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Close-range sensors are employed to observe glaciological processes that operate over short timescales (e.g. iceberg calving, glacial lake outburst floods, diurnal surface melting). However, under poor weather conditions optical instruments fail while the operation of radar systems below 17 GHz do not have sufficient angular resolution to map glacier surfaces in detail. This letter reviews the potential of millimetre-wave radar at 94 GHz to obtain high-resolution 3-D measurements of glaciers under most weather conditions. We discuss the theory of 94 GHz radar for glaciology studies, demonstrate its potential to map a glacier calving front and summarise future research priorities. [ABSTRACT FROM AUTHOR]

Volcanic plumes pose a hazard to health and society and a particular risk for aviation. Hazard mitigation relies on forecasting plume dispersion within the atmosphere over time. The accuracy of forecasts depends on our understanding of particle dispersion and sedimentation processes, as well as on the accuracy of model input parameters, such as the initial particle size distribution and concentrations of volcanic particles (i.e., volcanic ash) in the atmosphere. However, our understating of these processes and the accurate quantification of input parameters remain the main sources of uncertainty in plume dispersion modeling. It is usually impractical to sample volcanic plumes directly, but particle sedimentation can be constrained in the laboratory. Here, we describe the design of a new experimental apparatus for investigating the dynamics of free-falling volcanic particles. The apparatus can produce a sustained column of falling particles with variable particle concentrations appropriate to a volcanic plume. Controllable experimental parameters include particle size distributions, types, and release rates. A laser-illuminated macrophotography system allows imaging of in-flight particles and their interactions. The mass of landing particles is logged to inform deposition rates. Quantitative measurements include particle morphology characterization, settling velocities, flow rates, and estimation of concentrations. Simultaneous observations of particle interaction processes and settling dynamics through direct control over a wide range of parameters will improve our parameterization of volcanic plume dynamics. Although the apparatus has been specifically designed for volcanological investigations, it can also be used to explore the characteristics of free-falling particle columns occurring in both environmental and industrial settings. [ABSTRACT FROM AUTHOR]

Volcanic ash advisories are produced by specialised forecasters who combine several sources of observational data and volcanic ash dispersion model outputs based on their subjective expertise. These advisories are used by the aviation industry to make decisions about where it is safe to fly. However, both observations and dispersion model simulations are subject to various sources of uncertainties that are not represented in operational forecasts. Quantification and communication of these uncertainties are fundamental for making more informed decisions. Here, we develop a data assimilation method that combines satellite retrievals and volcanic ash transport and dispersion model (VATDM) output, considering uncertainties in both data sources. The methodology is applied to a case study of the 2019 Raikoke eruption. To represent uncertainty in the VATDM output, 1000 simulations are performed by simultaneously perturbing the eruption source parameters, meteorology, and internal model parameters (known as the prior ensemble). The ensemble members are filtered, based on their level of agreement with the ash column loading, and their uncertainty, of the Himawari–8 satellite retrievals, to produce a constrained posterior ensemble. For the Raikoke eruption, filtering the ensemble skews the values of mass eruption rate towards the lower values within the wider parameters ranges initially used in the prior ensemble (mean reduces from 1 to 0.1 Tg h -1). Furthermore, including satellite observations from subsequent times increasingly constrains the posterior ensemble. These results suggest that the prior ensemble leads to an overestimate of both the magnitude and uncertainty in ash column loadings. Based on the prior ensemble, flight operations would have been severely disrupted over the Pacific Ocean. Using the constrained posterior ensemble, the regions where the risk is overestimated are reduced, potentially resulting in fewer flight disruptions. The data assimilation methodology developed in this paper is easily generalisable to other short duration eruptions and to other VATDMs and retrievals of ash from other satellites. [ABSTRACT FROM AUTHOR]

The deformation style of active volcanoes can provide insight into the structural evolution of their edifices, volcanic activity and associated hazards. The Somma‐Vesuvius volcano is considered one of the most dangerous on the planet due to its proximity to the megacity of Naples (Southern Italy). Thus, understanding its deformation style and corresponding long‐term structural evolution are critical aspects for risk reduction. Although a large amount of data has already been collected about Somma‐Vesuvius, the deformation style affecting its volcanic edifice is still debated. Therefore, we devised an integrated approach to clarify the current state of deformation of this volcano. In particular, we combined analog experiments and finite element (FE) modeling to constrain the current deformation style affecting Somma‐Vesuvius and determine the physical parameters controlling its structural evolution. The analog models were built at a scale of 1:100,000 using sand mixtures (brittle analog) and polydimethylsiloxane (ductile analog). The FE models were implemented by considering a three‐dimensional time‐dependent fluid‐dynamic approach performed at both the analog model scale (1:100,000) and actual volcano scale (1:1). We obtained an FE model and a corresponding analog one that faithfully reproduced the observed deformation velocity patterns revealed by differential interferometric synthetic aperture radar (DInSAR) and GPS measurements at Somma‐Vesuvius. Overall, our results support the hypothesis that a combined gravitational spreading‐sagging process governs the deformation style of Somma‐Vesuvius. Plain Language Summary: Volcanic edifices of sufficient mass are capable of deforming substrata under their own weight; this deformation in turn can deform the volcanic edifices themselves. Identifying the deformation style characterizing a volcanic edifice is useful when considering the evolution of its volcanic activity. Vesuvius is considered one of the most dangerous volcanoes on the planet due to its proximity to the megacity of Naples (Southern Italy). Thus, understanding its deformation style and corresponding structural evolution are critical aspects for risk reduction. In order to analyze the deformation process of Vesuvius we used two different modeling techniques: analog modeling and Finite Element numerical modeling. The analog modeling approach allows us to reproduce real processes by using scaled models and media considered analog to natural materials under a physical point of view. The combination of analog and numerical modeling allowed us to constrain the current deformation style affecting Somma‐Vesuvius and to determine the physical parameters controlling its structural evolution. Finally, we compared our results with the observed deformation velocity patterns revealed by Differential Interferometric Synthetic Aperture Radar and GPS measurements at Vesuvius. Overall, the results support the hypothesis that a combined gravitational spreading‐sagging process governs the deformation style of Somma‐Vesuvius. Key Points: Analog and numerical modeling highlighted an active spreading‐sagging process at the Somma‐Vesuvius volcanoA comparison of models with differential interferometric synthetic aperture radar (DInSAR) deformation data validated the modeling proceduresThe spreading process affecting the Vesuvius volcano allowed inferring about the near‐future eruption style [ABSTRACT FROM AUTHOR]

