Studies on Three-Dimensional (3D) Accuracy Optimization and Repeatability of UAV in Complex Pit-Rim Landforms As Assisted by Oblique Imaging and RTK Positioning

Abstract

Unmanned Aerial Vehicles (UAVs) are a novel technology for landform investigations, monitoring, as well as evolution analyses of long−term repeated observation. However, impacted by the sophisticated topographic environment, fluctuating terrain and incomplete field observations, significant differences have been found between 3D measurement accuracy and the Digital Surface Model (DSM). In this study, the DJI Phantom 4 RTK UAV was adopted to capture images of complex pit-rim landforms with significant elevation undulations. A repeated observation data acquisition scheme was proposed for a small amount of oblique-view imaging, while an ortho-view observation was conducted. Subsequently, the 3D scenes and DSMs were formed by employing Structure from Motion (SfM) and Multi-View Stereo (MVS) algorithms. Moreover, a comparison and 3D measurement accuracy analysis were conducted based on the internal and external precision by exploiting checkpoint and DSM of Difference (DoD) error analysis methods. As indicated by the results, the 3D scene plane for two imaging types could reach an accuracy of centimeters, whereas the elevation accuracy of the orthophoto dataset alone could only reach the decimeters (0.3049 m). However, only 6.30% of the total image number of oblique images was required to improve the elevation accuracy by one order of magnitude (0.0942 m). (2) An insignificant variation in internal accuracy was reported in oblique imaging-assisted datasets. In particular, SfM-MVS technology exhibited high reproducibility for repeated observations. By changing the number and position of oblique images, the external precision was able to increase effectively, the elevation error distribution was improved to become more concentrated and stable. Accordingly, a repeated observation method only including a few oblique images has been proposed and demonstrated in this study, which could optimize the elevation and improve the accuracy. The research results could provide practical and effective technology reference strategies for geomorphological surveys and repeated observation analyses in sophisticated mountain environments.

Potential of Consumer-Grade Cameras and Photogrammetric Guidelines for Subsurface Utility Mapping

“…The availability of nadir photos might have helped tighten the self-calibration process by providing greater visibility of the utilities and surrounding environment due to its plan view perspective. This result draws similarity to existing works where a combination of nadir and oblique aerial images improved the measurement results (Amrullah et al, 2016;Bi et al, 2021).…”

“…Similarly, some authors explored how the use of oblique and nadir photos influences the precision and accuracy of photogrammetric measurements. For example, Amrullah et al (2016) and Bi et al (2021), concluded that measurements do improve with the combination of oblique and nadir images during an aerial survey.…”

Influence of AGL flight and off-nadir images on UAV-SfM accuracy in complex morphology terrains

“…Nowadays, thanks to UAVs equipped with onboard global navigation satellite system-real-time kinematic (GNSS RTK) receivers, the GCPs could be no longer needed as the camera stations at the time of exposure can be determined to centimetre accuracy and used to georeference the block and control its deformations. Related to this, (Štroner et al 2021) and (Bi et al 2021) observed a systematic elevation error that arises when using this technique, concluding that inclusion of oblique images decreases it significantly. In (Forlani et al 2018) reached horizontal accuracy lower than 2 cm in UAV photogrammetric project using an onboard GNSS RTK.…”

“…Furthermore, a combination of nadir and oblique images can produce a more precise self-calibrating bundle adjustment when using nadir images alone due to better internal calibration of the camera (James & Robson 2014;Harwin et al 2015), which largely determines the accuracy of photogrammetric products. In this regard, (Štroner et al 2021) and (Bi et al 2021) observed an improvement in the vertical accuracy of UAV photogrammetric project products using GNSS RTK techniques by including oblique images.…”

Extraction and Analysis of Spatial Feature Data of Traditional Villages Based on the Unmanned Aerial Vehicle (UAV) Image

“…. DSM, as a new generation of digital mapping products, has the advantages of high accuracy, rich information, and detailed elevation information [34,35]. In this study, DSM data were used as the basis for extracting terrain features of the Shuikou garden space.…”