Mike R James

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 multi-temporal digital elevation models to derive full 3D 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-day sequence from Sólheimajökull, Iceland, variations in horizontal ice velocity of ~10% were identified over timescales of ~25 days. An overall surface elevation decrease of ~3.0 m showed rate changes asynchronous 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 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.

Following a systematic series of laboratory experiments using different thermal imaging cameras, it is now possible to determine surface temperatures of volcanic rocks in the laboratory over a range of temperatures and viewing angles with a precision of 0.8%. Extracting useful temperature data in the field is more challenging. Current methods of dealing with pixel integrated temperatures make assumptions that

Strombolian activity produces gas-rich, magma-poor eruptions suggesting the separation and concentration of volcanic gases within the plumbing system. These gases are assumed to rise as relatively large bubble rafts or individual 'slug' bubbles that can cause detectable seismic activity on interaction with conduit geometry. Rising within the magma column, a gas bubble must expand appreciably in order to maintain magma-static pressure, for instance volume would increase by a factor of c. 200 for a 1 km rise to the magma-atmosphere interface. For a near-conduit-filling gas slug this expansion is one-dimensional (i.e. length-wise) and increases in rate non-linearly on approach to the surface. As they ascend, small gas slugs can expand sufficiently rapidly to maintain approximate magma-static pressure, but large gas slugs become dynamically overpressured. In laboratory experiments, these unsteady flows of gas and liquid generate pressure changes measurable below the gas phase. They also cause apparatus motion that does not apparently relate directly to these changes. Computational fluid dynamic (CFD) simulation of experiments reproduces the pressure changes within the liquid and allows visualisation of the viscous shear force exerted on the conduit wall around and above the slug as it rises and expands. CFD simulations at volcanic scale then give estimates of the various force contributions that could occur in the natural system. During the experiments, pressure change driven by slug expansion and burst was also measured in the ambient atmosphere above the upper liquid surface. We present experimental evidence of a range of burst processes that depend on the degree of gas overpressure in the slug. These processes range from the quiescent formation of a relatively long-lived liquid film that bursts some time after the gas slug has reached the liquid surface, through complex transitional behaviour where the meniscus detaches from the tube walls to form a bubble, to wholesale meniscus disruption when overpressure becomes appreciable. The nature of the atmospheric pressure response changes with the burst process, but is predominantly driven by two slug-depressurisation processes, namely the rise of the liquid-atmosphere interface and burst of an overpressured slug. We compare simulated results to existing seismic and acoustic measurements from a number of volcanic centres and discuss the insights this gives in understanding flow dynamics during strombolian eruptions as well as identifying where laboratory, numerical and field approaches could be more closely related to provide more accurate descriptions of the eruption process.

ABSTRACT Lava effusion rate is a critical parameter for flow models, with particular control over the potential maximum flow length attainable. However, effusion rate (or volume flux) can be extremely difficult to measure accurately in the field and is known to vary over a wide range of timescales. Here, we describe the application of computer vision and oblique photogrammetric techniques to both visible and thermal images of active aa flows in order to investigate distal flow processes at Mount Etna, Sicily, during the 2004-2005 eruption. Ground-based photogrammetric surveys were carried out (using a standard digital SLR camera) to produce repeated topographic datasets for calculation of volumetric lava flux at the flow fronts. Significant variations of the magma flux were detected (between ~0.05 and 0.35 m3s-1), and pulses of increased flux were visible in the distal channel region on timescales of several hours. The pulses are believed to result from more frequent flux changes which were observed in the vent region. They must thus also reflect the importance of some down- flow pulse coalescence process as well as short-period variations in effusion rate at the vent. Estimates of lava effusion rate can also be made from ground-based thermal data. The effect of the observed flow variations on the thermal data is described and the implications discussed in terms of observation frequency and distance. The lava flux at the vent was estimated to be around an order of magnitude larger than that at the flow fronts, suggesting that processes such as degassing, inflation and overflows were taking place in the unobserved medial part of the flow. Consequently, when considering the advance and stopping processes for these individual flow-fronts, it must be assumed that they were fed by a highly unsteady flux, which was volumetrically significantly lower than that at the vent.

