Abstract. The Digital Terrain Model (DTM) is essential in generating the topographic structure of an area by eliminating its external features. Conventional survey techniques are still employed to obtain accurate geographic data on the earth's surface. However, accurate land surveys are now achievable because of the development of Unmanned Aerial Vehicles (UAVs). This study aims to update the LiDAR DTM for Flood Modeling in Las Nieves, Agusan del Norte, using GNSS and UAS integration. The Static GNSS survey was carried out to collect precise points for the direct georeferencing of the DJI Phantom 4 GNSS-RTK UAV. There is a continuous investigation of the influence of flight parameters in creating DTM. Hence, this study also evaluates the effects of overlap percentage and flight altitude on the quality of the generated DTM. There were 16 flight plans prepared using various combinations of flight parameters. The UAS data collected was processed using the Structure from Motion. The quality of the DTM was assessed based on its accuracy and level of completeness to identify the optimal parameters for generating the data model. Based on the results of the accuracy and completeness of the DTMs, the optimal parameters for generating are 90% and 120 meters. Subsequently, the UAS-based DTM generated using this combination of flight parameters was utilized in updating the existing DTM of Las Nieves to create the flood model of the area. The flood model was generated using the hydrologic and hydraulic modeling of the HEC RAS Mapper.

Abstract. The municipality of Las Nieves is one of the most vulnerable areas prone to flooding, usually caused by tropical cyclones. With these, preparing for an emergency response is relatively significant, and one strategic response is planning for systematic management of evacuation shelters. Hence, this study assessed the spatial exposure to flooding, the spatial capacity and mapped the suitability of evacuation shelters using Geographic Information System and site suitability analysis. This study also developed a web-based application that will address the needs of the Las Nieves LGU, specifically in providing the locality with spatial information and other flood-related data. Weighted Overlay Analysis was performed based on Multi-Criteria Decision Making and weights from related literature to generate a suitability map for flood evacuation shelters. Factors included slope, elevation, land use/land cover, road networks, distance to water sources, distance to electrical supplies, and proximity to fuel stations. The results indicated that 85.1% of the areas were moderately suitable, 0.3% were highly suitable, and 14.6% were low suitable for flood evacuation sites in Las Nieves. From the analysis of 11 existing evacuation centers, there are ten evacuation centers moderately suitable for use during an emergency, while only the Maningalao Evacuation Center was not suitable because the location of the center was affected by the 100-year flood event. Moreover, the web mapping application developed in this study will help the stakeholders and community of Las Nieves to effectively manage evacuation centers and access real-time spatial data regarding evacuation centers' status and spatial capacity.

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