Assessing the effect of flight altitude and overlap on orthoimage generation for UAV estimates of coastal wetlands

There has been a constant increase in technology regarding unmanned aerial vehicle (UAV) systems in the last decade; yet the possibility of surveying large areas with such aerial platforms has been hampered by their low endurance. This situation has been exacerbated in low institutional capacity countries with high biodiversity coastal environments where constant monitoring is required for conservation purposes. The objective of this study was hence to determine an optimal flight mission over an initial area of 40 ha of coastal wetlands using a low-cost UAV system. The UAV missions were planned at altitudes of 60, 80, 100, and 120 m. Several orthoimages were generated with different along/across track overlaps (in intervals of 10% from 40 to 90%) in order to determine the minimum flight duration and computational efficiency. Results indicated that flight altitude and across track overlap were the most relevant variables in terms of flight distance, flight duration, number of generated images, and computer processing time. The optimal flight mission was found to be at 120-m altitude with an along/across track overlap of 60/50%. Based on the aforementioned parameters, we were able to survey a larger area of 200 ha with minimum flight duration and computer processing time. Our study highlights the feasibility and limitations of using a consumer-grade UAV in order to assess coastal wetland areas rapidly when field-based surveys approaches are not possible.