Volcanic ash advisories are produced by specialised forecasters who combine several sources of observational data and volcanic ash dispersion model outputs based on their subjective expertise. These advisories are used by the aviation industry to make decisions about where it is safe to fly. However, both observations and dispersion model simulations are subject to various sources of uncertainties that are not represented in operational forecasts. Quantification and communication of these uncertainties are fundamental for making more informed decisions. Here, we develop a data assimilation technique which combines satellite retrievals and volcanic ash transport and dispersion model (VATDM) output, considering uncertainties in both data sources. The methodology is applied to a case study of the 2019 Raikoke eruption. To represent uncertainty in the VATDM output, 1000 simulations are performed by simultaneously perturbing the eruption source parameters, meteorology and internal model parameters (known as the prior ensemble). The ensemble members are filtered, based on their level of agreement with Himawari satellite retrievals of ash column loading, to produce a posterior ensemble that is constrained by the satellite data and its uncertainty. For the Raikoke eruption, filtering the ensemble skews the values of mass eruption rate towards the lower values within the wider parameters ranges initially used in the prior ensemble (mean reduces from 1 Tg h-1 to 0.1 Tg h-1). Furthermore, including satellite observations from subsequent times increasingly constrains the posterior ensemble. These results suggest that the prior ensemble leads to an overestimate of both the magnitude and uncertainty in ash column loadings. Based on the prior ensemble, flight operations would have been severely disrupted over the Pacific Ocean. Using the constrained posterior ensemble, the regions where the risk is overestimated are reduced potentially resulting in fewer flight disruptions. The data assimilation methodology developed in this paper is easily generalisable to other short duration eruptions and to other VATDMs and retrievals of ash from other satellites. [ABSTRACT FROM AUTHOR]

Unmanned aerial vehicles (UAVs) and structure‐from‐motion photogrammetry enable detailed quantification of geomorphic change. However, rigorous precision‐based change detection can be compromised by survey accuracy problems producing systematic topographic error (e.g. 'doming'), with error magnitudes greatly exceeding precision estimates. Here, we assess survey sensitivity to systematic error, directly correcting topographic data so that error magnitudes align more closely with precision estimates. By simulating conventional grid‐style photogrammetric aerial surveys, we quantify the underlying relationships between survey accuracy, camera model parameters, camera inclination, tie point matching precision and topographic relief, and demonstrate a relative insensitivity to image overlap. We show that a current doming‐mitigation strategy of using a gently inclined (<15°) camera can reduce accuracy by promoting a previously unconsidered correlation between decentring camera lens distortion parameters and the radial terms known to be responsible for systematic topographic error. This issue is particularly relevant for the wide‐angle cameras often integrated into current‐generation, accessible UAV systems, frequently used in geomorphic research. Such systems usually perform on‐board image pre‐processing, including applying generic lens distortion corrections, that subsequently alter parameter interrelationships in photogrammetric processing (e.g. partially correcting radial distortion, which increases the relative importance of decentring distortion in output images). Surveys from two proglacial forefields (Arolla region, Switzerland) showed that results from lower‐relief topography with a 10°‐inclined camera developed vertical systematic doming errors > 0·3 m, representing accuracy issues an order of magnitude greater than precision‐based error estimates. For higher‐relief topography, and for nadir‐imaging surveys of the lower‐relief topography, systematic error was < 0·09 m. Modelling and subtracting the systematic error directly from the topographic data successfully reduced error magnitudes to values consistent with twice the estimated precision. Thus, topographic correction can provide a more robust approach to uncertainty‐based detection of event‐scale geomorphic change than designing surveys with small off‐nadir camera inclinations and, furthermore, can substantially reduce ground control requirements. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd [ABSTRACT FROM AUTHOR]

