Jim H. Chandler

Effective application of automated digital photogrammetry for geomorphological research

Developments in digital photogrammetry have provided geomorphologists with an automated tool to generate digital elevation models (DEMs) at exceedingly high densities. Although such software tools are available at low cost and run on far cheaper hardware than previous generations of photogrammetric instrumentation, some expertise is still required to derive accurate data. Various recommendations are provided that should enable the inexperienced user to make effective use of digital photogrammetry. Key issues discussed include the role of photo-control, the significance of checkpoints in the object space, and the importance of camera calibration data. An overview of self-calibration methods is provided, which is valuable in situations when a non-photogrammetric camera has been used. Accurate camera modelling also affects DEM quality and this issue is examined through assessing the impact of DEM inaccuracies upon derived data used for geomorphological enquiry. Although the automated software packages are designed primarily for use with satellite imagery or vertical aerial photography, it is explained how such software can be used for both close range and oblique imagery. The procedure requires rotating the object coordinate system defined by the photo-control points and the mathematics required to achieve this is provided. The procedure is used to derive the morphology of a soil surface using oblique, close-range imagery and demonstrates the effectiveness of this particular approach. Copyright

Three‐dimensional measurement of river channel flow processes using acoustic doppler velocimetry

This paper describes and assesses: (i) the use of a new instrument for the determination of three-dimensional flow velocities in natural rivers, the acoustic Doppler velocimeter (ADV); and (ii) a method for positioning and orienting such measurements relative to a single local coordinate system to relate flow velocity vectors with the bed and water surface. The ADV uses the Doppler shift principle to measure the velocity of small particles, assuming to be moving at velocities similar to the fluid. Velocity is resolved into three orthogonal components, and measured in a volume 5 cm below the sensor head, minimizing interference of the flow field, and allowing measurements to be made close to the bed. A simple method for positioning and orienting the instrument using digital tacheometry is described, and is used to obtain velocity measurements concurrently with measurements of both bed and water surface topography. The paper includes a preliminary field assessment of the ADV by comparing velocity profiles with those generated from Marsh McBirney electromagnetic current meters, and a full field assessment of the position and orientation methodology. These results suggest that the recommended methods in combination with an ADV are able to provide reliable mean three-dimensional velocity field information and accurate bed and surface topography. Copyright

Characterization of the Structure of River-Bed Gravels Using Two-Dimensional Fractal Analysis

This paper is concerned with the application of fractal analysis to understand the structure of water-worked gravel-bed river surfaces. High resolution digital elevation models, acquired using digital photogrammetric methods, allowed the application of two-dimensional fractal methods. Previous gravel-bed river studies have been based upon sampled profiles and hence one-dimensional fractal characterisation. After basic testing that bed elevation increments are Gaussian, the paper uses two-dimensional variogram surfaces to derive directionally dependent estimates of fractal dimension. The results identify mixed fractal behavior with two characteristic fractal bands, one associated with the subgrain scale and one associated with the grain scale. The subgrain scale characteristics were isotropic and sensitive to decisions made during the data collection process. Thus, it was difficult to differentiate whether these characteristics were real facets of the surfaces studied. The second band was anisotropic and not sensitive to data collection issues. Fractal dimensions were greater in the downstream direction than in other directions suggesting that the effects of water working are to alter the level of surface organisation, by increasing surface irregularity and hence roughness. This is an important observation as it means that water-worked surfaces may have a distinct anisotropic signal, revealed when using a fractal type analysis.