ABSTRACT We describe a photogrammetric approach used to determine the rheological properties of active lava flows based on stereo image sequences. Bulk rheological properties can be estimated from measurements of flow slope, velocity and dimensions and so, at flow-fronts, they can be calculated from sequential digital elevation models (DEMs) acquired as the flow advances over new ground. For useful flow parameters to be extracted, DEMs may need to be obtained at approximately minute intervals, over durations of up to multiple hours. To deliver such data, we use oblique stereo pair sequences captured by digital SLR cameras and a semi-automated DEM-generation pipeline. Although similar data could be acquired with a terrestrial laser scanner, with deployments in remote and hazardous regions the photogrammetric approach offers significant logistical advantages in terms of reduced equipment cost, bulk, weight and power requirements. We describe the application of the technique to an active lava flow on Mount Etna, Sicily, in 2006. Image sequences were acquired from two tripod-mounted cameras over a period of ~3 hours, as the flow-front advanced ~15 m. Photogrammetric control was provided by 11 targets placed in the scene, with their coordinates determined by dGPS. The cameras were synchronised by a shutter release cable and triggered by an external timer (intervalometer). Image pairs were obtained every minute with DEMs extraction carried out on every fourth epoch; 57 DEMs, with a 0.25-m resolution, were generated. We describe the challenges associated with data collection in this remote environment and the techniques required to automate the photogrammetric analysis and sequence-DEM generation.

ABSTRACT In order to quantify soil loss through gully erosion, accurate measurements of gully volume are required. However, gullys are usually extended features, often with complex morphologies and are challenging to survey appropriately and efficiently. Here we explore the use of a photo-based technique for deriving 3D gully models suitable for detailed erosion studies. Traditional aerial and oblique close-range photogrammetry approaches have been previously used to produce accurate digital elevation models (DEMs) from photographs. However, these techniques require expertise to carry out successfully, use proprietry software and usually need apriori camera calibration. The computer vision approach we adopt here relaxes these requirements and allows 3D models to be automatically produced from collections of unordered photos. We use a freely available 'reconstruction pipeline' (http://blog.neonascent.net/archives/bundler-photogrammetry-package/) that combines structure-from-motion and multi-view stereo algorithms (SfM-MVS) to generate dense point clouds (millions of points). The model is derived from photos taken from different positions with a consumer camera and is then scaled and georeferenced using additional software (http://www.lancs.ac.uk/staff/jamesm/software/sfm_georef.htm) and observations of some control points in the scene. The approach was tested on a ~7-m long sinous gully section (average width and depth ~2.4 and 1.2 m respectively) in Vertisol soils, near Cordoba, Spain. For benchmark data, the gully topography was determined with a terrestrial laser scanner (Riegl LMS-Z420i, with a cited range accuracy of 10 mm). 191 photos were taken with a Canon EOS 450D with a prime (fixed) 28 mm lens over a period of ~10 minutes. In order to georeference the SfM-MVS model for comparison with the TLS data, 6 control targets were located around the gully and their locations determined by dGPS. Differences between the TLS and SfM-MVS surfaces are dominated by areas of data gaps from either one of the techniques with some additional issues related to small amounts of vegetation on steep gully walls. The overall difference between the surfaces represents <2 % of the gully volume surveyed. This supports the viability of SfM-MVS as a useful tool for gully evaluations, capable of producing accurate DEMs at low cost.

