Abstract
Accurate waterbody mapping can support water-related environment monitoring and resource management. The Sentinel series satellites provide high-quality Synthetic Aperture Radar (SAR) and optical observations that are commonly used in waterbody mapping. However, owing to the 10-m spatial resolution of Sentinel data, previous studies mostly focused on the mapping of large waterbodies. In this work, we evaluated the performance of small waterbody mapping over urban and mountainous regions with two datasets, the average annual VH backscatter coefficients (VHavg), derived from the Sentinel-1A series, and the Modified Normalized Difference Water Index (MNDWI), derived from cloud-free Sentinel-2. A proven framework of waterbody mapping based on watershed segmentation and noise reduction was employed to assess the performance of the two datasets in waterbody identification. The validation was performed by comparing their results with 1-m spatial resolution reference waterbody data. Assessment metrics, including Precision, Recall, and F-measure, were employed. Results showed that: (1) the MNDWI outperformed the VHavg by 9 percentage points of the F-measure; (2) there was more room for results of VHavg to improve the accuracy through a combination with noise reduction; and (3) the potential smallest identifiable waterbody area (recall rate larger than 0.8) was larger than 104 m2.
Highlights
Surface waterbodies, including rivers, channels, ponds, lakes, and reservoirs, play an essential role in socioeconomic development and ecosystem balance, and provide irreplaceable natural resources for humans’ survival and development [1,2]
The water index-based method combines two or more spectral bands using various algebraic operations to enhance the discrepancy between the waterbody and land [8,9]. (2) Synthetic Aperture Radar (SAR) is an active remote sensing technique that uses microwave electromagnetic energy to form complex images of terrain reflectivity
According to the Guangzhou Water Resources Bulletin of 2018, the total waterbody area was estimated at 744 km2, which accounted for 10.05% of the city’s total land area [21]
Summary
Surface waterbodies, including rivers, channels, ponds, lakes, and reservoirs, play an essential role in socioeconomic development and ecosystem balance, and provide irreplaceable natural resources for humans’ survival and development [1,2]. The remotely sensed data can be divided into two categories based on their imaging principles. The typical optical data sources include Landsat-5/7/8 [4], Sentinel-2 [5], and GF-1/2 [6,7]. (2) Synthetic Aperture Radar (SAR) is an active remote sensing technique that uses microwave electromagnetic energy to form complex images of terrain reflectivity. The typical SAR data sources include Envisat [10], Radarsat-2 [11], and Sentinel-1 [12,13]. Comparing the two data sources, optical images are simple to interpret as they are closer to human vision. SAR images are relatively difficult to interpret as the data may vary depending upon many factors, including the incidence angle, terrain structure, and surface roughness.
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