This study investigates the optimisation of Unmanned Aerial Vehicle (UAV) photogrammetry and Global Positioning System Real-Time Kinematic (GPS-RTK) technologies for volume calculation in complex terrains, using the hillside of Universiti Teknologi MARA (UiTM) Perak as a case study. Although UAV photogrammetry is increasingly applied in topographic modelling, few studies have empirically compared its volumetric accuracy at different flight altitudes against GPS-RTK benchmarks in undulating landscapes. In this study, two UAV flight altitudes (80m and 100m) were evaluated to determine their impact on measurement precision, with results benchmarked against ground-truthed GPS RTK data. Processing through Pix4Dmapper and ArcGIS Pro revealed that UAV-based surveys achieved 91.87% accuracy at 80m compared to 87.57% at 100m, demonstrating the critical role of flight altitude in terrain modelling. The higher accuracy at lower altitudes is attributed to enhanced image resolution and improved capture of terrain details, despite marginally increased operational time. These findings position UAV technology as a viable, cost-efficient complement to traditional GPS RTK surveys, particularly for large-scale or inaccessible terrain mapping. For projects requiring sub-decimetre accuracy in undulating landscapes, an 80m flight altitude is recommended as the optimal balance between data quality and survey efficiency. This research provides empirically validated guidelines for integrating UAV photogrammetry into geospatial workflows across construction, environmental management, and land development sectors.