to produce the final shadow distribution map. The algorithm was tested by using the hyperspectral remote sensing images of Jinchuan River and Longjiang River in Nanjing. The results show that the RSSI shadow index can highlight the difference between shadow and water. The threshold determined by OTSU adaptively can better distinguish umbra, penumbra and water, and the overall recognition accuracy of shadow can reach more than 85%. This algorithm can effectively identify the water surface shadow on black and odor water and provide the technical support of data preprocessing for the subsequent qualitative and quantitative remote sensing monitoring for water.

Key words: shadow on water surface, hyperspectral image of UAV, shadow index, OTSU, umbra, penumbra, black and odor water

XU Jiafeng, LI Yunmei, XU Jie, LEI Shaohua, BI Shun, ZHOU Ling. Adaptive Threshold for Surface Shadow Detection of Black and Odor Water[J].Journal of Geo-information Science, 2020, 22(10): 1959-1970.DOI:10.12082/dqxxkx.2020.190489

Figures/Tables 17

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Tab. 2

Accuracy evaluation of each test scenario"

区域	生产者精度		用户精度		总体精度	Kappa系数
区域	$η s / %$	$η n / %$	$p s / %$	$p n / %$	$τ / %$	Kappa系数
测试场景1	85.27	82.89	93.25	67.00	86.89	0.83
测试场景2	89.57	82.26	89.05	82.98	87.30	0.88
测试场景3	85.70	95.22	96.74	91.78	90.38	0.95
测试场景4	90.04	96.49	84.56	91.15	88.61	0.92

Tab. 2

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References 27

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序号	仪器设备名称	主要技术指标
1	大疆M600 pro多旋翼无人机	旋翼数量：6个轴距：1133 mm 起飞重量：15.5 kg 最大载荷：6 kg 最大水平飞行速度：65 km/h（无风环境）最大续航时间：60 min（空载）18 min （5.5 kg载重）实用升限：4500 m
2	ZK-VNIR-FPG480机载高光谱成像仪	光谱范围：400~1000 nm 光谱分辨率：2.8 nm 光谱通道数：270个空间通道数：480个空间分辨率：0.08 m（120 m高度）（35 mm镜头）视场角：26° A/D转换：12 bits 最大帧频：100 fps 数据接口：GigE 最大功耗：20 W 外形尺寸：310×87×87 mm 重量：2.2 kg 成像方式：采用外置推扫连续成像,采集画幅无限制,扫描路线一次成图,影像无畸变

测试场景	阴影提取前				阴影提取后
测试场景	黑臭水体像元数/个	正常水体像元数/个	黑臭像元占比/%	正常水体像元占比/%	黑臭水体像元数/个	正常水体像元数/个	黑臭像元占比/%	正常水体像元占比/%
测试场景1	16 859	5622	74.99	25.01	7235	140	98.10	1.90
测试场景2	27 202	7450	78.50	21.50	13 040	112	99.15	0.85
测试场景3	13 612	12 885	51.37	48.63	10 445	533	95.14	4.86
测试场景4	17 827	6106	74.49	25.51	12 130	928	92.89	7.11

Adaptive Threshold for Surface Shadow Detection of Black and Odor Water

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