# An Effective Water Body Extraction Method with New Water Index for Sentinel-2 Imagery
## 4 Results
### Accuracy Assessment  
For the water extraction results of the four study areas in Figure 6, visual interpretation and a quantitative accuracy index are used to conduct accuracy assessments. The performance of water body extraction with different water indices in Chaka Salt Lake and Chain Lake show great differences; therefore, visual evaluation suggests that the extraction accuracy of the SWI is better than that of the NDWI in these two study areas. For Chaka Salt Lake, the SWI can not only extract salt water more completely than the NDWI but also the sensitivity of the SWI with respect to water turbidity renders it capable of distinguishing a small amount of water from bare soil; thus, the SWI has a better extraction performance than the NDWI in this regard. For Chain Lake, the smoothness and continuity of the SWI for extracting the lake boundary is improved compared to the NDWI extraction; thus, the water body extraction accuracy of the SWI is higher than that of the NDWI.  
The study areas of Taihu Lake and the Yangtze River cover various land cover types and water body types. The quantitative accuracy index method is employed to assess the water body extraction accuracy and the results are shown in Table 4. For the two study areas, the producer's accuracy, user's accuracy, overall accuracy, and Kappa coefficient of the SWI are superior to those of the NDWI, which suggests that the SWI can achieve better extraction performance than that of the NDWI. For Taihu Lake, since both indices can accurately extract most of the water, the differences in terms of the quantitative accuracy index between the NDWI and the SWI are small. For the Yangtze River, the accuracy of the NDWI is obviously lower than that of the SWI. This is because the NDWI classifies water bodies containing sediment as bare soil, buildings, or other nonwater land cover types, while the SWI can accurately extract these water body types. These results indicate that the extraction accuracy of the SWI is higher than that of the NDWI in the Yangtze River study area.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Study Area**} & \multicolumn{3}{c}{**Producer’s Accuracy**} & \multicolumn{2}{c}{**User’s Accuracy**} & \multicolumn{2}{c}{**Overall**} & \multirow{2}{*}{**Kappa**} \\  & & **Water** & **Nonwater** & **Water** & **Nonwater** & **Accuracy** & **Coefficient** \\ \hline \multirow{3}{*}{**Taihu Lake**} & NDWI & 95.80\% & 89.68\% & 79.72\% & 98.05\% & 91.50\% & 0.808 \\  & SWI & 99.11\% & 90.28\% & 79.86\% & 99.62\% & 92.75\% & 0.833 \\ \cline{1-1}  & NDWI & 96.43\% & 81.63\% & 72.65\% & 97.83\% & 86.6\% & 0.722 \\ \cline{1-1}  & SWI & 98.11\% & 89.63\% & 86.19\% & 98.63\% & 93\% & 0.868 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Accuracy verification in Taihu lake and Yangtze river.  
Figure 6: Water extraction performances and differences between the NDWI and SWI. ((**a,d,g,j**) represent the water bodies extracted by the NDWI in the Taihu Lake, Yangtze River, Chaka Salt Lake, and Chain Lake images, respectively. (**b,e,h,k**) represent the water bodies extracted by the SWI in the Taihu Lake, Yangtze River, Chaka Salt Lake, and Chain Lake images, respectively. (**c,f,i,l**) show the differences between the NDWI and SWI in the Taihu Lake, Yangtze River, Chaka Salt Lake, and Chain Lake images, respectively).