APPLICATION OF UAVLS TO RAPID GEOLOGICAL SURVEYS

Three-dimensional scanning with LiDAR has been widely used in geological surveys. The LiDAR with high accuracy is promoting geoscience quantification. And it will be much more convenient, efficient and useful when combining it with the Unmanned Aerial Vehicle (UAV). This study focuses on UAV-based Laser Scanning (UAVLS)geological field mapping, taking two examples to present advantages of the UAVLS in contrast with other mapping methods. For its usage in active fault mapping, we scanned the Nanpo village site on the Zhangxian segment of the West Qinling north-edge fault. It effectively removed the effects of buildings and vegetation, and uncovered the fault trace. We measured vertical offset of 1.3m on the terrace T₁ at the Zhang river. Moreover, we also scanned landslide features at the geological hazard observatory of Lanzhou University in the loess area. The scanning data can help understand how micro-topography affects activation of loess landslides. The UAVLS is time saving in the field, only spending about half an hour to scan each site. The amount of average points per meter is about 600, which can offer topography data with resolution of centimeter. The results of this study show that the UAVLS is expected to become a common, efficient and economic mapping tool.     

Application of Structure‐from‐Motion photogrammetry to river restoration

Structure‐from‐Motion (SfM) photogrammetry is now used widely to study a range of earth surface processes and landforms, and is fast becoming a core tool in fluvial geomorphology. SfM photogrammetry allows extraction of topographic information and orthophotos from aerial imagery. However, one field where it is not yet widely used is that of river restoration. The characterisation of physical habitat conditions pre‐ and post‐restoration is critical for assessing project success, and SfM can be used easily and effectively for this purpose. In this paper we outline a workflow model for the application of SfM photogrammetry to collect topographic data, develop surface models and assess geomorphic change resulting from river restoration actions. We illustrate the application of the model to a river restoration project in the NW of England, to show how SfM techniques have been used to assess whether the project is achieving its geomorphic objectives. We outline the details of each stage of the workflow, which extend from preliminary decision‐making related to the establishment of a ground control network, through fish‐eye lens camera testing and calibration, to final image analysis for the creation of facies maps, the extraction of point clouds, and the development of digital elevation models (DEMs) and channel roughness maps. The workflow enabled us to confidently identify geomorphic changes occurring in the river channel over time, as well as assess spatial variation in erosion and aggradation. Critical to the assessment of change was the high number of ground control points and the application of a minimum level of detection threshold used to assess uncertainties in the topographic models. We suggest that these two things are especially important for river restoration applications. Copyright © 2016 John Wiley & Sons, Ltd.     

无人机航摄相片倾角对立体高程扭曲的影响分析

通过对相片倾角对航测高程误差影响的理论分析,结合无人机低空摄影实际生产出现的问题,提出无人机低空摄影飞行姿态控制的指导性建议。     

Digital mapping of coastal boulders – high-resolution data acquisition to infer past and recent transport dynamics

Coastal boulder fields provide clues to long-term frequency-magnitude patterns of coastal flooding events and have the potential to play an important role in coastal hazard assessment. Mapping boulders in the field is time and labour-intensive, and work on intertidal reef platforms, as in the present study, is physically challenging. By addressing coastal scientists who are not specialists in remote sensing, this contribution reports on the possibilities and limitations of digital applications in boulder mapping in Eastern Samar, Philippines, where recent supertyphoons Haiyan and Hagupit induced high waves, coastal flooding and boulder transport. It is demonstrated how satellite imagery of sub-metre resolution (from Pléiades and WorldView-3 imagery) enables efficient analysis of transport vectors and distances of larger boulders, reflecting variation in latitudes of both typhoon tracks and approaching angles of typhoon-generated waves. During the investigated events, boulders with a-axes of up to 8 m were clearly identified to have been shifted for up to 32 m, mostly along the seaward margin of the boulder field. It is, however, hard to keep track of smaller boulders, and the length of a-axes and b-axes including their orientation is often impossible to map with sufficient accuracy. Orthophotographs and digital surface models created through the application of an unmanned aerial vehicle and the ‘Structure from Motion’ technique provide ultra-high-resolution data, and have the potential to not only improve the results of satellite image analysis, but also those from field mapping and may significantly reduce overall time in the field. Orthophotographs permit unequivocal mapping of a-axes and b-axes including their orientation, while precise values for c-axes can be derived from the respective digital surface models. Volume of boulders is best inferred from boulder-specific Structure from Motion-based three-dimensional models. Battery power, flight speed and altitude determine the limits of the area covered, while patches shielded by the boulders are difficult to resolve. For some tasks, field mapping remains mandatory and cannot be replaced by currently available remote sensing tools: for example, sampling for rock type, density and age dating, recording of lithological separation of boulders from the underlying geological unit and of geomorphic features on a millimetre to decimetre-scale, or documentation of fine-grained sediment transport in between the boulders in supratidal settings. In terms of future events, the digital products presented here will provide a valuable reference to track boulder transport on a centimetre to decimetre-scale and to better understand the hydrodynamics of extreme-wave events on a fringing reef coastline.     

