# Ground Subsidence Analysis in Tianjin (China) Based on Sentinel-1A Data Using MT-InSAR Methods
## 3 Results
### Comparison of Corresponding Points  
To further compare the displacement results derived from the two MT-InSAR techniques and analyze the accuracy of them, 1684 corresponding points (Figure 6, Table 3), with the same geographical coordinates, were selected in the study area for statistical analysis. The densities of the corresponding points in the high, medium and low subsidence areas increase sequentially, which are 3.26, 3.39 and 4.22 units/km\({}^{2}\), respectively. The maximum subsidence rate in the corresponding points is \(-\)107.47 mm/year, derived from the method of SBAS. For all the corresponding points in the whole study area, the average subsidence rate obtained by PS is \(-\)22.99 mm/year, while the average subsidence rate of SBAS is \(-\)20.21 mm/year, slightly less than PS. The subsidence rates  
Figure 5: **(a–d)** The Google Earth image and photos of ground subsidence in Wangqingtuo town, located in the high subsidence area; **(e–h)** the Google Earth image and photos of ground subsidence effects in Hebei University of Technology, located in the medium subsidence area. Figure 6: Corresponding points’ distribution of the two MT-InSAR methods, with the background of SAR intensity image of Sentinel-1A. of PS and SBAS in the high subsidence areas are \(-53.25\) mm/year and \(-53.92\) mm/year, respectively, which are significantly higher than those in the medium and low settlement areas. The histogram of the PS-SBAS velocity difference is shown in Figure 7. The proportion of the corresponding points whose PS subsidence rate minus SBAS is less than 0 accounted for 87.11% (Figure 7), suggesting that the deformation rate obtained by PS is higher than that of SBAS in the study area. In addition, 64% of the corresponding points' velocity difference is between \(-5\) and 0 mm/year. The good agreement between these two different MT-InSAR-based measurements can be checked by plotting the PS subsidence rate versus SBAS, as shown in Figure 8. On the whole, the linear correlation coefficient R of all the corresponding points reached 0.974, indicating the annual subsidence rate acquired by the two methods was highly consistent. Separately, the correlation coefficients of the high, medium and low subsidence areas are 0.707, 0.948 and 0.962, (Figure 8), showing the existence of more discrete points in the high subsidence area than in the medium and low subsidence areas. The average absolute value of the corresponding points' velocity differences in the high, medium and low subsidence areas are 5.18, 4.14 and 3.62 mm/year, and the points with rate differences more than 10 mm/year account for 13.71%, 1.45% and 0%, respectively. Therefore, the consistency of the subsidence velocity derived from the two MT-InSAR techniques increases from the high subsidence area to the low subsidence area. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{1}{c}{**H**} & \multicolumn{1}{c}{**M**} & \multicolumn{1}{c}{**L**} & \multicolumn{1}{c}{**Total**} \\ \hline Area (km\({}^{2}\)) & & 114.03 & 182.63 & 164.01 & 460.67 \\ \hline Numbers of corresponding points & & 372 & 620 & 692 & 1684 \\ \hline Minimum displacement rate (mm/year) & PS & \(-71.38\) & \(-50.61\) & \(-28.66\) & \(-71.38\) \\  & SBAS & \(-107.47\) & \(-47.2\) & \(-22.39\) & \(-107.47\) \\ \hline Maximum displacement rate (mm/year) & PS & \(-13.09\) & \(-3.15\) & 13.19 & 13.19 \\  & SBAS & \(-22.94\) & \(-0.44\) & 11.67 & 11.67 \\ \hline Average displacement rate (mm/year) & PS & \(-53.25\) & \(-26.98\) & \(-2.55\) & \(-22.99\) \\  & SBAS & \(-53.92\) & \(-22.85\) & \(-1.04\) & \(-20.21\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of corresponding points in the study area. Figure 7: Histogram of the LOS velocity difference between PS and SBAS. In order to further explore the spatial distribution of the corresponding points with different velocities in the study area, the points were divided into three levels (0-5; 5-10; \(>\) 10 mm/year) according to the magnitude of their velocity difference (Figure 9A).