Calculation of the sensible and latent heat (turbulent) fluxes is required in order to close the surface energy budget of glaciers and model glacial melt. The aerodynamic roughness length, z0, is a key parameter in the bulk approach to calculating sensible heat flux; yet, z0 is commonly considered simply as a tuning parameter or generalized between surfaces and over time. Spatially and temporally distributed observations of z0 over ice are rare. Both direct (from wind towers and sonic anemometers) and indirect (from microtopographic surveys) measurements of z0 are subject to sensitivities and uncertainties that are often unstated or overlooked. In this study, we present a quantitative evaluation of aerodynamic profile‐based and microtopographic methods and their effect on z0 using data collected from Storglaciären and Sydöstra Kaskasatjäkkaglaciären, Tarfala Valley, Arctic Sweden. Aggressive data filters discard most of the wind tower data but still produce realistic z0 values of 1.9 mm and 2 mm. Despite uncertainty introduced by scale and resolution dependence, microtopographic methods produced estimates of z0 comparable to wind tower values and those found on similar surfaces. We conclude that (1) in the absence of direct turbulent flux measurements from sonic anemometers, the profile and microtopographic methods provide realistic z0 values, (2) both 2D and 3D microtopographic methods are dependent on scale, resolution, and the chosen detrending method, and (3) careful calibration of these parameters could enable glacier‐wide investigations of z0 from remotely sensed data, including those increasingly available from satellite platforms. Key Points: Sensitivities and uncertainties in glacial aerodynamic roughness reviewed and analyzed using new field dataWind profile and microtopographic methods provide realistic values of z0 in absence of more robust eddy covariance dataDespite scale/resolution dependency, microtopographic estimates show good agreement with profiles and could be used to upscale studies [ABSTRACT FROM AUTHOR]

As a topographic modelling technique, structure‐from‐motion (SfM) photogrammetry combines the utility of digital photogrammetry with a flexibility and ease of use derived from multi‐view computer vision methods. In conjunction with the rapidly increasing availability of imagery, particularly from unmanned aerial vehicles, SfM photogrammetry represents a powerful tool for geomorphological research. However, to fully realize this potential, its application must be carefully underpinned by photogrammetric considerations, surveys should be reported in sufficient detail to be repeatable (if practical) and results appropriately assessed to understand fully the potential errors involved. To deliver these goals, robust survey and reporting must be supported through (i) using appropriate survey design, (ii) applying suitable statistics to identify systematic error (bias) and to estimate precision within results, and (iii) propagating uncertainty estimates into the final data products. © 2019 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Intertidal bars are common in mesotidal/macrotidal low‐to‐moderate energy coastal environments and an understanding of their morphodynamics is important from the perspective of both coastal scientists and managers. However, previous studies have typically been limited by considering bar systems two‐dimensionally, or with very limited alongshore resolution. This article presents the first multi‐annual study of intertidal alongshore bars and troughs in a macrotidal environment using airborne LiDAR (light detection and ranging) data to extract three‐dimensional (3D) bar morphology at high resolution. Bar and trough positions are mapped along a 17.5 km stretch of coastline in the northwest of England on the eastern Irish Sea, using eight complete, and one partial, LiDAR surveys spanning 17 years. Typically, 3–4 bars are present, with significant obliquity identified in their orientation. This orientation mirrors the alignment of waves from the dominant south‐westerly direction of wave approach, undergoing refraction as they approach the shoreline. Bars also become narrower and steeper as they migrate onshore, in a pattern reminiscent of wave shoaling. This suggests that the configuration of the bars is being influenced by overlying wave activity. Net onshore migration is present for the entire coastline, though rates vary alongshore, and periods of offshore migration may occur locally, with greatest variability between northern and southern regions of the coastline. This work highlights the need to consider intertidal bar systems as 3D, particularly on coastlines with complex configurations and bathymetry, as localized studies of bar migration can overlook 3D behaviour. Furthermore, the wider potential of LiDAR data in enabling high‐resolution morphodynamic studies is clear, both within the coastal domain and beyond. © 2019 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We present the evolution over 3 months of a 2016–2017 pāhoehoe flow at Kīlauea as it changed from a narrow sheet flow into a compound lava field fed by a stable system of tubes. The portion of the flow located on Kīlauea's coastal plain was characterized using helicopter‐based visible and thermal structure‐from‐motion photogrammetry to construct a series of georeferenced digital surface models and thermal maps on eight different days. Results reveal key influences on the emplacement and evolution of such long‐lived pāhoehoe flows. This region of the flow grew by ~12 × 106 m3 with a near‐constant time‐average discharge rate of 1.2–2.7 m3/s. The development of two tube systems is captured and shows an initial nascent tube enhanced by a narrow topographic confinement, which later inflated and created a topographic inversion that modulated the emplacement of a second flow lobe with its own tube system. The analysis of breakouts at various stages of the field's life suggests that the evolution of the thermal and morphological properties of the flow surface reflect its maturity. Thermal properties of breakouts were used to expand the empirical relationship of breakout cooling to longer timescales. This study contributes to the long‐term development and validation of more accurate predictive models for pāhoehoe, required during the management of long‐lasting lava flow crises in Hawai'i and elsewhere. Key Points: We document the first 3 months of a pāhoehoe flow, from its initial crust‐free channel into a compound field fed by a tube systemVisible and thermal aerial images describe the evolution of the morphology and surface temperature of the flow at various datesResults capture the relationship with topography and the partitioning of lava between the flow front, lateral breakouts, and flow inflation [ABSTRACT FROM AUTHOR]