Evaluating functional connectivity in a small agricultural catchment under contrasting flood events by using UAV

Concentrated erosion, a major feature of land degradation, represents a serious problem for soil and water resources management and a threat to ecosystems. Understanding the internal mechanisms (de-)coupling sediment pathways can improve the management and resilience of catchments. In this study, concentrated erosion and deposition forms were mapped accurately through field and aerial unmanned aerial vehicle (UAV) campaigns, in order to assess the evolution of connectivity pathways over a series of three contrasted and consecutive flood events occurring between October 2016 and January 2017 (return period ranging from 0.5 to 25 years) in a small Mediterranean agricultural catchment (Can Revull, Mallorca, Spain; 1.4 km²). In addition, a morphometric index of connectivity (IC) was used to identify the potential trajectories of different concentrated erosion forms and deposition areas. IC predictions were calibrated by identifying the optimal critical thresholds, i.e. those most consistent with field observations after each of the events studied. The results found that the index performed well in predicting the occurrence and the length/area of the different type of landforms, giving kappa (κ) coefficients of variation ranging between 0.21 and 0.92 and linear correlations R² between 0.33 and 0.72. The type of landform affected the correspondence of IC predictions and field observations, with lower thresholds the greater the magnitude of their associated geomorphic processes. Rainfall magnitude proved to be a very important factor controlling the development of erosion and deposition landforms, with large differences in length/area between the contrasted events. The evolution of the observed trajectories revealed feedback dynamics between the structural and functional connectivity of the catchment, in which morphological changes determined the spatial distribution of the processes’ activity in the successive events and vice versa. © 2020 John Wiley & Sons, Ltd.     

Monitoring soil surface roughness under growing winter wheat with low-altitude UAV sensing: Potential and limitations

Soil surface roughness (SSR) is an important factor in controlling sediment and runoff generation, influencing directly a wide spectrum of erosion parameters. SSR is highly variable in time and space under natural conditions, and characterizing SSR to improve the parameterization of hydrological and erosion models has proved challenging. Our study uses recent technological and algorithmic developments in capturing and processing close aerial sensing data to evaluate how high-resolution imagery can assist the temporally and spatially explicit monitoring of SSR. We evaluated the evolution of SSR under natural rainfall and growing vegetation conditions on two arable fields in Denmark. Unmanned aerial vehicle (UAV) photogrammetry was used to monitor small field plots over 7 months after seeding of winter wheat following conventional and reduced tillage treatments. Field campaigns were conducted at least once a month from October until April, resulting in nine time steps of data acquisition. Structure from motion photogrammetry was used to derive high-resolution point clouds with an average ground sampling distance of 2.7 mm and a mean ground control point accuracy of 1.8 mm. A comprehensive workflow was developed to process the point clouds, including the detection of vegetation and the removal of vegetation-induced point cloud noise. Rasterized and filtered point clouds were then used to determine SSR geostatistically as the standard deviation of height, applying different kernel sizes and using semivariograms. The results showed an influence of kernel size on roughness, with a value range of 0.2–1 cm of average height deviation during the monitoring period. Semivariograms showed a measurable decrease in sill variance and an increase in range over time. This research demonstrated multiple challenges to measuring SSR with UAV under natural conditions with increasing vegetation cover. The proposed workflow represents a step forward in tackling those challenges and provides a knowledge base for future research. © 2020 John Wiley & Sons, Ltd.     

