Unmanned Aerial Vehicle (UAV) based mapping in engineering geological surveys : considerations for optimum results

UAVs have been used in engineering for at least two decades, mainly focusing on structural health monitoring, geological surveys and site inspections, especially at cases where a rapid assessment is required, for example after a natural disaster. While there is a wide range of recognition algorithms for the automatic identification of structural damage, structural geological features etc. from the acquired images, the parameters affecting the resolution of these images are often overlooked. As a result, the UAV technology is not used at its full potential and at times, it is even regarded as leading to poor outcomes. This paper discusses the main parameters affecting the resolution of the images acquired by a UAV. We present a case study of the structural geological mapping of a coastal area carried out using two types of UAVs: a fixed wing and a hexacopter. A comparison between the structural geological maps based on the orthophotos and one produced using conventional techniques shows that the level of detail is the same and the time spent is at least 5 times less when using a UAV. The fixed wing is faster and therefore, can cover large areas while the copter gives better resolution images as it can fly at lower heights. The latter is cost and time effective only if it is used for surveys limited to small areas. The characterization of some structural geological features has not been possible based solely on the orthophotos. We show that in order to achieve the desired accuracy, a ground sample distance of at least half that value is required. We discuss technical aspects, such as the effect of topography and UAV orientation on the overlap value, the camera calibration, number of control points and lighting conditions, that should be taken into account prior to flying a UAV and provide recommendations on how to obtain optimum results, i.e. orthophotos that suit the needs of the project