<p><strong>Abstract.</strong> Consumer-grade Unmanned Aircraft Systems (UAS), and particularly Small Unmanned Aircraft (SUA) weighing less than 20&amp;thinsp;kg, have recently become very attractive for photogrammetric data acquisition across a wide range of applications. Compared to other more expensive remote-sensing technology, DJI Phantom series SUA provide a trade-off between cost, sensor quality, functionality and portability. Because of the significant interest in such systems, rigorous accuracy assessment of metric performance is crucial. This research investigates the capabilities of the Phantom 4 Pro (P4P) and the recently launched Phantom 4 RTK (P4RTK) SUA through both laboratory and in-situ assessments with multi-scale photogrammetric blocks. The study adopts self-calibrating bundle adjustments from conventional photogrammetry and from a Structure-from-Motion (SfM)-photogrammetric approach. Both systems deliver planimetric and vertical absolute accuracies of better than one and two pixels ground sampling distance, respectively, against independent check points. This can be achieved if the imaging network configuration includes a mixed range of nadir and oblique imagery and several ground control points are established as reference information. Ongoing analysis is investigating the strength of all bundle adjustment solutions. It is also evaluating the GNSS capabilities of the P4RTK SUA after post-processing raw observations of its trajectory. Findings from a comprehensive accuracy assessment can support non-experts in designing the pre-flight photogrammetric data acquisition plan and aid understanding of the performance of such popular off-the-shelf SUA.</p>

ABSTRACT:Landslides are hazardous events with often disastrous consequences. Monitoring landslides with observations of high spatio-temporal resolution can help mitigate such hazards. Mini unmanned aerial vehicles (UAVs) complemented by structure-from-motion (SfM) photogrammetry and modern per-pixel image matching algorithms can deliver a time-series of landslide elevation models in an automated and inexpensive way. This research investigates the potential of a mini UAV, equipped with a Panasonic Lumix DMC-LX5 compact camera, to provide surface deformations at acceptable levels of accuracy for landslide assessment. The study adopts a selfcalibrating bundle adjustment-SfM pipeline using ground control points (GCPs). It evaluates misalignment biases and unresolved systematic errors that are transferred through the SfM process into the derived elevation models. To cross-validate the research outputs, results are compared to benchmark observations obtained by standard surveying techniques. The data is collected with 6 cm ground sample distance (GSD) and is shown to achieve planimetric and vertical accuracy of a few centimetres at independent check points (ICPs). The co-registration error of the generated elevation models is also examined in areas of stable terrain. Through this error assessment, the study estimates that the vertical sensitivity to real terrain change of the tested landslide is equal to 9 cm.

Landslides represent hazardous phenomena, often with significant implications. Monitoring landslides with time‐series surface observations can indicate surface failure. Unmanned aerial vehicles (UAVs) employing compact digital cameras, in conjunction with structure‐from‐motion (SfM) and multi‐view stereo (MVS) image processing approaches, have become commonplace in the geoscience research community. These methods offer relatively low‐cost, flexible solutions for many geomorphological monitoring applications. However, conventionally ground control points (GCPs) are required for registration purposes, the provision of which is often expensive, difficult or even impracticable in hazardous and inaccessible terrain. In an attempt to overcome the reliance on GCPs, this paper reports research that has developed a morphology‐based strategy to co‐register multi‐temporal UAV‐derived products. It applies the attribute of curvature in combination with the scale‐invariant feature transform algorithm, to generate time‐invariant curvature features, which serve as pseudo‐GCPs. Openness, a surface morphological digital elevation model derivative, is applied to identify relatively stable ground regions from which pseudo‐GCPs are selected. A sensitivity threshold quantifies the minimum detectable change alongside unresolved biases and misalignment errors. The approach is evaluated at two study sites in the UK, first at Sandford with artificially induced surface change, and second at an active landslide at Hollin Hill, with multi‐epoch SfM‐MVS products derived from a consumer‐grade UAV. Elevation changes and annual displacement rates at dm‐level are estimated, with optimal results achieved over winter periods. The morphology‐based co‐registration strategy resulted in relative error ratios (i.e. mean error divided by average flying height) in the range 1:800–2500, comparable with those reported by similar studies conducted with UAVs augmented with real time kinematic (RTK)‐Global Navigation Satellite Systems. Analysis demonstrates the potential of the morphology‐based strategy for a semi‐automatic, and practical co‐registration approach to quantify surface motion. This can ultimately complement geotechnical and geophysical investigations and support the understanding of landslide behaviour, model prediction and construction of measures for mitigating risks. © 2018 John Wiley & Sons, Ltd.

Abstract. Unmanned aerial vehicles (UAVs) can provide observations of high spatio-temporal resolution to enable operational landslide monitoring. In this research, the construction of digital elevation models (DEMs) and orthomosaics from UAV imagery is achieved using structure-from-motion (SfM) photogrammetric procedures. The study examines the additional value that the morphological attribute of "openness", amongst others, can provide to surface deformation analysis. Image-cross-correlation functions and DEM subtraction techniques are applied to the SfM outputs. Through the proposed integrated analysis, the automated quantification of a landslide's motion over time is demonstrated, with implications for the wider interpretation of landslide kinematics via UAV surveys.

Moisture-induced landslides are a global geohazard; mitigating the risk posed by landslides requires an understanding of the hydrological and geological conditions present within a given slope. Recently, numerous geophysical studies have been attempted to characterise slow-moving landslides, with an emphasis on developing geoelectrical methods as a hydrological monitoring tool. However, landslides pose specific challenges for processing geoelectrical data in long-term monitoring contexts as the sensor arrays can move with slope movements. Here we present an approach for processing long-term (over 8 years) geoelectrical monitoring data from an active slow-moving landslide, Hollin Hill, situated in Lias rocks in the southern Howardian Hills, UK. These slope movements distorted the initial setup of the monitoring array and need to be incorporated into a time-lapse resistivity processing workflow to avoid imaging artefacts. We retrospectively sourced seven digital terrain models to inform the topography of our imaging volumes, which were acquired by either Unmanned Aerial Vehicle (UAV)-based photogrammetry or terrestrial laser ranging systems. An irregular grid of wooden pegs was periodically surveyed with a global position system, from which distortions to the terrain model and electrode positions can be modelled with thin plate splines. In order to effectively model the time-series electrical resistivity images, a baseline constraint is applied within the inversion scheme; the result of the study is a time-lapse series of resistivity volumes which also incorporate slope movements. The workflow presented here should be adaptable for other studies focussed on geophysical/geotechnical monitoring of unstable slopes.