Peatlands are important reserves of terrestrial carbon and biodiversity, and given that many peatlands across the UK and Europe exist in a degraded state, their conservation is a major area of concern and a focus of considerable research. Aerial surveys are valuable tools for habitat mapping and conservation and provide useful insights into their condition. We investigate how SfM photogrammetry-derived topography and habitat classes may be used to construct an estimate of carbon loss from erosion features in a remote blanket bog habitat. An autonomous, unmanned, aerial, fixed-wing remote sensing platform (Quest UAV 300™) collected imagery over Moor House, in the Upper Teesdale National Nature Reserve, a site with a high degree of peatland erosion. The images were used to generate point clouds into orthomosaics and digital surface models using SfM photogrammetry techniques, georeferenced and subsequently used to classify vegetation and peatland features. A classification of peatbog feature types was developed using a random forest classification model trained on field survey data and applied to UAV-captured products including the orthomosaic, digital surface model and derived surfaces such as topographic index, slope and aspect maps. Using the area classified as eroded peat and the derived digital surface model, we estimated a loss of 438 tonnes of carbon from a single gully. The UAV system was relatively straightforward to deploy in such a remote and unimproved area. SfM photogrammetry, imagery and random forest modelling obtained classification accuracies of between 42% and 100%, and was able to discern between bare peat, saturated bog and sphagnum habitats. This paper shows what can be achieved with low-cost UAVs equipped with consumer grade camera equipment and relatively straightforward ground control, and demonstrates their potential for the carbon and peatland conservation research community. [ABSTRACT FROM AUTHOR]

Structure-from-motion (SfM) photogrammetry techniques are now widely available to generate digital terrain models (DTMs) from optical imagery, providing an alternative to costlier options such as LiDAR or satellite surveys. SfM could be a useful tool in hazard studies because its minimal cost makes it accessible even in developing regions and its speed of use can provide updated data rapidly in hazard-prone regions. Our study is designed to assess whether crowd-sourced SfM data is comparable to an industry standard LiDAR dataset, demonstrating potential real-world use of SfM if employed for disaster risk reduction purposes. Three groups with variable SfM knowledge utilized 16 different camera models, including four camera phones, to collect 1001 total photos in one hour of data collection. Datasets collected by each group were processed using VisualSFM, and the point densities, accuracies and distributions of points in the resultant point clouds (DTM skeletons) were compared. Our results show that the point clouds are resilient to inconsistency in users' SfM knowledge: crowd-sourced data collected by a moderately informed general public yields topography results comparable in data density and accuracy to those produced with data collected by highly-informed SfM users or experts using LiDAR. This means that in a real-world scenario involving participants with a diverse range of expertise, topography models could be produced from crowd-sourced data quite rapidly and to a very high standard. This could be beneficial to disaster risk reduction as a relatively quick, simple and low-cost method to attain rapidly updated knowledge of terrain attributes, useful for the prediction and mitigation of many natural hazards. [ABSTRACT FROM AUTHOR]

An editorial is presented which discusses article related to photogrammetry in geography and the geosciences. Topics discussed include photogrammetry, a technique by which volumetric information describing surface structures; Structure from motion (FM) photogrammetry uses advances in computer vision algorithms; and conclude by evidencing the various ways in which SfM photogrammetric methods offer other advantages over terrestrial laser scanning for such applications.

Digital photogrammetry has experienced rapid development regarding the technology involved and its ease of use over the past two decades. We revisit the work of Jim Chandler who in 1999 published a technical communication seeking to familiarise novice users of photogrammetric methods with important theoretical concepts and practical considerations. In doing so, we assess considerations such as camera calibration and the need for photo-control and check points, as they apply to modern software and workflows, in particular for structure-from-motion (SfM) photogrammetry. We also highlight the implications of lightweight drones being the new platform of choice for many photogrammetry-based studies in the geosciences. Finally, we present three examples based on our own work, showing the opportunities that SfM photogrammetry offers at different scales and systems: at the micro-scale for monitoring geomorphological change, and at the meso-scale for hydrological modelling and the reconstruction of vegetation canopies. Our examples showcase developments and applications of photogrammetry which go beyond what was considered feasible 20 years ago and indicate future directions that applications may take. Nevertheless, we demonstrate that, in-line with Chandler's recommendations, the pre-calibration of consumer-grade cameras, instead of relying entirely on self-calibration by software, can yield palpable benefits in micro-scale applications and that measurements of sufficient control points are still central to generating reproducible, high-accuracy products. With the unprecedented ease of use and wide areas of application, scientists applying photogrammetric methods would do well to remember basic considerations and seek methods for the validation of generated products. [ABSTRACT FROM AUTHOR]

Terrestrial laser scanning (TLS) and structure-from-motion photogrammetry (SfMp) offer rapid, non-invasive surveying of in situ gravels. Numerous studies have used the point clouds derived from TLS or SfMp to quantify surface layer characteristics, but direct comparison of the methods for grain-scale analysis has received relatively little attention to date. Comparing equivalent products of different data capture methods is critical as differences in errors and sampling biases between the two methods may produce different outputs, effecting further analysis. The sampling biases and errors related to SfMp and TLS lead to differences in the point clouds produced by each method. The metrics derived from the point clouds are therefore likely to differ, potentially leading to different inputs for entrainment threshold models, different trends in surface layer development being identified and different trajectories for physical processes and habitat quality being predicted. This paper provides a direct comparison between TLS and SfMp surveys of an exposed gravel bar for three different survey periods following inundation and reworking of the bar surface during high flow events. The point clouds derived from the two methods are used to describe changes in the character of the surface layer between bar inundation events, and comparisons are made with descriptions derived from conventional pebble counts. The results found differences in the metrics derived using each method do exist, but the grid resolution used to detrend the surfaces and identify spatial variations in surface layer characteristics had a greater impact than survey method. Further research is required to understand the significance of these variations for quantifying surface texture and structure and for predicting entrainment thresholds and transport rates. [ABSTRACT FROM AUTHOR]

