## 3 Results
### Comparison of OBIA Results with Visual Interpretation  
The accuracy of the semi-automated OBIA results was assessed by comparison to results from visual interpretation. Table 5 summarizes the results of the accuracy assessment.  
Similar accuracy values were achieved for the OBIA classification among the selected images, whereby higher accuracies were reached for the landslide class compared to the lake class. Mixed pixels--or in our case mixed objects as a result of undersegmentation [73]--are an important issue in the identification and proportion estimation of classes in Landsat satellite scenes, since they cover more than one ground cover type and thus decrease the separability of classes [74]. We faced this problem especially in areas where small patches of vegetation within the landslide area exist or where revegetation leads to a sparse vegetation cover that influences the spectral reflectance on post-event images. This results in small differences in the OBIA landslide mapping compared to the visual interpretation. For the detected landslide-dammed lakes, classification uncertainties are mainly associated with mixed objects, partly shallow water areas with high sediment load or wet areas in the river bed.  
When interpreting the accuracy values it has to be considered that results from visual expert interpretation, especially for natural phenomena such as landslides, cannot constitute an entirely 'true' reference, as their creation depends on various factors such as the data used or the expertise of the interpreter [49].  
Figure 5: Time series of the Butangbunasi landslide area evolution and the dates of each typhoon event. The SSHWS category corresponds to the maximum category reached by the typhoon when crossing Taiwan. The mapped areas for the images corresponding to 12 December 1984 and 8 November 2018 are indicated. A locally estimated scatterplot smoothing (LOESS) curve (confidence interval 95%) is fitted to the time series for illustration purposes.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
** Landsat Image** & **Class** & \begin{tabular}{c} **OBIA** \\ **Mapping (ha)** \\ \end{tabular} & \begin{tabular}{c} **Manual** \\ **Mapping (ha)** \\ \end{tabular} & \begin{tabular}{c} **Difference** \\ **OBIA—MM (\%)** \\ \end{tabular} & \begin{tabular}{c} **Overlap** \\ **Area (ha)** \\ \end{tabular} & \
\begin{tabular}{c} **Producer’s** \\ **Accuracy (\%)** \\ \end{tabular} &
\begin{tabular}{c} **User’s** \\ **Accuracy (\%)** \\ \end{tabular} \\ \hline
4 December 2016 & Landsslide & 382.30 & 401.87 & \(-\)4.87 & 357.09 & 88.86 & 93.41 \\ (Landsat 8) & Lake & 8.38 & 8.24 & 1.68 & 6.44 & 78.19 & 76.91 \\ \hline
24 August 2008 & Landsslide & 164.19 & 189.04 & \(-\)13.15 & 155.92 & 82.48 & 94.96 \\ (Landsat 7) & Lake & 3.56 & 4.40 & \(-\)18.96 & 2.86 & 65.03 & 80.25 \\ \hline
27 September 2000 & Landsslide & 119.75 & 117.87 & 1.60 & 101.47 & 86.09 & 84.73 \\ (Landsat 5) & Lake & 6.88 & 6.04 & 13.77 & 4.32 & 71.50 & 62.85 \\ \hline \hline \end{tabular}
\end{table}
Table 5: OBIA and manual mapping (MM) results, the difference between OBIA and manual mapping results, overlapping area and producer’s and user’s accuracy for each of the three selected Landsat images.