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Open AccessArticle

Two-Step Urban Water Index (TSUWI): A New Technique for High-Resolution Mapping of Urban Surface Water

by Wei Wu 1,2, Qiangzi Li 1,*, Yuan Zhang 1,*, Xin Du 1 and Hongyan Wang 1
Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, No. 20 Datun Road, Chaoyang District, Beijing 100101, China
University of Chinese Academy of Sciences, Beijing 100049, China
Authors to whom correspondence should be addressed.
Remote Sens. 2018, 10(11), 1704;
Received: 19 September 2018 / Revised: 16 October 2018 / Accepted: 26 October 2018 / Published: 29 October 2018
Urban surface water mapping is essential for studying its role in urban ecosystems and local microclimates. However, fast and accurate extraction of urban water remains a great challenge due to the limitations of conventional water indexes and the presence of shadows. Therefore, we proposed a new urban water mapping technique named the Two-Step Urban Water Index (TSUWI), which combines an Urban Water Index (UWI) and an Urban Shadow Index (USI). These two subindexes were established based on spectral analysis and linear Support Vector Machine (SVM) training of pure pixels from eight training sites across China. The performance of the TSUWI was compared with that of the Normalized Difference Water Index (NDWI), High Resolution Water Index (HRWI) and SVM classifier at twelve test sites. The results showed that this method consistently achieved good performance with a mean Kappa Coefficient (KC) of 0.97 and a mean total error (TE) of 5.82%. Overall, classification accuracy of TSUWI was significantly higher than that of the NDWI, HRWI, and SVM (p-value < 0.01). At most test sites, TSUWI improved accuracy by decreasing the TEs by more than 45% compared to NDWI and HRWI, and by more than 15% compared to SVM. In addition, both UWI and USI were shown to have more stable optimal thresholds that are close to 0 and maintain better performance near their optimum thresholds. Therefore, TSUWI can be used as a simple yet robust method for urban water mapping with high accuracy. View Full-Text
Keywords: urban water mapping; water index; shadow detection; threshold stability urban water mapping; water index; shadow detection; threshold stability
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MDPI and ACS Style

Wu, W.; Li, Q.; Zhang, Y.; Du, X.; Wang, H. Two-Step Urban Water Index (TSUWI): A New Technique for High-Resolution Mapping of Urban Surface Water. Remote Sens. 2018, 10, 1704.

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