不同密度团头鲂(Megalobrama amblycephala)对轮叶黑藻(Hydrilla verticillata)和密刺苦草(Vallisneria denseserrulata)群落结构的影响

冠层型水草轮叶黑藻在富营养情况下,生长迅速,覆盖水面,对于草甸型水草密刺苦草具有明显的竞争优势.为控制轮叶黑藻的竞争优势,于湖北省鄂州市团头鲂原种场的一个池塘进行了原位围隔（10 m×10 m）实验,研究不同密度（0、0.5和1尾/m²）团头鲂牧食作用对轮叶黑藻和密刺苦草群落结构的影响,并通过无人机数字图像处理获取水草覆盖度信息.实验过程中,沉水植物总覆盖度未发生变化.研究发现,团头鲂选择性牧食使得密刺苦草生物量和覆盖度显著增加.随着牧食强度的加大,团头鲂对轮叶黑藻的控制效果显著,放养密度为1尾/m²的围隔中轮叶黑藻比例降幅最明显.群落中轮叶黑藻与密刺苦草的生物量之比迅速下降,由6.14减少至0.002,覆盖度之比由4.88左右减少至约1.44.图像处理结果与实际采样情况相吻合,且通过误差矩阵得到图像分类平均精度达到90%以上,表明无人机数字图像处理在一定条件下可以作为获取沉水植物覆盖度的一种有效辅助手段.实验结束后,1尾/m²组的围隔中密刺苦草成为优势种,其植株密度、株高、株重和单株叶片数均显著增加,草甸更加密实.实验结果表明,放养1尾/m²密度的团头鲂可以有效控制轮叶黑藻并维持密刺苦草种群优势,结果为团头鲂对湖泊沉水植物群落的控制提供了参考.     

基于微型无人机摄影技术的微构造信息提取研究——以博-阿断裂乌苏通沟东岸为例

利用微型无人机摄影测量技术,获取了博-阿断裂在乌苏通沟东岸的高精度地形、地貌数据,解译DEM数据,并结合野外调查工作,明确了断裂在乌苏通沟东岸冲洪积扇上19.3～31.1m的水平位错。分析获取的陡坎剖面,且对比陡坎两侧地貌的剥蚀程度,认为陡坎形成后受到后期水流的侵蚀,部分陡坎的高度在一定程度上被放大,断裂的实际垂直位错在0.7m左右。通过实例展示了无人机摄影技术在活动构造研究中的巨大潜力以及在微构造信息提取中的独特优势。     

基于小型无人机航拍影像的地表房屋建筑提取算法对比——以新疆阿图什市琼哈拉峻村为例

选取新疆阿图什市下辖的琼哈拉峻村为研究区,将小型旋翼无人机拍摄的图像作为数据源,分别采用面向对象以及面向像元2种影像分析方法对研究区的房屋建筑进行提取,并对2种算法的提取结果进行对比,分析了各自的优势。结果表明:面向对象方法可以有效地去除椒盐噪声对分类带来的影响,保证房屋形态的完整性,但影响内部相似的光谱、纹理信息若对应多种物体则会导致影像对象的错分。在面向像元的提取方法中加入了改进的数学形态学算法,可有效的抑制椒盐噪声,保持建筑物边缘的连续性与完整性,较好地解决了面向对象方法中部分农田与房屋出现错分的问题。     

基于无人机倾斜摄影的强震区公路高位危岩崩塌形成机制及稳定性评价

强震区公路高位危岩崩塌具有极高隐蔽性和危害性,传统的接触式勘察方法难以有效调查震后位于公路两侧高陡斜坡体上的危岩崩塌体。提出一种基于无人机的倾斜摄影测量技术,该技术采用无人机超低空采集高位危岩体的高清影像数据,构建三维空间模型,从而提取危岩体特征参数,为危岩体稳定性分析提供数据支撑。利用无人机对某高速公路危岩崩塌地质灾害路段进行调查,对无人机倾斜摄影测量成果进行分析,明确调查区19处危岩崩塌体特征和崩塌成因机制,在此基础上评价典型崩塌体稳定性。并且使用RocFall软件模拟分析典型危岩体崩落运动轨迹,研究高位危岩崩塌对公路的危险性。研究成果对强震区山区公路高位危岩崩塌勘察、稳定性评价工作具有重要的参考价值。