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Structure from motion (SfM) permits a fast and inexpensive way to obtain data necessary for geotechnical characterization of rock slope. Unlike scanline survey, a traditional technique, SfM can reduce the risk, time, cost, bias in measurement and improve the accessibility, quantity and quality of data collection. A small and low cost Unmanned Aerial Vehicle (UAV), DJI Phantom 4 Pro mounted with 20 megapixels camera is utilized to capture high quality images in this study with Real-Time Kinematic Global Positioning System (RTK-GPS) being used to obtain coordinates of 10 Ground Control Points (GCP) distributed evenly on the rock slope. Images registered and aligned with GCP render better accuracy results compared to images aligned without GCP. With photogrammetry software, dense point cloud, digital surface model (DSM) and orthophotos can be generated. 3D dense cloud reconstructed in the setting of high accuracy, with millions of points of a rock surface, can be imported into CloudCompare to extract the geological planes using FACET plugin. The major discontinuity sets can be observed and the orientation (dip/dip direction) of the discontinuity sets obtained from the algorithm is accurate and reliable for future geotechnical work such as rock slope stability analysis.

Unmanned Aerial Vehicles (UAV) photogrammetry is as precise as conventional survey which able to produce centimetre accuracy. It is considerably quicker and gives brief portrayal of geography. Given ideal regulatory conditions this innovation will supersede current techniques for most mapping and studying applications. UAV can be an important technology to extract useful information of the earth surface in short period of time for example Digital Surface Models (DSM) and other information. The final products are highly dependent on the choice of values on various parameter for the flight planning, and for the processing of the data. The aim of this research is to study the effect of Ground Control Point (GCP) influence on the final accuracy of the output with difference flying altitude. All photogrammetric procedure was applied using Agisoft PhotoScan a Structure from motion (Sfm) software from aligning photos to build Digital Elevation Model (DEM) and georeferenced orthophoto. In this research, we have used total of 16 GCP's where the coordinates measured with L1 RTK GPS receiver. Analysis was carried out to show the importance of GCP for mapping using low cost UAV. The analysis of both qualitative and quantitative were carried out to verify that the use of GCP is essential to produce a centimetre accuracy photogrammetric output. Utilization of GCP in mapping is significant because the output obtained from photogrammetry comes with actual measurements of the model that we have mapped.