Automated extraction of grain-size data from gravel surfaces using digital image processing

This paper describes and tests a method for the automated extraction of grain-size data from digital imagery. It combines two basic image processing methods for this purpose: grey-scale thresholding to create a binary image and watershed segmentation to grow edges on the binary image to allow the identification of individual grains. The method is subject to rigorous testing in terms of edge detection and automatic measurement of grain-size information from the edge images, and is also compared with the results obtained from simple direct clast sampling. The edge detection methods are tested with respect to manually-identified edges. This suggests that simple thresholding of raw imagery produces grain-size estimates that are: (i) in excellent agreement with manual estimates, above a critical particle size defined by the scale of the photography; (ii) downgraded with the inclusion of additional edge information from analysis of high resolution digital elevation models (DEMs); and (iii) not affected by the use of raw imagery as opposed to imagery that has been rectified to deal with geometric, tilt and relief distortion effects. The automated ellipse-based measurement method is shown to produce a good estimate of two-dimensional a- and b- axes as they appear as long and short axes on the edge images. Thus, the research shows that it can be used to map and quantify very rapidly spatial variations in grain-size characteristics, although it cannot deal with the longrecognised problem of the relationship between two-dimensional planform grain-size estimates and actual a- and b- axes obtained by direct grain sampling.

Monitoring River-Channel Change Using Terrestrial Oblique Digital Imagery and Automated Digital Photogrammetry

Imagery acquired using a high-resolution digital camera and ground survey has been used to monitor changes in bed topography and plan form, and to obtain synoptic water-surface and flow-depth information in the braided, gravel-bed Sunwapta River in the Canadian Rockies. Digital images were obtained during daily low flows during the summer meltwater season to maximize the exposed bed area and to map the water surface on the days with the highest flows. Images were acquired from a cliff-top 125 m above and at a distance of 235 m from the riverbed and used to generate high-resolution orthophotos and digital elevation models (DEMs) at a ground resolution of 0.2 m, within an area 80 × 125 m. The creation of DEMs from oblique and nonmetric imagery using automated digital photogrammetry can be difficult, but a solution based on rotation of coordinates is described here. Independent field verification demonstrated that root mean square accuracies of 0.045 m in elevation were achieved. The ground survey data representing riverbed topography were merged with photogrammetric DEMs of the exposed bars. The high-flow water surface could not be surveyed directly because wading was dangerous but was derived by ground survey of selected accessible points and photogrammetry. The DEMs and depth map provide high-resolution, continuous data on the channel morphology and will be the basis for subsequent two-dimensional flow-modeling of velocity and shear stress fields. The experience of using digital photogrammetry for monitoring river-channel change allows the authors to identify other potential benefits of using this technique for fluvial research and beyond.

Morphological Estimation of the Time-Integrated Bed Load Transport Rate

Recent research has suggested that given the problems associated with more conventional methods of estimating the bed load transport rate in dynamic rivers, measurement of the transfer of bed material, as indicated by changes in river channel form, may be an appropriate alternative. Thus far, this methodology has been successfully employed to estimate medium-term bed load transport rates at the reach scale. This paper illustrates the potential of such a technique and extends it further through application to the within-reach scale and estimation of spatially distributed patterns of bed load transport. It uses a data collection technique that allows the rapid field acquisition of digital terrain models of river channel form. Intercomparison of digital terrain models from separate time periods allows calculation of both the total volumes and the distributed volumes of erosion and deposition. The total volumes can be combined with upstream information on sediment supply to assess the contribution of a particular reach to the total bed load transport rate. The distributed information can be combined with a likely direction of movement of material in transport to estimate the distributed pattern of the time-averaged bed load transport rate. Both of these approaches show a good correspondence with observed patterns of the bed load transport rate.

Discharge and sediment supply controls on erosion and deposition in a dynamic alluvial channel

Research on dynamic alluvial channels has recognised the influence on river channel change of both discharge and sediment supply, although it has proved difficult to measure the latter. This paper presents the first accurate data from a dynamic alluvial channel that describe the interrelated effect on channel morphological change of both discharge and sediment supply variations over different timescales. Reliable information on the spatial patterns of erosion and deposition were obtained using combined photogrammetry and tacheometric survey of the meltwater stream of a glacierised catchment over a 5 week period. Over the full time-period, the morphological evidence suggested the passage of a wave of sediment. Counter-intuitively, the period of aggradation was associated with increasing diurnal peak and base flow discharge magnitude, while degradation occurred during a period of steady peak diurnal discharge. This suggests that the channel change was controlled by variation in upstream sediment supply. Over a shorter timescale, information on diurnal changes in morphology reveals that although discharge was responsible for some aspects of channel change, this was modified by the effects of upstream sediment supply. Consideration of the spatial patterns of erosion and deposition at the within-reach scale reinforces this point, with spatial aspects of morphological change being driven by discharge and sediment supply fluctuations, but modified by spatial feedbacks associated with internal channel morphology. The implications of these findings for fluvial geomorphology in general are considered.

