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“...subcentimeter to centimeter level. It varied in this investigation depending on the objective of the mapping for each landslide. The distance to target varied from a few tens to approximately 150 m. Although technically one person was needed to operate the UAV, at least two were needed to safely fly the UAV and deploy and measure the GCPs used for reliable model development. Data acquisition time (or flight time), shown in Table 1, was the additional time beyond field deployment. Based on experience, approximately 3 to 10 times the data acquisition time was needed in the office for data analysis and accurate 3-D model development, with the amount of time...”
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“...OBSERVATIONS OF LANDSLIDES CAUSED BY THE APRIL 2015 GORKHA, NEPAL, EARTHQUAKE
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affected by the level of detail as well as the accuracy required in the developed model. Development of the 3-D model required SfM methodology, as presented by Westoby et al. (2012), using Pix4d and Agisoft Photoscan software. Detailed discussion of the procedures used in this study is found in Greenwood et al. (2016) for two selected landslides.
In addition to computation and data analysis time, a significant amount of time was invested in obtaining permission from the Nepalese government to fly the UAVs in the specific areas. The statistics reported in Table 1, especially those related to ground-based and UAV-based assessments, are the estimates for this specific expedition; they generally vary depending on the conditions in the field, the type of data required, the UAV and imagery technology used, and the experience of the deployment team members.
FIELD OBSERVATIONS
Numerous landslides were observed in...”
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“...SI06
ZEKKOS ET AL.
(a)
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Figure 8. (a) Aerial and (b) land-based view of damage caused by rock fall at the Melamchi water supply tunnel near Timbu.
Figure 9. (a) Terrace slopes near Syabru Besi in the Trishuli area; (b) terrace slopes near Melamchi town in the Indrawati area; (c) 3-D reconstruction of a terrace slope using UAV-enabled SfM near Timbu in the Indrawati area.
DEBRIS FLOWS
During field deployment in October 2015, a major debris flow was observed next to Timbu village. Testimonials from locals confirmed that the debris flow took place during the 2015 monsoon season and that there were at least three separate flow events. One of them dammed the river for approximately three minutes before collapsing. Satellite-based orthophotos of the location of the debris before and after the flow are shown in Figures 10a and 10b, respectively. An orthophoto of the base of the debris flow, based on UAV imagery, is shown in Figure 10c, and a DEM is shown in Figure lOd. The total area of the...”
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