Mark W. Smith

Structure from motion photogrammetry in physical geography

Accurate, precise and rapid acquisition of topographic data is fundamental to many sub-disciplines of physical geography. Technological developments over the past few decades have made fully distributed data sets of centimetric resolution and accuracy commonplace, yet the emergence of Structure from Motion (SfM) with Multi-View Stereo (MVS) in recent years has revolutionised three-dimensional topographic surveys in physical geography by democratising data collection and processing. SfM-MVS originates from the fields of computer vision and photogrammetry, requires minimal expensive equipment or specialist expertise and, under certain conditions, can produce point clouds of comparable quality to existing survey methods (e.g. Terrestrial Laser Scanning). Consequently, applications of SfM-MVS in physical geography have multiplied rapidly. There are many practical options available to physical geographers when planning a SfM-MVS survey (e.g. platforms, cameras, software), yet, many SfM-MVS end-users are uncertain as to the errors associated with each choice and, perhaps most fundamentally, the processes actually taking place as part of the SfM-MVS workflow. This paper details the typical workflow applied by SfM-MVS software packages, reviews practical details of implementing SfM-MVS, combines existing validation studies to assess practically achievable data quality and reviews the range of applications of SfM-MVS in physical geography. The flexibility of the SfM-MVS approach complicates attempts to validate SfM-MVS robustly as each individual validation study will use a different approach (e.g. platform, camera, georeferencing method, etc.). We highlight the need for greater transparency in SfM-MVS processing and enhanced ability to adjust parameters that determine survey quality. Looking forwards, future prospects of SfM-MVS in physical geography are identified through discussion of more recent developments in the fields of image analysis and computer vision.

Applying flow resistance equations to overland flows

Resistance to flow determines routing velocities and must be adequately represented both within stream channels and over hillslopes when making predictions of streamflow and soil erosion. The limiting assumptions inherent in flow resistance equations can be relaxed if the spatial and temporal scale over which they are applied is restricted. This requires a substantial methodological advance in the study of overland flows over natural surfaces. It is suggested that terrestrial laser scanning will allow a greater understanding of overland flow hydraulics and present opportunities to investigate resistance to flow over complex morphologies. The Darcy-Weisbach, Chézy and Manning equations are the most widely used empirical equations for the calculation of flow velocity in runoff and erosion models. These equations rest on analyses originally developed for one-dimensional pipe flows and assume conditions which are not met by overland flows. The following assumptions are brought into question: flow can be described as uniform; flow is parallel to the surface; flow is of a constant width and the boundary to the flow is longitudinally uniform; grain roughness is homogeneous over the wetted perimeter and can be considered as random; form roughness and other sources of flow resistance can be ignored; resistance is independent of flow depth; and resistance can be modelled as a function of the Reynolds number. A greater appreciation of the processes contributing to resistance to overland flows must be developed. This paper also presents a brief history of the development of flow resistance equations.

Structure from Motion in the Geosciences

Structure from Motion with Multi View Stereo provides hyperscale landform models using images acquired from standard compact cameras and a network of ground control points. The technique is not limited in temporal frequency and can provide point cloud data comparable in density and accuracy to those generated by terrestrial and airborne laser scanning at a fraction of the cost. It therefore offers exciting opportunities to characterise surface topography in unprecedented detail and, with multi-temporal data, to detect elevation, position and volumetric changes that are symptomatic of earth surface processes. This book firstly places Structure from Motion in the context of other digital surveying methods and details the Structure from Motion workflow including available software packages and assessments of uncertainty and accuracy. It then critically reviews current usage of Structure from Motion in the geosciences, provides a synthesis of recent validation studies and looks to the future by highlighting opportunities arising from developments in allied disciplines. This book will appeal to academics, students and industry professionals because it balances technical knowledge of the Structure from Motion workflow with practical guidelines for image acquisition, image processing and data quality assessment and includes case studies that have been contributed by experts from around the world.

Toward a dynamic representation of hydrological connectivity at the hillslope scale in semiarid areas

Smith, M. W., Bracken, L. J. (2010). Toward a dynamic representation of hydrological connectivity at the hillslope scale in semiarid areas. Water Resources Research, 46, Article Number: W12540.

3-D uncertainty-based topographic change detection with structure-from-motion photogrammetry : precision maps for ground control and directly georeferenced surveys

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 3-D topographic change. The method uses novel 3-D precision maps that describe the 3-D photogrammetric and georeferencing uncertainty, and determines change through an adapted state-of-the-art fully 3-D point-cloud comparison (M3C2; Lague, et al., 2013), 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 3-D 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-½ 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.

Aerodynamic roughness of glacial ice surfaces derived from high resolution topographic data

This paper presents new methods of estimating the aerodynamic roughness (z0) of glacier ice directly from three-dimensional point clouds and digital elevation models (DEMs), examines temporal variability of z0, and presents the first fully distributed map of z0 estimates across the ablation zone of an Arctic glacier. The aerodynamic roughness of glacier ice surfaces is an important component of energy balance models and meltwater runoff estimates through its influence on turbulent fluxes of latent and sensible heat. In a warming climate these fluxes are predicted to become more significant in contributing to overall melt volumes. Ice z0 is commonly estimated from measurements of ice surface microtopography, typically from topographic profiles taken perpendicular to the prevailing wind direction. Recent advances in surveying permit rapid acquisition of high-resolution topographic data allowing revision of assumptions underlying conventional z0 measurement. Using Structure from Motion (SfM) photogrammetry with Multi-View Stereo (MVS) to survey ice surfaces with millimeter-scale accuracy, z0 variation over 3 orders of magnitude was observed. Different surface types demonstrated different temporal trajectories in z0 through 3 days of intense melt. A glacier-scale 2 m resolution DEM was obtained through terrestrial laser scanning (TLS), and subgrid roughness was significantly related to plot-scale z0. Thus, we show for the first time that glacier-scale TLS or SfM-MVS surveys can characterize z0 variability over a glacier surface potentially leading to distributed representations of z0 in surface energy balance models.