ABSTRACT Data for detailed digital elevation models (DEMs) are usually collected by expensive laser-based techniques, or by photogrammetric methods that require expertise and specialist software. However, recent advances in computer vision research now permit 3D models to be automatically derived from unordered collections of photographs, offering the potential for significantly cheaper and quicker DEM production. Here, we assess the accuracy of this approach for geomorphological applications using examples from a coastal cliff and a volcanic edifice. The reconstruction process is based on a combination of structure-from-motion and multi-view stereo algorithms (SfM-MVS). Using multiple photographs of a scene taken from different positions with a consumer-grade camera, dense point clouds (millions of points) can be derived. Processing is carried out by automated 'reconstruction pipeline' software downloadable from the internet, e.g. http://blog.neonascent.net/archives/bundler-photogrammetry-package/. Unlike traditional photogrammetric approaches, the initial reconstruction process does not require the identification of any control points or initial camera calibration and is carried out with little or no operator intervention. However, such reconstructions are initally un-scaled and un-oriented so additional software (http://www.lancs.ac.uk/staff/jamesm/software/sfm_georef.htm) has been developed to permit georeferencing. Although this step requires the presence of some control points or features within the scene, it does not have the relatively strict image acquisition and control requirements of traditional photogrammetry. For accuracy, and to allow error analysis, georeferencing observations are made within the image set, rather than requiring feature matching within the point cloud. In our coastal example, 133 photos taken with a Canon EOS 450D and 28 mm prime lens, from viewing distances of ~20 m, were used to reconstruct a ~60 m long section of eroding cliff. The resulting surface model was compared with data collected by a Riegl LMS-Z210ii terrestrial laser scanner. Differences between the surfaces were dominated by the varying effects of occlusions on the techniques, and systematic distortion of the SfM-MVS model along the length of the cliff could not be resolved over the ±15 mm precision of the TLS data. For a larger-scale example, a ~1.6 km wide region over the summit of Piton de la Fournaise volcano was reconstructed using 133 photos taken with a Canon EOS D60 and 20 mm prime lens, from a microlight aircraft (with a representative viewing distance of 1.0 km). In this case, the resulting DEM showed an RMS error of 1.0 m when compared with the results from traditional photogrammetry and some areas of systematic error were evident. Such errors were minimised by reprocessing the SfM-MVS results with a more sophisticated camera model than is integrated into the reconstruction pipeline. In combination, the results indicate that, with a good, convergent image set, SfM-MVS can be anticipated to deliver relative precisions of 1:1000 or better, for geomorphological applications. However, under certain conditions, the restricted camera model used can result in detectable error. We highlight the requirement for new network design tools that will help optimise image collection, facilitate error visualisation and allow a user to determine whether their image network is fit for purpose.

ABSTRACT The dynamics of obsidian lava flow emplacement remain poorly understood as active obsidian lavas are seldom seen. In contrast with well-documented basaltic lavas, we lack observational data on obsidian flow advance and temporal evolution. The ongoing silicic eruption at Puyehue-Cordón Caulle volcanic complex (PCCVC), southern Chile provides an unprecedented opportunity to witness and study obsidian lava on the move. The eruption, which started explosively on June 4th 2011, has since June 20 generated an active obsidian flow field that remains active at the time of writing (January 2012), with an area of ~6 km2, and estimated volume of ~0.18 km3. We report on observations, imaging and sampling of the north-western lava flow field on January 4th and 10th 2012, when vent activity was characterised by near-continuous ash venting and Vulcanian explosions (Schipper et al, this session) and was simultaneously feeding the advancing obsidian flow (Castro et al, this session). On January 4th the north-western lava flow front was characterised by two dominant facies: predominant rubbly lava approximately 30-40 m thick and mantled by unstable talus aprons, and smoother, thinner lobes of more continuous lava ~50 m in length that extended roughly perpendicular to the overall flow direction, forming lobes that protrude from the flow margin, and lacked talus aprons. The latter lava facies closely resembled squeeze-up structures in basaltic lava flows[1] and appeared to originate from and overlie the talus apron of the rubbly lava. Its upper surface consisted of smooth, gently folded lava domains cut by crevasse-like tension gashes. During ~2 hours of observation the squeeze-up lava lobe was the most frequent location of small-volume rockfalls, which occurred at ~1-10 minute intervals from the flow front and indicated a locus of lava advance. On January 10th the squeeze-up lava lobes had evolved significantly, with disruption and breakage of smooth continuous lava surfaces to form blocky lava domains. Gravitational collapse of lobe toes had created an incipient talus apron that had markedly advanced. In contrast, the rubbly lava had undergone only modest evolution, reflecting continued rockfall and subtle advance of its well-developed talus apron. Visualisation of the lava morphology and evolution was assisted by 3D models of the lava flow front, produced by an automated photo-reconstruction technique (SfM-MVS, a combination of structure from motion and multi-view stereo algorithms), and >1000 digital images taken at the scene. Additionally samples were collected from the rubbly lava and squeeze-up lava lobe facies. Sample textures, geochemistry and volatile concentrations will provide further insight into the evolving physical and chemical state of the lava. Our observations indicate that endogenous growth plays a major role in obsidian lava flow advance, with effective thermal insulation of lava that emerges from squeeze-ups close to the flow margin. This has important implications for the longevity, mobility and hazard potential of obsidian flows and indicates striking similarities with the dynamics of basaltic lava flow emplacement. [1]Applegarth L.J. et al. 2010 Bull. Volcanol. 72, 641-656.

