Calibration of UAV Flight Parameters to Inspect the Deterioration of Heritage Façades Using Orthogonal Arrays

Heritage façades are relevant to the historical preservation and identity of a city. Their deterioration can cause the devaluation of the urban environment, together with accidents caused by possible detachment of their different elements; therefore, maintaining them is crucial. It is important to have early warning systems in place, systems which would indicate potential damage and deterioration of such façades, which can be caused by factors such as humidity, earthquakes, the passage of time, or even people. This is often done through visual inspection; however, visual inspection is often affected by external factors, such as the financing of the on-site inspection and the availability of experts. Unmanned aircraft vehicles (UAVs) are a practical and economical tool that has facilitated and improved imaging by regulating parameters such as camera angle, capture distance and image overlap, thus allowing improvements in photogrammetry techniques for virtual reconstruction processes. Although the implementation of this technology has been studied in recent years, research has focused primarily on horizontal structures, such as roads. In the case of vertical structures, flight parameters are traditionally defined by trial and error, without any methodological procedure. Therefore, this paper proposes a methodology to calibrate UAV flight parameters to build a photogrammetric 3D model of a vertical structure. This proposal is based on the development of a series of experiments configured through Taguchi orthogonal arrays, which is a method that significantly reduces the number of experiments required. This methodology is applied to a case study of a façade with architectural heritage features. The results obtained offer a flight plan and the optimal parameters to achieve efficiency in the capture of images in an inspection of heritage façades, thus achieving the optimisation of resources and time.