The Role of Small Format Aerial Photographs for First Response in Landslide Event
Authors: Ulfa F. , Sartohadi J. 
Published: November 2019

Abstract
The first response phase during a disaster is the most crucial part of disaster management. It is necessary for the decision maker to know the information about disaster condition, the current affected location and further the potential affected location. In a disaster such as landslide, the specific information is needed to determine the current condition of the material flow. Aerial photograph is a helpful tool to identify the disaster condition since it is considered as a low cost tool compared to direct field observation. The current condition produced from aerial photograph can be used to determine further potentially affected area or to determine potential location to build the emergency shelter or to determine the route of evacuation and so on. In Indonesia, the main focus of first emergency response is how to find the victim as soon as possible without considering the current condition of the hazard. Without knowing the current condition of the hazard, a bad implication such as inducing further landslides might happen, which increase the number of victims. In fact, the current condition analysis is much helpful in deciding the proper action in first response, including finding the victims without increasing other implications. Thus, the aerial photos are considered as an option to identify the current situation of landslide events. Therefore, the study is aimed to measure the effectiveness of the landslide first response tool, especially small scale aerial photographs based on the time and the process of photo extraction. To achieve the main objective, there are 3 specific objectives including: 1) to capture the landslide area extent, including the area that used for modelling process, 2) to know the minimum duration taken to prepare the aerial photograph, 3) to measure the effectiveness of aerial photograph as a first response tool. The study was trying to find the duration of post landslide management when it used aerial photograph to help the management. The duration resulted by image acquisition and processing will be assessed whether it is effective or not to be a tool for post landslide management. The aerial photograph acquisition and processing was processed in several basic steps by using certain devices. The method used is differentiated into two steps which is data acquisition using DGI Phantom-4 then further processed by using the Agisoft Photoscan Software, which is processed on ASUS N551. Both processes will be evaluated based on its steps, results and duration taken for the whole process. Then the shortest duration of the process was used for post landslide management, including situation analysis, first response and civil protection. To evaluate the effectiveness of the aerial photograph, the critical analysis of landslide first response was conducted by literature review to know how long the optimal duration since landslide occurrence until the first response. The ideal duration from critical analysis and the duration of landslide first response using aerial photograph was compared. Finally, the effectiveness of aerial photograph utilization in post landslide management can be evaluated. UAV called DJI Phantom 4 was used to obtain the aerial photograph, while to operate the flight of UAV, DJI Phantom 4 was connected to android application namely Pix4D. The application setting is adjusted, such as 70% overlap percentage, flight height around 60-100 m, 90 degree camera angle and slow UAV speed. To cover the landslide area extent, 5 times of flight were needed. The first 2 flights acquired the head of the landslide with the purpose to get the information, whether the evacuation process is safety enough or not. In total, the duration of all flights is only 15 minutes. In reality the whole acquisition process took about 2 hours. It happened due to the preparation of flight as well as the field condition that is not easy to be reached because of the landslide occurrence itself. Subsequently, Agisoft 1.1 was used to process the whole aerial photo taken from the flight. Among the five times of flight, the first two which consist of the landslide head photos were processed directly in the field so that the evacuation step can be started as soon as possible. There are five steps in photo processing including importing photos, alignment, creating dense cloud, building mesh and building texture. It took the shortest processing duration about 38 minutes and 20 seconds with 4.12 cm resolution. In total, aerial photograph acquisition as well as the processing takes around 3 hours for 2.2 ha extent area. By those results, it can be concluded that the using of aerial photograph is effective enough, then further can be used in decision making including situation analysis, first response and civil protection. The aerial photograph will help the stakeholder as decision maker to know the first response step that they will take.