# Exploring the Impact of Multitemporal DEM Data on the Susceptibility Mapping of Landslides
## 5 Discussion
### Advantages and Limitations  
The elevation, topography, and slope--depending on the DEM--are usually the main factors affecting susceptibility [23]. The existing studies assume that the influencing factors for susceptibility are constant over time [15; 54]. However, the DEM data always change significantly over time owing to landscape reshaping, such as landslides and engineering construction [30; 31; 55]. A susceptibility assessment is, therefore, closely related to the data source [56], and as such the susceptibility maps of the study area are not consistent.  
Despite this, the existing studies have analyzed the susceptibility of landslides using a single time-related data source without considering whether the date of the data source is consistent with the study date [57; 58]. Therefore, it is necessary to understand the temporal evolution of influencing factors (e.g., topography) and the influencing mechanisms of the factors on the susceptibility assessment [59; 60]. The changes in influencing factors are significant in earthquake- and landslide-prone areas [61], such as the study area. Therefore, more research regarding the impacts of factors on the susceptibility of landslides needs to be undertaken. It is better not to evaluate susceptibility simply using constant factors.  
The present study performs a preliminarily analysis of the impact of multitemporal DEM on susceptibility and shows that DEM variation has an impact on the susceptibility assessment using DEM-dependent factors. The analysis undertaken here can extend the knowledge regarding the  
Figure 11: Changes in the assessment levels of historical landslide events from 2000 to 2009.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{2}{c}{**Values**} & \multicolumn{5}{c}{**2000**} \\ \cline{3-6}  & & **Level I** & **Level II** & **Level III** & **Level IV** \\ \hline \multirow{6}{*}{2009} & \multirow{2}{*}{Number} & Level I & 68 & 25 & 4 & 8 \\  & & Level II & 2 & 35 & 28 & 20 \\  & & Level III & 0 & 13 & 23 & 47 \\  & & Level IV & 0 & 0 & 9 & 59 \\ \hline \multirow{6}{*}{2009} & \multirow{2}{*}{Percentage} & Level I & 19.94\% & 7.33\% & 1.17\% & 2.35\% \\  & & Level II & 0.59\% & 10.26\% & 8.21\% & 5.87\% \\  & & Level III & 0 & 3.81\% & 6.74\% & 13.78\% \\  & & Level IV & 0 & 0 & 2.64\% & 17.3\% \\ \hline \multirow{6}{*}{
\begin{tabular}{c} RNA \\ (\(10^{-3}\)/km\({}^{2}\)) \\ \end{tabular} } & Level I & 3.73 & 6.36 & 5.80 & 15.50 \\  & & Level II & 0.59 & 3.93 & 9.50 & 36.83 \\ \cline{1-1}  & & Level III & 0.00 & 2.69 & 2.57 & 7.31 \\ \cline{1-1}  & & Level IV & 0.00 & 0.00 & 1.97 & 4.77 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Numbers and percentages of historical landslide events at various assessment levels and RNA values of various assessment levels.  
impacts of multitemporal data on susceptibility and can aid preliminary studies on the mechanism causing the influence of factor variation.  
DEM variation closely relates to local geotechnical properties, which change the susceptibility assessment of landslides. However, the reasons for DEM variation are debated. The mechanism explaining the consequent influences of geotechnical properties on DEM variation remains a scientific challenge. In addition, considering the large spatial extent and period (about 77,000 km\({}^{2}\), 17 years) in the present study, changes between the three susceptibility maps could not be completely explained by DEM variation. There are also data limitations and difficulties in analyzing the interactions of various factors.