## 4 Conclusions  
This study experimentally evaluated the applicability of digital image photogrammetry in South Korea for the rapid measurement of MSE wall deformation in restricted-access and controlled areas. The results are summarized as follows.  
1. The cause of the facing crack of the MSE wall in this research's case study site was evaluated based on the results of the electrical resistivity survey. The evaluation results confirmed that an abnormal area occurred in the corner of the structurally vulnerable MSE wall due to the groundwater infiltration of the original ground and reinforced earth mass. In other words, it was observed that it caused the facing crack and deformation of the MSE wall.
2. In order to evaluate the applicability of digital image photogrammetry in restricted-access and controlled areas, the displacement of the MSE wall was monitored using the traditional monitoring method and digital image photogrammetry. The mon  
Figure 14: Error rate analysis: (**a**) traditional monitoring method vs. digital image photogrammetry (digital camera), 7-month monitoring period; (**b**) traditional monitoring method vs. digital image photogrammetry (digital camera), 4-month monitoring period; (**c**) traditional monitoring method vs. digital image photogrammetry (cellphone camera), 7-month monitoring period.  
itoring results showed that the displacement values showed similar elapsed time trends for both methods, but digital image photogrammetry results exhibited larger displacements than the traditional monitoring results. Nevertheless, the error of the digital camera applied for digital image photogrammetry was lower than that of the cellphone camera.
3. In order to evaluate the accuracy of digital image photogrammetry, the error rate was analyzed. The results showed that the error at a specific location was similar between the digital camera and the cellphone camera. However, it was found that digital image photogrammetry using a digital camera with a consistent error occurrence tendency is highly applicable to rapid structural deformation monitoring in restricted-access and controlled areas.
4. It was found that research on the effect of the camera's pixels on the error was necessary in order to improve the accuracy and error resolution of digital image photogrammetry using a digital camera. In addition, the effect of the aligned image on the accuracy of the measurement coordinates in the 3D transformation of the 2D image acquired from the digital camera must be studied. It was also evaluated that the position of the digital camera may have contributed to the error rate of the measurement results. Therefore, research should continue to evaluate the limitations of the image acquisition distance and angle of the digital camera.  
Conceptualization, C.-H.C. and G.H.; methodology, C.-H.C. and J.-G.H.; software, C.-H.C.; validation, J.-G.H. and G.H.; formal analysis, C.-H.C. and G.H.; investigation, C.-H.C. and J.-G.H.; resources, C.-H.C.; data curation, J.-G.H. and G.H.; writing--original draft preparation, C.-H.C. and G.H.; writing--review and editing, J.-G.H. and G.H.; visualization, G.H.; supervision, J.-G.H.; project administration, C.-H.C. and J.-G.H; funding acquisition, G.H. All authors have read and agreed to the published version of the manuscript.  
Not applicable.  
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors.  
The authors declare no conflicts of interest.