ABSTRACT Collecting ground-based images of environmental scenes usually results in images with significant depth of field and strongly oblique views of the objects of interest. These factors complicate quantitative analysis by introducing large changes in scale within images and, in the case of thermal data, varying the corrections required to account for atmospheric attenuation. We report on the use of photogrammetric and machine vision techniques to combine spatial data with thermal imagery in order to allow distance, velocity and appropriately corrected thermal data to be obtained. Topographic information was determined by photogrammetry, using visible images from a consumer-grade digital SLR camera which were collected simultaneously with the thermal images. Orientation of the thermal images (i.e. calculation of camera position and pointing direction) then allowed viewing distance corrections to be applied to the thermal images on a pixel-by-pixel basis. Rectification of the corrected images allows them to be presented in geographic coordinates and therefore facilitates correlation with other datasets for analysis. Examples are given from the 2004 to 2005 eruption of Mount Etna, Sicily. Images were collected from flow front regions just south of Monte Centenari, approximately 2 km from the active vent. From the viewing positions occupied, the lava flowed towards the camera, with distances to the flow being between ~50 and 400 m. We describe the viewing distance corrections applied to the thermal data and illustrate how sequence analysis in a 3D spatial context can be used to determine flow profiles and lava flux rates. Significant variations in flux can be correlated with periods of levee building, breaching and ogive formation.

The early stages of effusive volcanic eruptions, during which lava flows are lengthening, are often closely monitored for hazard management. Processes involved in lengthening are therefore relatively well understood, and lava flow development during this phase can be modelled with some success[1,2]. However, activity may continue after the lavas have reached their maximum length, leading to flow inflation, breakouts and

The AVTIS remote sensing instrument is a custom built millimetre wave sensor that has been developed as a practical field tool for remote sensing of volcanic terrain at active lava domes. We present validation of the MMW radiometry of a volcanic scene by comparison with coincident infrared imagery.

In order to increase our understanding of the processes involved in the evolution of lava flow fields, detailed and frequent assessments of the activity and the topographic change involved are required. Although topographic data of sufficient accuracy and resolution can be acquired by airborne lidar, the cost and logistics generally prohibit repeats at the daily (or more frequent) intervals necessary

ABSTRACT We report on the use of the All-weather Volcano Topography Imaging Sensor (AVTIS) 94 GHz dual mode radar/radiometric imager for environmental monitoring. The FMCW radar yields 3D maps of the terrain whilst the passive radiometer records brightness temperature maps of the scene. AVTIS is a low power portable instrument and has been used operationally to survey terrain at ranges up to 6 km. AVTIS was originally developed for the ground-based measurement of active volcanoes and has been used successfully to measure the Arenal Volcano in Costa Rica and the Soufrière Hills Volcano on Montserrat. However, additional environmental remote sensing applications are emerging for this technology and we will present details of how the instrument is used to perform terrain mapping and thermal surveys of outdoor scenes. The extraction of digital elevation maps is the primary function of the AVTIS radar mode. We review this process covering range drift compensation, radar cross section (RCS) histogram analysis and thresholding, and georeferencing to GPS. Additionally, we will present how careful calibration enables RCS imaging of terrain and the extraction of the intrinsic reflectivity of the terrain material (normalized RCS, or sigma-nought) which can potentially be used to classify terrain types. We have validated the passive mode imagery against infrared thermal imagery and they show good agreement once the differences in spatial resolution are accounted for. This comparison also reveals differences in propagation due to obscurants (steam, gas, ash) in the two wavebands.

Acquiring accurate digital elevation models (DEMs) of growing lava domes is critical for hazard assessment. However, most techniques require expertise and time (eg photogrammetry) or expensive equipment (eg laser scanning and radar-based techniques ...

The tiny Caribbean island of Montserrat has been in a state of crisis since the Soufrière Hills Volcano (SHV) began its current eruption in July 1995. With its main town, Plymouth, destroyed by pyroclastic flows in 1997, the islanders who have remained have had to rebuild their society on the northern half of the island under varying degrees of threat

Understanding the processes involved with the advance of lava flows is critical for improving hazard assessments at many volcanoes. Here, we describe the application of computer vision and oblique photogrammetric techniques to visible and thermal images of active lavas in order to investigate flow processes at Mount Etna, Sicily and in Hawaii. Photogrammetric surveys were carried out to produce repeated topographic datasets for calculation of volumetric lava flux at the flow-fronts. Photogrammetry is an established technique for the investigation of change in landform over time, relying typically on vertical aerial imagery or more unusually, on oblique imagery from aircraft or terrestrial platforms (1). This extended abstract describes experiences in processing data from terrestrial digital photogrammetric surveys of lava flows acquired with digital photogrammetric SLR cameras at a number of sites which have active lava flows. In each case the objective was to ascertain flow evoluti...

