# Optimization of DG-LRG Water Extraction Algorithm Considering Polarization and Texture Information
## 2 Research Area and Data Pre-Processing
### Data Source and Pre-Processing
#### 2.2.2 Data Pre-Processing  
The obtained GF-3 image could not be directly used for water extraction, and required image pre-processing, such as radiometric correction, multi-look transformation, filtering, and geocoding.  
1. Radiometric Correction:  
The radar sensor measures back-scatter, which is the ratio of the transmitted pulse to the intensity of the received signal. Due to the influence of multi-source errors, radiation  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Basic Information** \\ \hline Imaging Mode & FSII \\ Polarization mode & Dual polarization (HH + HV) \\ Data type & SLC \\ Resolution/m & 10 \\ Imaging width/km & 100 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Basic parameters of GF-3.  
Figure 1: Geographic Location and Topographic Relief of the Study Area: (**a**) Position within China; (**b**) Location in Beijing Municipality; (**c**) Elevation Map Highlighting Northwest-to-Southeast Gradient.  
error is present in the SLC image. In order to accurately reflect the echo characteristics of ground objects, it is necessary to convert the input signal into radar back-scatter coefficients. After radiometric correction and normalization of SAR images to a unified standard, the back-scatter intensity information obtained is not affected by the observation geometry of the SAR data, which is helpful for comparison and analysis.  
2. Multi-look:  
The SLC image of the GF-3 satellite overlaps with the radar echo signal scattered by a single pixel, causing noise in the data. To improve the performance of the GF-3 image and improve the back-scatter estimation accuracy for each pixel, multi-look processing is performed. The SLC data after multi-look processing has improved resolution data, but spatial resolution is decreased. In this experiment, multi-look averaging was applied after radiometric calibration but before geocoding, using the ENVI SARScap5.6.2 toolbox's Multi-looking operator with default settings. We applied four looks in the azimuth and range directions. This is consistent with the GF-3 FSII mode's typical configuration.  
3. Filtering:  
According to the characteristics of coherent noise in the image, the Lee filtering method is used to filter and process the image. The smoothing effect of this method is weak, it can suppress the noise of the GF-3 image, and it can better preserve the texture information of the image, which is conducive to water extraction.  
In this experiment, we employed the ENVI SARScap toolbox's Refined Lee Filtering operator. A \(7\times 7\) sliding window was chosen to balance noise suppression and edge preservation. Larger windows (e.g., \(9\times 9\)) over-smooth textures, while smaller windows (\(5\times 5\)) retain noise.  
4. Geocoding.  
Geocoding refers to the conversion of SAR data from slant-range coordinate systems to geographic coordinate systems. The process of geocoding involves three steps: the first is initial assumption of geometric transformation; the second is improving the geometric transformations; and the third is re-sampling one coordinate system to another. We selected ALOS DEM data with a resolution of 12.5 m for geocoding. The pre-processing result is shown in Figure 2  
Figure 2: Pre-processed SAR image. This image is a grayscale image in a geographic coordinate system, representing back-scatter. The blue line represents the administrative district of Fangshan.