Digital topographic mapping and modelling using low altitude unmanned aerial vehicle

Accurate topographic surveys that rely on traditional field-based methods (e.g. Total Station and Global Positioning System) are generally time consuming, challenging and costly, especially if it involves mapping large or remote areas where access could be difficult. A low-cost and user-friendly low altitude unmanned aerial vehicle (UAV) is emerging as an alternative to traditional field-based topographic methods. The automation in the image acquisition and image processing stages of the UAV technology allows for precision, efficiency and consistency in data acquisition in the field, saves time and cost, reduces the vulnerability of crew members to risk in the field and the production of topographic products such as digital elevation models and orthophotos with ease. To demonstrate such potential, DJI Phantom 4, a quad-rotor unmanned aerial vehicle, equipped with a 12.4-megapixel digital camera was used to capture 464 highly overlapping (80% overlap) and high-resolution images of an area of 10.87 hectares at an altitude of 50 m above the ground. Agisoft PhotoScan Professional, a structure-from-motion photogrammetric solution was used to generate orthomosaic and digital surface model of the study area. The 3D models were exported to ArcMap for feature extraction. The horizontal and vertical accuracies of the generated UAV solution were computed by comparing the coordinates of 20 ground control points (GCPs) with coordinates measured using the RTK GNSS method. The generated 3D models achieved the 1990 ASPRS accuracy standards for digital geospatial data with root mean square errors computed as 0.033 m and 0.042 m for horizontal and vertical accuracy respectively. It is evident from this study that low altitude UAV photogrammetry with the aid of ground control points is suitable for producing and updating large scale class 1 map up to a scale of 1:200 with a contour interval of 0.1 m. This study shows that geospatial UAV-derived data possesses similar practical accuracy to RTK GNSS and Total Station, which are usually used for topographic, cadastral and engineering survey works.