Piton de la Fournaise is one of the world's most active and visited volcanoes. Its summit crater (Cratère Dolomieu), the main tourist attraction, underwent a major caldera collapse in 2007 and its rim is not yet stabilized. In order to assess the caldera rim instability risk for visitors, we followed its structural evolution from 2007 to 2015. Using aerial photogrammetry campaigns, we mapped the unstable sites very precisely, carried out a quantitative analysis of the temporal evolution of these instabilities, and assessed the risks for visitors. Considering the 2008–2015 period, four sites close to the crater's edge showed significant horizontal ground motion (0.5–2 m), fracture widening (average of 0.3–0.56 m) and large-scale mass wasting volumes (total of 1.8±0.1 × 106 m3). We infer two different processes at work: (1) to the west and north, toppling of the basalt units occurs after periods of fracture widening due to the combined effect of magmatic intrusions and long-term inflation/deflation cycles; (2) to the south and east, parts of the caldera rim slowly slide towards the caldera centre, with significant accelerations during periods of enhanced volcanic activity (in 2008–2010 and 2014–2015). The official observation platform is the most stable zone to overlook the Cratère Dolomieu. By contrast, the most frequently visited area of the rim (northwest) outside the official platform is also the most unstable. [ABSTRACT FROM AUTHOR]

Relict landforms provide a wealth of information on the evolution of the modern landscape and climate change in the past. To improve understanding of the origin and development of these landforms we need better spatial measurements across a variety of scales. This can be challenging using conventional surveying techniques due to difficulties in landform recognition on the ground (e.g. weak visual/topographic expression) and spatially variable areas of interest. Here we explore the appropriateness of existing remote sensing datasets (aerial LiDAR and aerial photography) and newly acquired unmanned aerial vehicle (UAV) imagery of a test site on the upland of Dartmoor in SW England (Leeden Tor) for the recognition and automated mapping of relict patterned ground composed of stripes and polygons. We find that the recognition of these landforms is greatly enhanced by automated mapping using spectral two-dimensional imagery. Image resolution is important, with the recognition of elements (boulders) of <1 m maximised from the highest resolution imagery (UAV red-green-blue (RGB)) and recognition of landforms (10–100 m scale) maximised on coarser resolution aerial imagery. Topographic metrics of these low relief (0.5 m) landforms are best extracted from structure-from-motion (SfM) processed UAV true-colour imagery, and in this context the airborne LiDAR data proved less effective. Integrating automated mapping using spectral attributes and SfM-derived digital surface models from UAV RGB imagery provides a powerful tool for rapid reconnaissance of field sites to facilitate the extraction of meaningful topographic and spatial metrics that can inform on the origin of relict landform features. Care should be given to match the scale of features under consideration to the appropriate scale of datasets available. [ABSTRACT FROM AUTHOR]

Structure from Motion (SfM) is a tool being increasingly utilised in geosciences for high-resolution three-dimensional mapping of landscapes. However, a number of authors have demonstrated that broad-scale systematic deformations, in the form of 'doming' and 'bowling', can occur when applied to linear (low-amplitude, feature-limited) topographies. In such contexts, a more rigorous lens calibration and ground control point acquisition process is required, which means that application of SfM to environments such as tidal flats or desert plains can be challenging. Uncertainties in elevation models generated through SfM were investigated here in the context of the low elevation, micro-topographic environment of saltmarsh. Eight digital surface models (DSMs) were generated for a saltmarsh site in the Deben Estuary (Suffolk, UK) using imagery acquired by a low-cost consumer grade unmanned aerial system (UAS). The results provide clear illustration of the systematic bowling effect following self-calibration during bundle adjustment. This was due to poor estimations of distortion parameters in the camera model. Deformation was most pronounced when UAS-GPS data were used for georeferencing. The use of dGPS-determined ground control points improved the DSM, but did not fully mitigate the deformations. By introducing a pre-calibrated model, derived using a typical checkerboard routine, deformation was significantly mitigated. These results were tested in both the commercial Agisoft PhotoScan® and open-source Micmac software. When self-calibration was used, Micmac generated significantly more accurate DSMs because a more complex lens distortion model could be implemented. The results show that when mapping flat topographies, pre-calibration of the camera model out-performs self-calibration. However, if pre-calibration is not possible, a complex distortion model (such as Micmac's Four model) can be utilised to limit deformation. The results of the software analysis concluded there is no one-size fits all software solution, and therefore customisable open-source systems offer many potential benefits. [ABSTRACT FROM AUTHOR]

