Purpose. Develop an optimal algorithm that will increase the accuracy of determining the coordinates of the terrain when using the aerial process applying an unmanned aerial vehicle (UAV). Method. The minimization of function based on the condition of collinearity is performed, which clarifies the elements of external orientation (EZO) of digital images and leads to an increase in the accuracy of the spatial coordinates of the points of objects. The proposed function is the sum of the squares of the differences between the calculated and measured reference points on the corresponding digital images. The sequence of implementation of the proposed algorithm is that taking into account the condition of the minimum of this function makes it possible to obtain a system of six nonlinear equations for EZO. The process of determining EZO is performed in two ways: in the first case, the function G is minimized directly by one of the numerical methods, and in the second - obtained as a solution of a system of equations, which gives refined EZO values based on initial approximations obtained directly from UAV telemetry. Modified conditions of the minimum of the function G in which there are no differentiation operations are used to control the accuracy of EZO determination. As a result, we obtain the final values of the EZO at the time of shooting. Results. An algorithm has been developed and tested on mock-ups on real examples, which allows to increase the accuracy of calculating the coordinates of terrain points when using UAVs for the aerial photography process. Scientific novelty. Formulas are obtained, which increase the accuracy of creating topographic materials by digital stereophotogrammetric method. Practical significance. The implementation of the developed algorithm will significantly increase the accuracy of processing large-scale orthophotos and topographic plans created on the basis of aerial photography from UAVs.
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