Abstract
The use of synthetic aperture radar (SAR) imagery is generally considered to be an effective method for detecting surface water. Among various supervised/unsupervised classification methods, a SAR-intensity-based histogram thresholding method is widely used to distinguish waterbodies from land. A SAR texture-based automatic thresholding method is presented in this article. The use of texture images substantially enhances the contrast between water and land in intensity images. It also makes the method less sensitive to incidence angles than intensity-based methods. A modified Otsu thresholding algorithm is applied to selected sub-images to determine the optimal threshold value. The sub-images were selected using k-means results to ensure a sufficient number of pixels for both water and land classes. This is critical for the Otsu algorithm being able to detect an optimal threshold for a SAR image. The method is completely unsupervised and is suitable for large SAR image scenes. Tests of this method on a Radasat-2 image mosaicked from 8 QuadPol scenes covering the Spritiwood valley in Manitoba, Canada, show a substantial increase in land–water classification accuracy over the commonly used SAR intensity thresholding method (kappa indices are 0.89 vs. 0.79). The method is less computationally intensive and requires less user interaction. It is therefore well suited for detecting waterbodies and monitoring their dynamic changes from a large SAR image scene in a near-real time environment).
Highlights
Freshwater is crucial for much of life on Earth and is an essential part of the natural environment
After checking the weather data for the acquisition dates of the Synthetic aperture radar (SAR) images and visually examining the wind effects, we selected the HV-polarized image for the Radarsat-2 QuadPol mosaic data as the input to the proposed method
This study presents an automatic texture thresholding method for waterbody detection using large SAR image scenes
Summary
Freshwater is crucial for much of life on Earth and is an essential part of the natural environment. Almost 9%, or 900,000 km, of Canada’s total landmass is covered by freshwater in the form of lakes, rivers, streams, etc. There are over 2 million lakes in Canada, of which 910,400 have an area greater than 0.1 km 516 have an area greater than 1 km, and 564 have an area greater than 100 km (Minns et al 2008). Lakes and rivers are important for water supply, fishing, and recreation. Many large lakes in Canada support commercial and subsistence fisheries which provide vital sources of food and income for people, especially the First Nations (Minns et al 2008). The inland waterbodies play an important role in the terrestrial water cycles and surface water budget (Wang, Huang, Rivera, et al 2014; Wang, Huang,Yang, et al 2014; Wang, McKenney, et al 2014), which strongly affect the atmosphere and surface/subsurface processes such as cloud development (Molders and Raabe 1996), surface albedo (Wang et al 2006), evapotranspiration (Wang et al 2013), stream flow (Koster and Milly 1997), and
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