Developments in photogrammetry; the geomorphological potential:

Current emphasis in geomorphology recognizes the need for the accurate representation of topographic form, reflected in the growth of digital terrain and elevation modelling. A key requirement of such strategies is the efficient acquisition of information in an appropriate form and at an appropriate resolution to the landform under consideration. The traditional use of photographs in geomorphology has been for interpretation, but developments in photogrammetry may allow the full advantages of the photograph as a means of acquiring and storing quantitative information to be used. The photograph can provide information on all areas visible on a photograph; the information is acquired retrodictively; the photograph preserves the spatial relationship of morphological units; the collection of photographs requires minimal landform contact; the photograph records extra explanatory information; and photographs can be obtained at an appropriate temporal resolution to the landform under investigation. However, optical and mechanical limitations imposed by traditional photogrammetric approaches have prevented its rigorous and widespread application to geomorphology. Developments within photogrammetry, notably the analytical approach, now open up wider geomorphological possibilities. The analytical approach overcomes these limitations through the use of an interactive mathematical model at the stage of photographic analysis. The obtained information is in a form directly suited to the construction of digital terrain or elevation models. This technique can be used both for landform monitoring and for the analysis of archival photographs to reconstruct historical landform change.

Modeling the dynamics of soil erosion and size-selective sediment transport over nonuniform topography in flume-scale experiments

Soil erosion and the associated nutrient fluxes can lead to severe degradation of surface waters. Given that both sediment transport and nutrient sorption are size selective, it is important to predict the particle size distribution (PSD) as well as the total amount of sediment being eroded. In this paper, a finite volume implementation of the Hairsine-Rose soil erosion model is used to simulate flume-scale experiments with detailed observations of soil erosion and sediment transport dynamics. The numerical implementation allows us to account for the effects of soil surface microtopography (measured using close range photogrammetry) on soil erosion. An in-depth discussion of the model parameters and the constraints is presented. The model reproduces the dynamics of sediment concentration and PSD well, although some discrepancies can be observed. The calibrated parameters are also consistent with independent data in the literature and physical reason. Spatial variations in the suspended and deposited sediment and an analysis of model sensitivity highlight the value of collecting distributed data for a more robust validation of the model and to enhance parametric determinacy. The related issues of spatial resolution and scale in erosion prediction are briefly discussed.

Historical aerial photographs for landslide assessment: two case histories

This paper demonstrates the value of historical aerial photographs for assessing long-term landslide evolution. The study focused on two case histories, the Mam Tor and East Pentwyn landslides. In both case histories we explored the variety of data that could be derived relatively easily using an ordinary PC desktop, commercially available software and commonly available photographic material. The techniques to unlock qualitative and quantitative data captured in the photographic archive were based on the principles of aerial photo-interpretation and photogrammetry. The products created comprised geomorphological maps, automatically derived digital elevation models (DEMs), displacement vectors and animations. The measured horizontal displacements of the Mam Tor landslide ranged from 0.09 to 0.74 m a−1 between 1953 and 1999, which was verified by independent survey data. Moreover, the observed displacement patterns were consistent with photo-interpreted geomorphological information. The photogrammetric measurements from the East Pentwyn landslide (horizontal displacements up to 6 m a−1 between 1971 and 1973) also showed a striking resemblance to independent data. In both case histories, the vertical accuracy was insufficient for detecting significant elevation changes. Nevertheless, DEMs proved to be a powerful tool for visualization. Overall, the results in this study validated the techniques used and strongly encourage the use of historical photographic material in landslide studies.