## 3 Results and Discussion  
### Study Area and Background  
The city of Wuhan (Figure 2) is situated in the east of Hubei province, China and lies in the eastern Jianghan Plain, at the intersection of the middle reaches of the Yangtze River. Its climate is humid subtropical, with abundant rainfall and four distinct seasons. From 2011 to 2016, the construction of a new tunnel and its ancillary buildings took place on East Lake. After the new tunnel was completed, the ancillary buildings were removed in 2016. Moreover, in July 2016, a 50-year return period of rainfall occurred, and the rainfall was significantly higher than the average annual precipitation. Wuhan was affected by continuous heavy rain, and some areas were seriously flooded and dramatically changed. The use of optical sensors was limited by this severe weather. In this study, our aim was to monitor the changes associated with the tunnel construction on East Lake from three temporal PolSAR images (2011, 2015, and 2016) and to use bi-temporal PolSAR images (2015 and 2016) to detect the changes after the heavy rainfall.  
Figure 2: Location of the study area.  
Three C-band quad-polarimetric RADARSAT-2 (single look complex) images of Wuhan were acquired on 7 December 2011, 25 June 2015, and 6 July 2016. The nominal pixel spacings in the azimuth and range directions were 5.12 \(\times\) 4.73 m (7 December 2011 and 25 June 2015) and 4.86 \(\times\) 4.73 m (6 July 2016). The swath width was 25 \(\times\) 25 km, and the beams of the images were FQ21 (7 December 2011 and 25 June 2015) and FQ27 (6 July 2016), with the incidence angle ranging from 40.16\({}^{\circ}\) to 41.58\({}^{\circ}\) (7 December 2011 and 25 June 2015) and 45.23\({}^{\circ}\) to 46.49\({}^{\circ}\) (6 July 2016). The repeat cycle was 24 days. The preprocessing of temporal PolSAR data is important for change detection. In this study, the preprocessing consisted of radiometric calibration, speckle filtering, and image co-registration. After the radiometric calibration, the pixel values of the temporal PolSAR images are directly related to the radar backscatter of the scene. This condition is necessary for the comparison of PolSAR images acquired at different dates. Speckle filtering and image co-registration were also performed. Two free open-source software packages--the Next ESA SAR Toolbox and the PolSARpro SAR Data Processing and Educational Tool--were used in the preprocessing of the SAR datasets. The co-registered images of 2011 and 2015 were of 5058 \(\times\) 5696 pixels, and the co-registered images of 2015 and 2016 were of 4906 \(\times\) 5114 pixels.  
The Pauli-RGB images (\(|S_{hh}-S_{vv}|\) for red, \(|S_{hv}|\) for green, and \(|S_{hh}+S_{vv}|\) for blue) are shown in Figure 3a-c. Due to the large study areas, long intervals (2011-2015), and inclement weather conditions (heavy rainfall in July 2016), the regions labeled by the three red boxes are used to give a detailed assessment, to verify the efficiency of the proposed method. These labeled regions involve bridge, city, forest, and water bodies. Region 1 is East Lake, Region 2 is Yanxi Lake, and Region 3 is Yandong Lake. The ground reference maps over the three test sites were obtained based on the corresponding optical images obtained from Google Earth (version 7.18) and field surveys conducted by researchers.