Understanding lava flow processes is important for interpreting existing lavas and for hazard assessments. Although substantial progress has been made for basaltic lavas our understanding of silicic lava flows has seen limited recent advance. In particular, the formation of lava flow breakouts, which represent a characteristic process in coolinglimited basaltic lavas, but has not been described in established models of rhyolite emplacement. Using data from the 2011-2012 rhyolite eruption of Puyehue-Cordón Caulle, Chile, we develop the first conceptual framework to classify breakout types in silicic lavas, and to describe the processes involved in their progressive growth, inflation, and morphological change. By integrating multiscale satellite, field, and textural data from Cordón Caulle, we interpret breakout formation to be driven by a combination of pressure increase (from local vesiculation in the lava flow core, as well as from continued supply via extended thermally preferential pathways) and a weakening of the surface crust through lateral spreading and fracturing. Small breakouts, potentially resulting more from local vesiculation than from continued magma supply, show a domed morphology, developing into petaloid as inflation increasingly fractures the surface crust. Continued growth and fracturing results in a rubbly morphology, with the most inflated breakouts developing into a cleft-split morphology, reminiscent of tumulus inflation structures seen in basalts. These distinct morphological classes result from the evolving relative contributions of continued breakout advance and inflation. The extended nature of some breakouts highlights the role of lava supply under a stationary crust, a process ubiquitous in inflating basalt lava flows that reflects the presence of thermally preferential pathways. Textural analyses of the Cordón Caulle breakouts also emphasize the importance of late-stage volatile exsolution and vesiculation within the lava flow. Although breakouts occur across the compositional spectrum of lava flows, the greater magma viscosity is likely to make late-stage vesiculation much more important for breakout development in silicic lavas than in basalts. Such late-stage vesiculation has direct implications for hazards previously recognized from silicic lava flows, enhancing the likelihood of flow front collapse, and explosive decompression of the lava core. [ABSTRACT FROM AUTHOR]

Measurements of glacier ice cliff evolution are sparse, but where they do exist, they indicate that such areas of exposed ice contribute a disproportionate amount of melt to the glacier ablation budget. We used Structure from Motion photogrammetry with Multi-View Stereo to derive 3-D point clouds for nine ice cliffs on Khumbu Glacier, Nepal (in November 2015, May 2016 and October 2016). By differencing these clouds, we could quantify the magnitude, seasonality and spatial variability of ice cliff retreat. Mean retreat rates of 0.30–1.49 cm d−1 were observed during the winter interval (November 2015–May 2016) and 0.74–5.18 cm d−1 were observed during the summer (May 2016–October 2016). Four ice cliffs, which all featured supraglacial ponds, persisted over the full study period. In contrast, ice cliffs without a pond or with a steep back-slope degraded over the same period. The rate of thermo-erosional undercutting was over double that of subaerial retreat. Overall, 3-D topographic differencing allowed an improved process-based understanding of cliff evolution and cliff-pond coupling, which will become increasingly important for monitoring and modelling the evolution of thinning debris-covered glaciers. [ABSTRACT FROM PUBLISHER]

Aerial image capture has become very common within the geosciences due to the increasing affordability of low-payload (<20 kg) unmanned aerial vehicles (UAVs) for consumer markets. Their application to surveying has subsequently led to many studies being undertaken using UAV imagery and derived products as primary data sources. However, image quality and the principles of image capture are seldom given rigorous discussion. In this contribution we firstly revisit the underpinning concepts behind image capture, from which the requirements for acquiring sharp, well-exposed and suitable image data are derived. Secondly, the platform, camera, lens and imaging settings relevant to image quality planning are discussed, with worked examples to guide users through the process of considering the factors required for capturing high-quality imagery for geoscience investigations. Given a target feature size and ground sample distance based on mission objectives, the flight height and velocity should be calculated to ensure motion blur is kept to a minimum. We recommend using a camera with as large a sensor as is permissible for the aerial platform being used (to maximise sensor sensitivity), effective focal lengths of 24-35 mm (to minimise errors due to lens distortion) and optimising ISO (to ensure the shutter speed is fast enough to minimise motion blur). Finally, we give recommendations for the reporting of results by researchers in order to help improve the confidence in, and reusability of, surveys through providing open access imagery where possible, presenting example images and excerpts and detailing appropriate metadata to rigorously describe the image capture process. [ABSTRACT FROM AUTHOR]

Terrestrial time-lapse photography offers insight into glacial processes through high spatial and temporal resolution imagery. However, oblique camera views complicate measurement in geographic coordinates, and lead to reliance on specific imaging geometries or simplifying assumptions for calculating parameters such as ice velocity. We develop a novel approach that integrates time-lapse imagery with multitemporal DEMs to derive full three-dimensional coordinates for natural features tracked throughout a monoscopic image sequence. This enables daily independent measurement of horizontal (ice flow) and vertical (ice melt) velocities. By combining two terrestrial laser scanner surveys with a 73 days sequence from Sólheimajökull, Iceland, variations in horizontal ice velocity of ~10% were identified over timescales of ~25 days. An overall decrease of ~3.0 m surface elevation showed asynchronous rate changes with the horizontal velocity variations, demonstrating a temporal disconnect between the processes of ice surface lowering and mechanisms of glacier movement. Our software, ‘Pointcatcher’, is freely available for user-friendly interactive processing of general time-lapse sequences and includes Monte Carlo error analysis and uncertainty in projection onto DEM surfaces. It is particularly suited for analysis of challenging oblique glacial imagery, and we discuss good features to track, both for correction of camera motion and for deriving ice velocities. [ABSTRACT FROM PUBLISHER]

During volcanic eruptions, measurements of the rate at which magma is erupted underpin hazard assessments. For eruptions dominated by the effusion of lava, estimates are often made using satellite data; here, in a case study at Mount Etna (Sicily), we make the first measurements based on terrestrial laser scanning (TLS), and we also include explosive products. During the study period (17-21 July 2012), regular Strombolian explosions were occurring within the Bocca Nuova crater, producing a ~50 m-high scoria cone and a small lava flow field. TLS surveys over multi-day intervals determined a mean cone growth rate (effusive and explosive products) of ~0.24 m3·s-1. Differences between 0.3-m resolution DEMs acquired at 10-minute intervals captured the evolution of a breakout lava flow lobe advancing at 0.01-0.03 m3·s-1. Partial occlusion within the crater prevented similar measurement of the main flow, but integrating TLS data with time-lapse imagery enabled lava viscosity (7.4 × 105 Pa·s) to be derived from surface velocities and, hence, a flux of 0.11 m3·s-1 to be calculated. Total dense rock equivalent magma discharge estimates are ~0.1-0.2 m3·s-1 over the measurement period and suggest that simultaneous estimates from satellite data are somewhat overestimated. Our results support the use of integrated TLS and time-lapse photography for ground-truthing space-based measurements and highlight the value of interactive image analysis when automated approaches, such as particle image velocimetry (PIV), fail. [ABSTRACT FROM AUTHOR]

