## 4 Measurement and Results/Discussion
### I-180 Slope
The intermittent red spots (at the top failure line and the bottom) indicate localized but particularly high deformation areas. The survey is based on two results, one before the failure and another after the failure. The general trend of deformation in the UAS-based method aligns with the TS-survey outcomes. However, the magnitude of deformation is comparatively higher in the UAS-based approach when compared to the TS survey, but it is in line with DSS results showing similar pot-bellied deformation at the downstream side (the zone with light blue color). This disparity between TS and UAS method could be attributed to differences in measurement principles and dates employed for the TS survey and DSS. Notably, weather conditions (i.e., temperature, rain, and wind) were crucial factors for DSS, TS, and UAS-based measurements. To summarize the findings of the deformation measurement techniques, it is challenging to determine which method is superior for this particular site. In light of the possible deformation profile of the slope after failure, ranging from sub 10 cm to almost 1 m, as demonstrated by [PERSON] et al. [28], all three techniques possess an inclusive sensitivity in the mm range, and indicate the location of the failure zone accurately. Therefore, a detailed error analysis was unnecessary. However, there are differences in measurement convenience that can be noted, as described below. The DSS was easy to install in the field and take measurement readings. The fiber optic cable was easy to purchase and the price was reasonable enough to be easily acquired, particularly when considering the number of data points one could obtain. The measuring method was based on valid Optical Physics; however, Brillouin scatter and Raman scatter might be coupled in cases when the temperature fluctuation was high, then the coupling of the mechanical strain and the thermal strain might have an unignorable effect on the measured deformation. Currently, it is not possible for practicing engineers to distinctly differentiate between mechanical strain and thermal strain. Notably, the DSS method has a distinct characteristic of measuring the relative deformation between anchor points, rather than the absolute deformation measured in TS and UAS-based measurements. To measure absolute deformation, reference anchor points located outside of the deformation zone may need to be installed so that the relative deformation could be calibrated to the total deformation. DSS fiber optic cables required more delicate handling than traditional copper cables. Their appearance was similar to that of typical electrical cables. Inexperienced users tended to handle them like a copper cable and ended up with a damaged cable, failing to provide any data or reliable data. Splicing the fiber optic cable was not easy in the field, particularly when environmental factors such as cold weather and dusty conditions were taken into consideration. A fiber scope, a professional-grade splicer, and a DSS interrogator (readout unit) were often used to inspect the damage in the fiber, but they might be cumbersome to use or expensive, prohibiting wide applications. After overcoming these difficulties, DSS needed about one hour to complete a set of measurements, which was the quickest method among the three techniques. A trivial but additional benefit is that the technique needed only one operator. In addition, the DSS technique may be set to self-monitoring mode, and able to take the measurement and transmit the data wirelessly regardless of the weather condition, even after the ground was covered by deep snow. Surveying is a conventional method and was supposed to be an easy technique. However, the accuracy of the result may heavily depend on the skill and experience of the surveyors. This technique required two persons, and it took a longer amount of time compared to DSS and UAS-based techniques. In addition, this technique might not be an ideal fit for rugged terrains.