In order to improve our understanding of how lavas flow, cool and stop, accurate and frequent DEMs and associated temperature measurements of active flows are required. We describe the use of terrestrial photogrammetric techniques which allow detailed measurements to be carried out rapidly, frequently and over relevant spatial scales. Furthermore, the equipment required is sufficiently small and light to be easily deployed in remote areas. Images of lava flows from Etna (Sicily) and Hawai'i have been acquired, representing cases involving different length scales, observation distances and advance rates. On Etna, flow-front regions and distal channels of aa flows were studied over distances of up to 400 m. Advance rates were relatively slow (< 4 m hr-1) over flow-fronts ~7 m in height and up to ~30 m in width. The slow rate of change allowed topographic surfaces to be constructed from images collected from multiple locations using a single camera. Sequential surfaces were use...

Understanding the way that lavas cool is an important step in the development and validation of models of lava flows, and thermal imaging cameras are playing an important role in this investigation. Recent technological advances have led to the development of uncooled thermal imaging cameras, increases in data-acquisition rates and a new generation of high frame rate cameras. During August 2004, a hand-held FLIR ThermaCAM S40 was used to collect several thousand 30 Hz thermal images from 4 active lava flows on the Pulama pali fault scarp, Kilauea volcano, Hawaii. Short observation distances (3 to 10 m) resulted in the collection of high spatial (4-13 mm) resolution thermal data. The images acquired cover the formation, cooling, inflation and budding of new pahoehoe lobes and the formation of a range of surface textures on a small (35 m long) open channel flow. The images reveal the temperatures at which these flows developed surface textures such as a ductile crust and ropes, togeth...

Abstract Many explosive terrestrial volcanic eruptions are accompanied by lightning and other atmospheric electrical phenomena. The plumes produced generate large perturbations in the surface atmospheric electric potential gradient and high charge densities have been measured on falling volcanic ash particles. The complex nature of volcanic plumes (which contain gases, solid particles, and liquid drops) provides several possible charging mechanisms. For plumes rich in solid silicate particles, fractoemission ( ...

Page 1. Journal of the Geological Society, London, Vol. 155, 1998, pp. 587–590. Printed in Great Britain By monitoring perturbations of the natural atmospheric electric potential gradient, it is possible to detect and track particle laden volcanic plumes. ...

ABSTRACT Electrostatic phenomena have long been associated with the explosive eruption of volcanoes. Lightning generated in volcanic plumes is a spectacular atmospheric electrical event that requires development of large potential gradients over distances of up to kilometres. This process begins as hydrated liquid rock (magma) ascends towards Earth's surface. Pressure reduction causes water supersaturation in the magma and the development of bubbles of supercritical water, where deeper than c. 1000 m, and water vapour at shallower depths that drives flow expansion. The generation of high strain rates in the expanding bubbly magma can cause it to fracture in a brittle manner, as deformation relaxation timescales are exceeded. The brittle fracture provides the initial charge separation mechanism, known as fractoemission [1]. The resulting mixture of charged silicate particles and ions evolves over time, generating macro-scale potential gradients in the atmosphere and driving processes such as particle aggregation. For the silicate particles, aggregation driven by electrostatic effects is most significant for particles smaller than c. 100 μm. Aggregation acts to change the effective aerodynamic behaviour of silicate particles [2], thus altering the sedimentation rates of particles from volcanic plumes from the atmosphere. The presence of liquid phases also promotes aggregation processes [3] and lightning.

Title: Comment on ``It takes three to tango: 2. Bubble dynamics in basaltic volcanoes and ramifications for modeling normal Strombolian activity'' by J. Suckale, BH Hager, LT Elkins-Tanton, and J.-C. Nave. Authors: James, Mike R.; Llewellin, Edward W.; Lane, Stephen J. ...

The following exercise was compiled as part of the IAVCEI ‘Drone’ workshop, held on 13th August, 2017 in Portland, USA. Completing the exercise should enable you to:
• Process UAV image data in PhotoScan to create DEMs and orthomosaics.
• Refine your SfM processing approach to increase reproducibility through rigorous consideration of outliers, error, appropriate camera models etc.
• Discuss processed results in terms of measurement precision, and identify appropriate solutions for increasing survey performance if required.