Recent developments in low-cost structure-from-motion (SfM) technologies offer new opportunities for geoscientists to acquire high-resolution soil microtopography data at a fraction of the cost of conventional techniques. However, these new methodologies often lack easily accessible error metrics and hence are difficult to evaluate. In this research, a framework was developed to evaluate a SfM approach for soil microtopography measurement through assessment of uncertainty sources and quantification of their potential impact on overall 3D reconstruction. Standard deviations of camera interior orientation parameters estimated from SfM self-calibration within five different soil surface models were several orders of magnitude larger than precisions expected from pattern-based camera calibration. Sensitivity analysis identified the principal point position as the dominant source of calibration-induced uncertainty. Overall, surface elevation values estimated from both technologies were similar in magnitude with a root mean square ( RMS) of elevation difference of 0·2 mm. Nevertheless, the presence of deformation in either SfM or traditional photogrammetric point clouds highlights the importance of quality assurance safeguards (such as a judicious choice of control points) in SfM workflows for soil microtopography applications. [ABSTRACT FROM AUTHOR]

With the increasing role that unmanned aerial systems (UAS) are playing in data collection for environmental studies, two key challenges relate to harmonizing and providing standardized guidance for data collection, and also establishing protocols that are applicable across a broad range of environments and conditions. In this context, a network of scientists are cooperating within the framework of the Harmonious Project to develop and promote harmonized mapping strategies and disseminate operational guidance to ensure best practice for data collection and interpretation. The culmination of these efforts is summarized in the present manuscript. Through this synthesis study, we identify the many interdependencies of each step in the collection and processing chain, and outline approaches to formalize and ensure a successful workflow and product development. Given the number of environmental conditions, constraints, and variables that could possibly be explored from UAS platforms, it is impractical to provide protocols that can be applied universally under all scenarios. However, it is possible to collate and systematically order the fragmented knowledge on UAS collection and analysis to identify the best practices that can best ensure the streamlined and rigorous development of scientific products. [ABSTRACT FROM AUTHOR]

Viscosity is one of the most important physical properties controlling lava flow dynamics. Usually, viscosity is measured in the laboratory where key parameters can be controlled but can never reproduce the natural environment and original state of the lava in terms of crystal and bubble contents, dissolved volatiles, and oxygen fugacity. The most promising approach for quantifying the rheology of molten lava in its natural state is therefore to carry out direct field measurements by inserting a viscometer into the lava while it is flowing. Such in-situ syn-eruptive viscosity measurements are notoriously difficult to perform due to the lack of appropriate instrumentation and the difficulty of working on or near an active lava flow. In the field, rotational viscometer measurements are of particular value as they have the potential to measure the properties of the flow interior rather than an integration of the viscosity of the viscoelastic crust + flow interior. To our knowledge only one field rotational viscometer is available, but logistical constraints have meant that it has not been used for 20 yr. Here, we describe new viscosity measurements made using the refurbished version of this custom-built rotational viscometer, as performed on active pāhoehoe lobes from the 61G lava flow of Kilauea's Pu'u ‘Ō‘ō eruption in 2016. We successfully measured a viscosity of ∼380 Pa s at strain-rates between 1.6 and 5 s −1 and at 1144 °C. Additionally, synchronous lava sampling allowed us to provide detailed textural and chemical characterization of quenched samples. Application of current physico-chemical models based on this characterization (16 ± 4 vol.% crystals; 50 ± 6 vol.% vesicles), gave viscosity estimates that were approximately compatible with the measured values, highlighting the sensitivity of model-based viscosity estimates on the effect of deformable bubbles. Our measurements also agree on the range of viscosities in comparison to previous field experiments on Hawaiian lavas. Conversely, direct comparison with sub-liquidus rheological laboratory measurements on natural lavas was unsuccessful because recreating field conditions (in particular volatile and bubble content) is so far inaccessible in the laboratory. Our work shows the value of field rotational viscometry fully-integrated with sample characterization to quantify three-phase lava viscosity. Finally, this work suggests the need for the development of a more versatile instrument capable of recording precise measurements at low torque and low strain rate, and with synchronous temperature measurements. [ABSTRACT FROM AUTHOR]

