Recently, analysis and decision-making based on spatiotemporal unmanned aerial vehicle (UAV) high-resolution imagery are gaining significant attention in smart agriculture. Constructing a spatiotemporal dataset requires multiple UAV image mosaics taken at different times. Because the weather or a UAV flight trajectory is subject to change when the images are taken, the mosaics are typically unaligned. This paper proposes a two-step approach, composed of global and local alignments, for spatiotemporal alignment of two wide-area UAV mosaics of high resolution. The first step, global alignment, finds a projection matrix that initially maps keypoints in the source mosaic onto matched counterparts in the target mosaic. The next step, local alignment, refines the result of the global alignment. The proposed method splits input mosaics into patches and applies individual transformations to each patch to enhance the remaining local misalignments at patch level. Such independent local alignments may result in new artifacts at patch boundaries. The proposed method uses a simple yet effective technique to suppress those artifacts without harming the benefit of the local alignment. Extensive experiments validate the proposed method by using several datasets for highland fields and plains in South Korea. Compared with a recent work, the proposed method improves the accuracy of alignment by up to 13.21% over the datasets.
This study proposes a method for finding defects in heat-resistant structures without causing any damage to them, by utilizing THz waves. THz waves, also called Terahertz waves, are a portion of the electromagnetic spectrum with a frequency range of 0.1 to 10 THz, which lies between the microwave and infrared frequencies. These waves have unique properties that make them useful for nondestructive testing of various materials such as plastics, composites, and heat-resistant materials. The proposed method involves mounting a THz wave scanner on a multi-axis stage and using it to detect defects in conical- and cylindrical-shaped heat-resistant structures. Once the scan angle and height are set, the scan path is automatically calculated, and the THz wave scanner follows this path to perform the scan. As an initial step, the time taken for the THz wave to penetrate the heat-resistant structure and reach the adhesive layer was calculated. The time windowed data corresponding to the adhesive layer was then examined. Signal processing was applied to remove noise from the data to achieve more precise defect visualization. The THz signal to which the signal processing was applied was then reconstructed into a three-dimensional matrix, and the result corresponding to the adhesive layer was visualized in the same shape as the planar figure of the actual specimen. Those areas of the visualized image reflecting characteristics that deviated from those associated with a normal signal were identified as suffering from debonding defects. We performed a visual inspection of the adhesive layer of the actual heat-resistant structure and compared it to the images obtained by our method. There was a notable agreement between the two, thereby confirming that the technique presented in this paper is a viable means of inspecting heat-resistant structures for debonding defects.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.