Quantifying the extent of soil erosion at a fine spatial resolution can be time consuming and costly; however, proximal remote sensing approaches to collect topographic data present an emerging alternative for quantifying soil volumes lost via erosion. Herein we compare terrestrial laser scanning (TLS), and both unmanned aerial vehicle (UAV) and ground photography (GP) structure-from-motion (SfM) derived topography. We compare the cost-effectiveness and accuracy of both SfM techniques to TLS for erosion gully surveying in upland landscapes, treating TLS as a benchmark. Further, we quantify volumetric soil loss estimates from upland gullies using digital surface models derived by each technique and subtracted from an interpolated pre-erosion surface. Soil loss estimates from UAV and GP SfM reconstructions were comparable to those from TLS, whereby the slopes of the relationship between all three techniques were not significantly different from 1:1 line. Only for the TLS to GP comparison was the intercept significantly different from zero, showing that GP is more capable of measuring the volumes of very small erosion features. In terms of cost-effectiveness in data collection and processing time, both UAV and GP were comparable with the TLS on a per-site basis (13.4 and 8.2 person-hours versus 13.4 for TLS); however, GP was less suitable for surveying larger areas (127 person-hours per ha(-1) versus 4.5 for UAV and 3.9 for TLS). Annual repeat surveys using GP were capable of detecting mean vertical erosion change on peaty soils. These first published estimates of whole gully erosion rates (0.077 m a(-1)) suggest that combined erosion rates on gully floors and walls are around three times the value of previous estimates, which largely characterize wind and rainsplash erosion of gully walls. Copyright (c) 2017 John Wiley \& Sons, Ltd.

Structure-from-motion (SfM) photogrammetry is revolutionising the collection of detailed topographic data, but insight into geomorphological processes is currently restricted by our limited understanding of SfM survey uncertainties. Here, we present an approach that, for the first time, specifically accounts for the spatially variable precision inherent to photo-based surveys, and enables confidence-bounded quantification of 3D topographic change. The method uses novel 3D precision maps that describe the 3D photogrammetric and georeferencing uncertainty, and determines change through an adapted state-of-the-art fully 3D point-cloud comparison (M3C2), which is particularly valuable for complex topography. We introduce this method by: (1) using simulated UAV surveys, processed in photogrammetric software, to illustrate the spatial variability of precision and the relative influences of photogrammetric (e.g. image network geometry, tie point quality) and georeferencing (e.g. control measurement) considerations; (2) we then present a new Monte Carlo procedure for deriving this information using standard SfM software and integrate it into confidence-bounded change detection; before (3) demonstrating geomorphological application in which we use benchmark TLS data for validation and then estimate sediment budgets through differencing annual SfM surveys of an eroding badland. We show how 3D precision maps enable more probable erosion patterns to be identified than existing analyses, and how a similar overall survey precision could have been achieved with direct survey georeferencing for camera position data with precision half as good as the GCPs'. Where precision is limited by weak georeferencing (e.g. camera positions with multi-metre precision, such as from a consumer UAV), then overall survey precision can scale as n(-1/2) of the control precision (n=number of images). Our method also provides variance-covariance information for all parameters. Thus, we now open the door for SfM practitioners to use the comprehensive analyses that have underpinned rigorous photogrammetric approaches over the last half-century. Copyright (c) 2017 John Wiley \& Sons, Ltd.

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.

Strombolian volcanism is a ubiquitous form of activity, driven by the ascent and bursting of bubbles of slug morphology. Whilst considerable attention has been devoted to understanding the behaviour of individual slugs in this regime, relatively little is known about how inter-slug interactions modify flow conditions. Recently, we reported on high temporal frequency strombolian activity on Etna, in which the larger erupted slug masses were followed by longer intervals before the following explosion than the smaller bursts (Pering et al., 2015). We hypothesised that this behaviour arose from the coalescence of ascending slugs causing a prolonged lag before arrival of the next distinct bubble. Here we consider the potential importance of inter-slug interactions for the dynamics of strombolian volcanism, by reporting on the first study into the behaviour of trains of ascending gas slugs, scaled to the expansion rates in volcanic conduits. This laboratory analogue study illustrates that slugs in trains rise faster than individual slugs, and can be associated with aspects of co-current flow. The work also highlights that coalescence and inter-slug interactions play an important role in modulating slug train behaviour. We also report, for the first time, on slug coalescence driven by vertical expansion of the trailing slug, a process which can occur, even where the leading slug base ascent velocity is greater than that of the trailing slug. (C) 2017 The Authors. Published by Elsevier B.V.