The interpretation of geophysical measurements at active volcanoes is vital for hazard assessment and for understanding fundamental processes such as magma degassing. For Strombolian activity, interpretations are currently underpinned by first-order fluid dynamic models which give relatively straightforward relationships between geophysical signals and gas and magma flow. However, recent petrological and high-speed video evidence has indicated the importance of rheological stratification within the conduit and, here, we show that under these conditions, the straightforward relationships break down. Using laboratory analogue experiments to represent a rheologically-stratified conduit we characterise the distinct variations in the shear stress exerted on the upper sections of the flow tube and in the gas pressures measured above the liquid surface, during different degassing flow configurations. These signals, generated by varying styles of gas ascent, expansion and burst, can reflect field infrasonic measurements and ground motion proximal to a vent. The shear stress signals exhibit timescales and trends in qualitative agreement with the near-vent inflation–deflation cycles identified at Stromboli. Therefore, shear stress along the uppermost conduit may represent a plausible source of near-vent tilt, and conduit shear contributions should be considered in the interpretation of ground deformation, which is usually attributed to pressure sources only. The same range of flow processes can produce different experimental infrasonic waveforms, even for similar masses of gas escape. The experimental data resembled infrasonic waveforms acquired from different vents at Stromboli associated with different eruptive styles. Accurate interpretation of near-vent ground deformation, infrasonic signal and eruptive style therefore requires detailed understanding of: a) spatiotemporal magma rheology in the shallow conduit, and b) shallow conduit geometry, as well as bubble overpressure and volume. [ABSTRACT FROM AUTHOR]

Bubble nucleation and growth dynamics exert a primary control on the explosivity of volcanic eruptions. Numerous theoretical and experimental studies aim to capture the complex process of melt vesiculation, whereas textural studies use vesicle populations to reconstruct magma behaviour. However, post-fragmentation vesiculation in rhyolitic bombs can create final quenched bubble (vesicle) textures that are not representative of the nature of fragmenting magma within the conduit. To examine bubble growth in hydrous rhyolitic bombs, we have used heated stage microscopy to directly observe vesiculation of a Chaitén rhyolite melt (with an initial dissolved water content of ~0.95 wt %) at atmospheric pressure and magmatic temperatures upon reheating. Thin wafers of obsidian were held from 5 min up to two days in the heated stage at temperatures between 575 °C and 875 °C. We found that bubble growth rates, measured through changes in bubble diameter, increased with both temperature and bubble size. The average growth rate at the highest temperature of 875 °C is ~1.27 μm s−1, which is substantially faster than the lowest detected growth rate of ~0.02 μm s−1 at 725 °C; below this temperature no growth was observed. Average growth rate V r follows an exponential relationship with temperature, T and inferred melt viscosity η , where V r = 5.57 × 10−7e0.016 T and V r = 3270e−1.117 η . Several stages of evolving bubble morphology were directly observed, including initial relaxation of deformed bubbles into spheres, extensive growth of spheres, and, at higher temperatures, close packing and foam formation. Bubble deformation due to bubble-bubble interaction and coalescence was observed in most experiments. We use our simple, experimentally-determined relationship between melt viscosity and bubble growth rates to model post-fragmentation vesicle growth in a cooling 1 m-diameter rhyolitic bomb. The results, which indicate negligible vesicle growth within 2–3 cm of the bomb surface, correspond well with the observed dense margin thickness of a Chaitén bomb of comparable dimensions. The experiments described can be used to effectively reconstruct the post-fragmentation vesiculation history of bombs through simple analytical expressions which provide a useful tool for aiding in the interpretation of pumiceous endmember textures in hydrous rhyolitic bombs. • Suite of high temperature bubble growth experiments performed on a rhyolitic bomb from the 2008 eruption of Chaitén volcano. • Hot-stage microscopy allowed the tracing of in-situ bubble growth at different temperatures. • Bubble growth rates placed in context of a cooling volcanic bomb to determine the amount of post-fragmentation vesiculation. • Our model recreates textures observed in metre scale volcanic bombs. [ABSTRACT FROM AUTHOR]

• UAV-SfM soil loss measurements were validated by direct sediment collection. • Photogrammetric precision estimates defined spatially variable levels of detection. • DEMs of difference illustrated the diffuse erosion patterns through time. • Accurate 3-D soil surface models were constructed from UAV-SfM data. Sheet erosion is common on agricultural lands, and understanding the dynamics of the erosive process as well as the quantification of soil loss is important for both soil scientists and managers. However, measuring rates of soil loss from sheet erosion has proved difficult due to requiring the detection of relatively small surface changes over extended areas. Consequently, such measurements have relied on the use of erosion plots, which have limited spatial coverage and have high operating costs. For measuring the larger erosion rates characteristic of rill and gully erosion, structure-from-motion (SfM) photogrammetry has been demonstrated to be a valuable tool. Here, we demonstrate the first direct validation of UAV-SfM measurements of sheet erosion using sediment collection data collected from erosion plots. Three erosion plots (12 m × 4 m) located at Lavras, Brazil, with bare soil exposed to natural rainfall from which event sediment and runoff was monitored, were mapped during two hydrological years (2016 and 2017), using a UAV equipped with a RGB camera. DEMs of difference (DoD) were calculated to detect spatial changes in the soil surface topography over time and to quantify the volumes of sediments lost or gained. Precision maps were generated to enable precision estimates for both DEMs to be propagated into the DoD as spatially variable vertical uncertainties. The point clouds generated from SfM gave mean errors of ~2.4 mm horizontally (xy) and ~1.9 mm vertically (z) on control and independent check points, and the level of detection (LoD) along the plots ranged from 1.4 mm to 7.4 mm. The soil loss values obtained by SfM were significantly (p < 0.001) correlated (r2 = 95.55%) with those derived from the sediment collection. These results open up the possibility to use SfM for erosion studies where channelized erosion is not the principal mechanism, offering a cost-effective method for gaining new insights into sheet, and interrill, erosion processes. [ABSTRACT FROM AUTHOR]