## 3 Results
### The Sliding Behavior Comparing with Slope Inclinometer  
The study site was investigated for more than ten years. However, the continuous GNSS measurement was quite expensive before that. The borehole is the traditional method to understand underground geology and deformation measurement. Two boreholes, BH1 and BH2, are adopted in this study, as shown in Figure 1. The depths of BH1 and BH2 are 100 m and 80 m, respectively. The measurement of deformation is initiated from the far bottom of the hole. The algorithm assumes that the far bottom is not moving and measures the tilt angle in two perpendicular directions tilt angle. Then the horizontal displacement at each depth can be derived by tilt angle times depth. The horizontal displacement can be used as a reference compared with the SBAS method when the GNSS is limited. The horizontal ground surface displacements are plotted in Figure 9 with daily rainfall. There are two horizontal directions measured, which are A direction and B direction. The two directions are perpendicular to each other. The direction of horizontal displacement is hard to identify because the underground pipe could be twisted. Thus, we merge two directions into one displacement. The results show that A direction is a controlled direction in this case. The slope inclinometer was initiated to measure from 2011. There were two significant events in 2012 and 2017, respectively. The two events were caused by heavy rainfall of more than 1000 mm. Because SBAS can only detect ground deformation, this study uses ground surface displacements with time as a reference. \begin{table}
\begin{tabular}{c c c} \hline \hline  & \multicolumn{2}{c}{**GNSS1 Location**} \\ \cline{2-3}  & **Slope (degree)** & **Aspect (degree)** \\  & 15.6 & 88.9 \\  & **Ascending orbit** & **Descending orbit** \\ \hline
**Error Mean (mm)** & 13.21 & 84.74 \\
**Error Standard Deviation (mm)** & 10.14 & 37.82 \\
**Correlation coefficient** & 0.95 & \(-\)0.69 \\ \hline \hline \end{tabular}
\begin{tabular}{c c c} \hline \hline  & \multicolumn{2}{c}{**GNSS2 Location**} \\ \cline{2-3}  & **Slope (degree)** & **Aspect (degree)** \\  & 19.8 & 169.8 \\
**Ascending orbit** & **Descending orbit** \\ \hline
**Error Mean (mm)** & 24.43 & 25.76 \\
**Error Standard Deviation (mm)** & 15.17 & 17.52 \\
**Correlation coefficient** & 0.51 & \(-\)0.20 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The slope (steepness) and aspect (azimuth) of GNSS stations and related DInSAR comparison errors. A concept to convert horizontal displacement to vertical displacement is proposed in this research. The idea of this concept is shown in Figure 10. The slope indicator measures a horizontal displacement in referenece to the deep ground surface. With an average slope at the hole, the vertical displacement at the same point can be estimated based on Equation (2). The recommended distance (radius) to calculate slope degree is from the center of the hole to the radius larger than the depth of the inclinometer. \[\tan\theta=\frac{V}{H}\to V=H\times\tan\theta \tag{2}\]  
where, \(\theta\): average slope at slope indicator; \(H\): ground surface displacement from slope indicator; \(V\): estimated vertical displacement. Figure 9: The measured ground surface horizontal displacement from slope inclinometer. The converted vertical displacement is plotted in Figure 11. The vertical movement derived from ESA Sentinel-1 is also plotted in the same period to compare. The result shows that this method can be used for estimating the vertical displacement trends correctly at BH1. However, the displacement comparison at BH2 is only consistent with the deformation trend, not accurate. The method proposed is based on landslide monitoring, in which the observation point is moving down in general. The process should be carefully checked in case the observation point is moving up (uplift). This method allows comparing DInSAR and slope indicator data, especially traditional monitoring sites without GNSS or leveling data.