3D flight planning for UAV-based photogrammetry in urban areas with an emphasis on solving problems caused by extreme scale differences and occlusion points

The use of accurate and up-to-date photogrammetric products as a basis for urban planning is very important because cities are complex and dynamic physical and social systems that are constantly changing. Urban planning experts and scientists need a wide range of spatial maps and technologies to make decisions to advance its benefits. This requires providing fast and accurate mapping methods. One of the low-cost and accurate mapping method is UAV-based photogrammetry but the limitations of this method require special solutions for developing the system in urban areas. Currently, most UAV photogrammetry projects are carried out in urban areas, without considering changes in the elevation of the ground and the heights of various features, as well as occlusion points. All three of these cases have a great impact on the accuracy and quality of output products in areas with altitude features. The developed three-dimensional flight planning in this article, in addition to paying attention to the height of various features such as buildings and terrain, makes the scale as uniform as possible and prevents the UAV from hitting obstacles. The flight design also includes a new technique such as oblique or semi-oblique imaging to reduce occlusion points. The proposed method was implemented in an urban area. The results of the algorithm were evaluated based on the accuracy, and quality of point clouds and output products, the accuracy of the checkpoints and comparison with the expected standard errors, scale stability in images, density of point clouds, and reduction of occlusion points. The results show the high quality of the products and the planimetric accuracy of 3.6 cm and the altitude accuracy of 7 and 4 cm at the requested scale of 1.750. Also, the results show that oblique images have a significant effect in generating information in the connection between walls and the ground and the density of building facades in the area. The results show that the design and implementation of the UAV photogrammetric flight planning algorithm based on the topographic model and features is successful.