Habitat Hydrology and Geomorphology Control the Distribution of Malaria Vector Larvae in Rural Africa

Background Larval source management is a promising component of integrated malaria control and elimination. This requires development of a framework to target productive locations through process-based understanding of habitat hydrology and geomorphology. Methods We conducted the first catchment scale study of fine resolution spatial and temporal variation in Anopheles habitat and productivity in relation to rainfall, hydrology and geomorphology for a high malaria transmission area of Tanzania. Results Monthly aggregates of rainfall, river stage and water table were not significantly related to the abundance of vector larvae. However, these metrics showed strong explanatory power to predict mosquito larval abundances after stratification by water body type, with a clear seasonal trend for each, defined on the basis of its geomorphological setting and origin. Conclusion Hydrological and geomorphological processes governing the availability and productivity of Anopheles breeding habitat need to be understood at the local scale for which larval source management is implemented in order to effectively target larval source interventions. Mapping and monitoring these processes is a well-established practice providing a tractable way forward for developing important malaria management tools.

Mapping Hotspots of Malaria Transmission from Pre-existing Hydrology, Geology and Geomorphology Data in the Pre-elimination Context of Zanzibar, United Republic of Tanzania.

BackgroundLarval source management strategies can play an important role in malaria elimination programmes, especially for tackling outdoor biting species and for eliminating parasite and vector populations when they are most vulnerable during the dry season. Effective larval source management requires tools for identifying geographic foci of vector proliferation and malaria transmission where these efforts may be concentrated. Previous studies have relied on surface topographic wetness to indicate hydrological potential for vector breeding sites, but this is unsuitable for karst (limestone) landscapes such as Zanzibar where water flow, especially in the dry season, is subterranean and not controlled by surface topography.MethodsWe examine the relationship between dry and wet season spatial patterns of diagnostic positivity rates of malaria infection amongst patients reporting to health facilities on Unguja, Zanzibar, with the physical geography of the island, including land cover, elevation, slope angle, hydrology, geology and geomorphology in order to identify transmission hot spots using Boosted Regression Trees (BRT) analysis.ResultsThe distribution of both wet and dry season malaria infection rates can be predicted using freely available static data, such as elevation and geology. Specifically, high infection rates in the central and southeast regions of the island coincide with outcrops of hard dense limestone which cause locally elevated water tables and the location of dolines (shallow depressions plugged with fine-grained material promoting the persistence of shallow water bodies).ConclusionsThis analysis provides a tractable tool for the identification of malaria hotspots which incorporates subterranean hydrology, which can be used to target larval source management strategies.

Terrestrial laser scanning to deliver high-resolution topography of the upper Tarfala valley, arctic Sweden

Abstract Alpine valleys are experiencing rapidly changing physical, biological and geochemical processes as glacier masses diminish, snowfall patterns change and consequently as hillslopes and valley-floor landforms and sediments adjust. Measurement and understanding of these processes on a valley, landform and surface scale requires topographic data with sufficient spatial coverage and spatial resolution to resolve sources, fluxes and storages of sediment. Most ideally such topographic data will be of a resolution sufficient to resolve important spatial heterogeneity in land cover, topography and surface texture, for example. This study presents the first high-resolution (1 m grid cell size) and freely available topography for the upper part of the Tarfala valley, arctic Sweden. The topography was obtained using terrestrial laser scanning and a bespoke workflow is presented to most efficiently cover a 9.3 km2 area. The unprecedented spatial resolution of this topography, which is 15 times greater than that previously available, reveals a suite of alpine landforms. These landforms span multiple glacier forefields, a variety of bedrock surfaces, various hillslopes and types of mass movement, and valley floor glacial, fluvial and periglacial sediments, for example. Primary and second-order derivatives of this elevation data, and vertical transects are given and will assist future classification of landforms and thus assist future targeted field campaigns. Overall, this study presents (1) baseline data from which future re-surveys will enable quantitative analysis of a dynamic landscape, and (2) an efficient workflow that is readily transferable to any scientific study at any other site. Both of these project outputs will find widespread usage in future alpine studies.

Heterogeneous water storage and thermal regime of supraglacial ponds on debris-covered glaciers

The water storage and energy transfer roles of supraglacial ponds are poorly constrained, yet they are thought to be important components of debris-covered glacier ablation budgets. We used an unmanned surface vessel (USV) to collect sonar depth measurements for 24 ponds to derive the first empirical relationship between their area and volume applicable to the size distribution of ponds commonly encountered on debris-covered glaciers. Additionally, we instrumented nine ponds with thermistors and three with pressure transducers, characterising their thermal regime and capturing three pond drainage events. The deepest and most irregularly-shaped ponds were those associated with ice cliffs, which were connected to the surface or englacial hydrology network (maximum depth = 45.6 m), whereas hydrologically-isolated ponds without ice cliffs were both more circular and shallower (maximum depth = 9.9 m). The englacial drainage of three ponds had the potential to melt ~100 ± 20 × 103 kg to ~470 ± 90 × 103 kg of glacier ice owing to the large volumes of stored water. Our observations of seasonal pond growth and drainage with their associated calculations of stored thermal energy have implications for glacier ice flow, the progressive enlargement and sudden collapse of englacial conduits, and the location of glacier ablation hot-spots where ponds and ice cliffs interact. Additionally, the evolutionary trajectory of these ponds controls large proglacial lake formation in deglaciating environments.