Abstract
Due to the large number of technological developments in recent years, UAS systems are now used for monitoring purposes and in projects with high precision demand, such as 3D model-based creation of dams, reservoirs, historical monuments etc. These unmanned systems are usually equipped with an automatic pilot device and a digital camera (photo/video, multispectral, Near Infrared etc.), of which the lens has distortions; but this can be determined in a calibration process. Currently, a method of “self-calibration” is used for the calibration of the digital cameras mounted on UASs, but, by using the method of calibration based on a 3D calibration object, the accuracy is improved in comparison with other methods. Thus, this paper has the objective of establishing a 3D calibration field for the digital cameras mounted on UASs in terms of accuracy and robustness, being the largest reported publication to date. In order to test the proposed calibration field, a digital camera mounted on a low-cost UAS was calibrated at three different heights: 23 m, 28 m, and 35 m, using two configurations for image acquisition. Then, a comparison was made between the residuals obtained for a number of 100 Check Points (CPs) using self-calibration and test-field calibration, while the number of Ground Control Points (GCPs) variedand the heights were interchanged. Additionally, the parameters where tested on an oblique flight done 2 years before calibration, in manual mode at a medium altitude of 28 m height. For all tests done in the case of the double grid nadiral flight, the parameters calculated with the proposed 3D field improved the results by more than 50% when using the optimum and a large number of GCPs, and in all analyzed cases with 75% to 95% when using a minimum of 3 GCP. In this context, it is necessary to conduct accurate calibration in order to increase the accuracy of the UAS projects, and also to reduce field measurements.
Highlights
The calibration process of a digital camera is necessary to obtain metric information of the three-dimensional world using two-dimensional images
In order to bring the results into a desired coordinate system, Ground Control Points (GCPs) are used as constraints in a bundle adjustment process
The calibration parameters for the digital camera integrated into the “DJI Phantom 3 Standard” Unmanned Aerial System (UAS) platform calculated using the proposed test-field calibration and the “3DF Zephyr Pro” software for three different heights, namely 23 m, 28 m, and 35 m, and for two image configurations only with oblique images and with a combination of nadiral and oblique images, have been tested
Summary
The calibration process of a digital camera is necessary to obtain metric information of the three-dimensional world using two-dimensional images. For the calibration of digital cameras installed on UAS systems, the “self-calibration” or “auto-calibration” method is used, i.e., a method that refers to the process of calculating the camera’s internal and external orientation parameters using only image point correspondences [6]. There are some problems with the self-calibration mentioned in [9], such as correlations between parameters, images without relative roll angles, incomplete use of image format, use of high distortion lenses, lack of camera stability, and missing scale information in the viewing direction. Most of the cameras mounted on UASs are consumer-grade digital cameras Those non-metric cameras with large distortions need to be calibrated at the same focal length and focus, as in the projects in which cameras are used [10]. If the large distortions are not determined with high accuracy, the UAS image block will be oriented with less accuracy, and will lead to further accuracy losses of the resulted products: point clouds, meshes, DEM, DSM, or orthophotos
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
