Photogrammetric UAV sees a surge in use for high-resolution mapping, but its use to map terrain under dense vegetation cover remains challenging due to a lack of exposed ground surfaces. This paper presents a novel object-oriented classification ensemble algorithm to leverage height, texture and contextual information of UAV data to improve landscape classification and terrain estimation. Its implementation incorporates multiple heuristics, such as multi-input machine learning-based classification, object-oriented ensemble, and integration of UAV and GPS surveys for terrain correction. Experiments based on a densely vegetated wetland restoration site showed classification improvement from 83.98% to 96.12% in overall accuracy and from 0.7806 to 0.947 in kappa value. Use of standard and existing UAV terrain mapping algorithms and software produced reliable digital terrain model only over exposed bare grounds (mean error = −0.019 m and RMSE = 0.035 m) but severely overestimated the terrain by~80% of mean vegetation height in vegetated areas. The terrain correction method successfully reduced the mean error from 0.302 m to −0.002 m (RMSE from 0.342 m to 0.177 m) in low vegetation and from 1.305 m to 0.057 m (RMSE from 1.399 m to 0.550 m) in tall vegetation. Overall, this research validated a feasible solution to integrate UAV and RTK GPS for terrain mapping in densely vegetated environments.
A scheme for generating and/or eliminating the orbital angular momentum (OAM) of radio frequency (RF) signals based on the true time delay (OTTD) system is proposed for the first time. The central structure of the scheme is the OTTD unit connected with a circular antenna array which can provide or compensate the helical phase variation precisely, and therefore can form or eliminate the OAM radio beam with different states (the topological charges) conveniently. Other prominent advantages of the proposed system are support for multiplexing/demultiplexing the OAM states carried on the RF signals with the same frequency and support for exchange of the OAM states carried on two RF signals. This can be regarded as the analog of various other multiplexing technologies for increasing the capacity and efficiency in radio communication systems. Theoretical analysis and numerical simulations demonstrate the feasibility of the proposed scheme.
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