## 5 Discussion
### Correlation Analysis  
By acquiring corresponding points of three methods in the four zones, a comparative analysis of the correlation between the proposed method and conventional methods were carried out, as shown in Figure 10. According to the comparative results from PS-InSAR and the proposed method shown in Figure 10a-d, the correlation was poor in the blue curved circles in Figure 10a,c. The mainly reason is that the PS-InSAR technique is a single-master-based interferometric method, there are some interferograms with a large spatio-temporal baseline in this method relative to the multi-master-based interferometric method (the interferometric method in this paper that combined multiple techniques), which consequently affects the coherence [39]. Additionally, the long-time scale contains a much larger displacement, resulting in poor coherence of the interferogram. In contrast, the proposed method in this paper can obtain a sufficient quantity of CPs including regions with large displacement, which leads to a certain degree of difference in the correlation between the two methods in regions with large displacement. Considering the results of the correlation analysis with SBAS shown in Figure 10e-h, the overall correlation was higher in all four zones compared with that from PS-InSAR (as the correlation coefficient R from SBAS-InSAR is 0.991, 0.9984, 0.9973, and 0.9958 for A, B, C, and D zones shown in (e-h), respectively, greater than 0.8988, 0.9878, 0.9665, and 0.9587 for A, B, C, and D zones shown in (a-d) from PS-InSAR). The main reason is that SBAS sets a spatio-temporal baseline threshold to ensure the coherence of the interferometric pair while avoiding the inclusion of large-magnitude displacement due to the wide time span of the interferometric pair. It can eliminate the errors caused by spatio-temporal decoherence to a certain extent and can obtain higher CPs in large displacement regions. The proposed method is also able to obtain sufficient CPs for large shape variables. Therefore, the correlation between the two methods can be consistent. However, there is a poor correlation for some points with large displacements in A zone (blue curve circles in Figure 10e). The region mainly covering the town area, where a large quantity of PS points with stable scattering characteristics can be obtained with high coherence (high coherence of PS points in dense building areas), and this can lead to high proportion of PS point phase values in the integrated phase values, resulting in an underestimation of large displacement. In general, the displacement monitoring accuracy obtained by the proposed method was comparable to that of the PS-InSAR and SBAS-InSAR in the region with small displacement values, whereas the proposed method was able to achieve comparable accuracy with SBAS in the large displacement region. In terms of displacement magnitude monitoring capability, this method is able to obtain results that are closer to those of SBAS-InSAR technique.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Methods** & **in A** & **in B** & **in C** & **in D** \\ \hline PS-InSAR & 15.1 & 6.2 & 5.98 & 17.47 \\ SBAS-InSAR & 1.65 & 2.44 & 3.88 & 2 \\ \hline \end{tabular}
\end{table}
Table 3: Statistics on the increase in the quantity of CPs < 20 mm/year from the proposed method to the conventional time series InSAR techniques.