UNIFIED DIGITAL ELEVATION MODEL DEVELOPMENT FOR ERELU RESERVOIR AND ITS CATCHMENT AREA, OYO WEST LOCAL GOVERNMENT AREA, OYO STATE

This study focuses on the development of a high-accuracy Unified Digital Elevation Model (UDEM) for the Erelu Reservoir and its catchment in Oyo State, Nigeria, integrating terrestrial GNSS measurements and bathymetric data from a Single Beam Echo Sounder (SBES) to produce a seamless representation of both land and underwater terrain. Field data were collected using dual-frequency UniStrong G970II GNSS receivers operating in RTK mode and a South SDE-28S+ SBES with continuous GPS referencing, following a structured workflow that included reconnaissance, instrument calibration, synchronized topo-bathymetric acquisition, preprocessing, interpolation, and model validation. Data integration and surface generation were performed in ArcGIS 10.7.1, Surfer 16, Global Mapper 24, Hypack 2023, and Microsoft Excel using Kriging algorithms, with all datasets referenced to the Minna Datum (Clarke 1880, UTM Zone 31N). The resulting UDEM achieved an RMSE of 0.027 m and a Total Vertical Uncertainty of ±0.137m, meeting IHO S-44 and SURCON accuracy standards. Derived products—including contour, slope, aspect, hillshade, and 3D terrain models—revealed a concave reservoir basin, steep northern and eastern slopes acting as major sediment-source zones, gentler western and southern terrains that promote infiltration and deposition, and well-defined erosional corridors and drainage pathways. The unified model demonstrated significant improvement over global DEMs in terrain continuity, vertical precision, and morphometric detail. Overall, the study shows that integrating locally acquired GNSS and bathymetric datasets enhances the reliability of elevation modelling and provides a robust foundation for hydrological modelling, sediment budgeting, flood risk assessment, and sustainable reservoir and watershed management. It recommends periodic topo-bathymetric surveys, institutional capacity building, and the establishment of centralized geospatial repositories for improved water resource governance in Nigeria.