## 4 Validation and Discussion of Land Subsidence Results
### Deformation Result Validation  
The ground displacement results are validated through three steps: 1. analyzing the standard deviation of the annual sedimentation; 2. comparing the deformation results from Sentinel-1 data and TerraSAR images; and 3. matching the results from the SBAS-InSAR and StaMPS technologies.  
1. Standard deviation verification Internal coincidence verification refers to the analysis of the phase model of the time series InSAR to determine the influence degree of the main error terms, that is, the theoretical accuracy evaluation that the time series InSAR can achieve. As is shown in Figure 7b, the average deformation rate of the experimental year is marked by a difference in the distribution map. It can be concluded that most of the errors range from \(-6.5\) to \(-1\) mm. The errors in the coastal parts of the JS District and Dishui Lake vary greatly because of the low coherence in coastal areas. In densely built-up places, the error is nearly neglected, and the experimental results are basically consistent with the actual situation.
2. TerraSAR displacement result comparison In order to further verify the feasibility of land subsidence in Shanghai using the SBAS-InSAR technology, this study monitored time series in some areas of Shanghai with nine scenes (2020-2021) from the high-resolution descending TerraSAR data using SBAS technology. Since the orbit types of the data from the two satellites are different, Sentinel-1A data are the ascending orbit data, and TerraSAR data are the descending orbit data; thus, the LOS deformation time series results obtained using the two datasets were first cropped to the public area and resampled. Figure 9 shows the annual average deformation rate of the Sentinel-1A and TerraSAR data, and the maximum annual average deformation rates were \(-39.5\) mm/\(a\) and \(-33\) mm/a, respectively. The subsidence mainly occurs in the MH District, the PD New Area coastal area, and the southern area. Meanwhile, it is possible to make errors in the recognition and operation of deformation points while selecting more target points in coastal areas and farmland areas of low coherence. From the results in the figure, it can be concluded that the standard deviation interval of the distribution of the annual average deformation rate was relatively consistent, and the monitoring results of the two different data sources were in good agreement at the level of deformation variables.
3. Results comparison with different SAR techniques We applied two different techniques to compare the annual average deformation rate results during the same time period in the same study areas. Firstly, the annual average deformation of PS points obtained via StaMPS technology was interpolated inversely. Finally, the difference between the two technologies was obtained. As is shown in Figure 10, the displacement difference results are mostly between approximately \(-1.8\) and \(0.8\) mm/a. Combined with the historical settlement data and previous research results in Shanghai, the deformation status has a high consistency, which shows the reliability of using the SBAS technology to study the time series in Shanghai.