This paper proposes a coastal erosion monitoring system for beach erosion management, which we demonstrate on natural and artificial pocket gravel beaches in Croatia. The approach uses low-cost Structure-from-Motion (SfM) photogrammetric imaging and multi -view stereo (MVS) to produce high -resolution 3D beach models for detecting morphological changes and erosion occurrence. Coastal state indicators, such as the shoreline position and subaerial beach volume, are derived from the 3D models and used to quantify changes between surveys. The method is illustrated through two case studies and, to our knowledge, these are the first repetitive measurements taken on the Croatian eastern Adriatic Coast (CEAC). In case of the natural Brse4 beach, beach rotation was found to be a response to natural forcing from waves of various incident directions. For the artificial Dugi Rat beach, which loses sediment every winter and is subsequently re-nourished every spring, monitoring showed that beach nourishment is of limited durability. Both case studies showed that the SfM-MVS technique is suitable for the rapid and frequent acquisition of 3D survey data, from which quantitative coastal indicators can be derived to inform future coastal management interventions. Significantly, this low-cost data acquisition has a great potential for regular beach management survey. The introduction of beach monitoring in Croatia is timely because emerging Integrated Coastal Zone Management (ICZM) practices will require data-based approaches. Moreover, rare natural pocket beaches and the ever-increasing number of artificial beaches are extremely vulnerable to natural and man-made changes. Adaptive beach management, based on systematic monitoring data, should be included in the ICZM, and we detail how SfM-MVS-based monitoring can be used at different levels of the ICZM. Implementing robust ICZM monitoring will require broad considerations and consultation with all stakeholders, so we propose that SfMMVS beach surveys should be initially integrated into the existing monitoring practices for CEAC sea water bathing quality. Extension of the existing database with rapidly-gathered low-cost 3D beach survey data, from a number of targeted beaches, could be used to provide a crucial baseline for the ICZM and strategic coastal monitoring of the CEAC.

Volcanic ash clouds can present an aviation hazard over distances of thousands of kilometres and, to help to mitigate this hazard, advanced numerical models are used to forecast ash dispersion in the atmosphere. However, forecast accuracy is usually limited by uncertainties in initial conditions such as the eruption rate and the vertical distribution of ash injected above the volcano. Here, we demonstrate the potential of the Telematics Earth Observation Mission (TOM) picosatellite formation, due for launch in 2020, to provide valuable information for constraining ash cloud dispersion models through simultaneous image acquisition from three satellites. TOM will carry commercial frame cameras. Using photogrammetric simulations, we show that such data should enable ash cloud heights to be determined with a precision (similar to 30-140 m depending on configuration) comparable to the vertical resolution of lidar observations (30-180 m depending on the cloud height). To support these estimates, we processed photographs taken from the International Space Station of the 2009 Sarychev Peak eruption, as a proxy for TOM imagery. Structure-from-motion photogrammetric software successfully reconstructed the 3-D form of the ascending ash cloud, as well as surrounding cloud layers. Direct estimates of the precision of the ash cloud height measurements, as well as comparisons between independently processed image sets, indicate that a vertical measurement precision of similar to 200 m was achieved. Image sets acquired at different times captured the plume dynamics and enabled a mean ascent velocity of 14 m s(-1) to be estimated for regions above 7 km. In contrast, the uppermost regions of the column (at a measured cloud top height of similar to 11 km) were not ascending significantly, enabling us to constrain a 1-D plume ascent model, from which estimates for the vent size (50 m) and eruption mass flux (2.6 x 10(6) kg s(-1)) could be made. Thus, we demonstrate that nanosatellite imagery has the potential for substantially reducing uncertainties in ash dispersion models by providing valuable information on eruptive conditions.

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 pahoehoe lobes from the 61G lava flow of Kilauea's Pu'u `O'o eruption in 2016. We successfully measured a viscosity of similar to 380 Pas at strain-rates between 1.6 and 5 s(-1) and at 1144 degrees 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. (C) 2018 Elsevier B.V. All rights reserved.

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 cooling-limited basaltic lavas, but has not been described in established models of rhyolite emplacement. Using data from the 2011-2012 rhyolite eruption of Puyehue-CordOn 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 multi-scale satellite, field, and textural data from Cordon 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 Cordon 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.

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.