True orthophoto mosaic generation: a simple and fast method

UAV-based images are now widely used to generate large-scale orthophoto mosaics. The generation of an orthophoto mosaic is divided into two stages: image registration and image stitching. Using the DSM associated with each image, orthophotos are generated throughout the image registration. The single orthophotos generated are then joined to each other step by step in the second stage, utilizing various methods of image stitching. Image stitching methods depend on the complex and challenging processes of image matching and seamline determination. In addition, the registration and stitching of images in UAV-based mapping projects with a significant number of images is a time-consuming process. In this study, a straightforward method is provided for creating large-scale true orthophoto mosaics from UAV-based images without the requirement to generate single orthophotos, image registration and seamline network determination. Instead of registrating the images and then stitching them step-by-step, this method processes the DSM of the entire area and all of the images simultaneously. First, for each DSM point, the optimal image is determined from among all the visible images based on an optimization procedure. Optimization is based on two criteria: distance from nadir point and distance from the projection center. Using the determined optimal images, the differential rectification procedure is then run, and the orthophoto mosaic cells are filled. The results of this investigation demonstrated that the proposed method yielded a mosaic with minimal changes along the seamline. In addition, the proposed method is compared with the conventional orthophoto mosaic production method, which is based on image matching and determination of seamlines. Evaluations indicate that the proposed method is able to increase the production rate of orthophoto mosaic by 39% and 45% in dataset 1 and 2 respectively. Additionally, the geometric accuracy calculated using the checkpoints in the orthophoto mosaic generated by the suggested method has decreased by an average of 2 cm, indicating more precise results.