The development of an automated correction �procedure for digital photogrammetry for the study of wide, shallow, gravel-bed rivers

Abstract

This paper develops an automated correction procedure for dealing with point errors associated with through-water photogrammetry, for application in the study of clear-water, shallow gravel-bed rivers. The procedure involves combining digital photogrammetry and image analysis techniques to: (i) correct for the effects of refraction at an air±water interface; and (ii) eliminate and reinterpolate points where the bed has not been`seen'. The correction procedure was applied to raw digital elevation models (DEMs) generated using digital photogrammetry from 1:3000 scale aerial photography of a small reach of the North Ashburton River, New Zealand. The accuracy of corrected and uncorrected DEMs is evaluated using an independent data set. A measure of`geomorphological usefulness' as well as DEM external reliability is obtained from calculations of water depth distributions and mean bed level. Results show that digital photogrammetry, used in conjunction with image analysis techniques, can successfully be used for extracting high-resolution DEMs of gravel river beds. In exposed areas, errors are small and random, tending to cancel out over large numbers of points. Where water is shallow, and following correction, point elevation errors are statistically no different from those for exposed zones. In deeper water, despite an improvement following application of the correction procedure, elevation errors scale with water depth. The geomorphological potential of photogrammetric survey of large, gravel river beds is demonstrated by the ease and accuracy of calculations of water depth distribution (important for the assessment of a river's ecological and recreational characteristics) and mean bed level (important for the calculation of reach-scale sediment volumes). ). Third, there is an increasing recognition of the failings associated with using process-based studies to determine and predict changes in river form and sediment transport rates. This has led to calls for a`reversal' of the traditional methodology whereby processes are instead inferred from changes in channel morphology (e.g. Goff and Ashmore, 1994;Lane et al., 1995a;Ashmore and Church, 1998). At a more practical level, there is also a need to understand the dynamics of river channels to help inform and advise river management decisions.Central to a DEM-based approach is the acquisition of high-resolution topographic information. This must be distributed across the full area of interest, be of a quality commensurate with the scale of study, and collected and re-collected sufficiently rapidly for there to be only minimal change during the measurement process. Recent developments have made photogrammetry an increasingly attractive methodology for this purpose Chandler, 1999), and digital photogrammetry has been successfully used to derive digital elevation models (DEMs) in fluvial environments (e.g. Pyle et al., 1997;Butler et al., 1998;Lane, 1998;Stojic et al., 1998). In particular, automation of the data collection process allows very rapid and inexpensive DEM a...

Quantification of the relation between surface morphodynamics and subsurface sedimentological product in sandy braided rivers

“…First, digital elevation data from only the wet points were subtracted from the interpolated water surface to yield raw depth estimates. Second, these depths were corrected to account for the effects of light refraction at the water surface (Westaway et al, 2000). Third, these corrected depth estimates were paired with the logarithm of the pixel brightness (dN) values from the associated orthophotographs.…”

“…Similarly, photogrammetric and laser scanning techniques that are capable of generating digital elevation models (DEMs) at the kilometre scale (e.g. Westaway et al, 2000;Chandler et al, 2002;Brasington et al, 2003;Charlton et al, 2003;Lane et al, 2003Lane et al, , 2010 have enabled the river morphodynamics to be quantified over a much greater area.…”

Through‐water terrestrial laser scanning of gravel beds at the patch scale

“…Digital photogrammetry (Lane, 2000;Westaway et al, 2000) and airborne light detection and ranging (LiDAR; Charlton et al, 2003;Kinzel et al, 2007) can be used to remotely acquire reach-scale digital elevation models (DEMs) but each of these is subject to large uncertainties when surveying submerged topography (Brasington et al, 2003;Lane et al, 2003). Optical reflectance depth monitoring (Winterbottom and Gilvear, 1997;Fonstad and Marcus, 2005) spectral remote sensing (Legleiter et al, 2009) and bathymetric green LiDAR (Guenther et al, 2000;Bailly et al, 2010) present alternative surveying strategies for the submerged zone but large uncertainties remain.…”

The Accuracy and Reliability of Traditional Surface Flow Type Mapping: Is it Time for a New Method of Characterizing Physical River Habitat?

“…We derived quantitative data from the sUAS-SfM process in the form of refraction corrected (RC) water depth and point cloud roughness, both as raster datasets. Woodget et al (2015) provide further detail concerning water depth estimation and the required refraction correction procedure, as initially proposed by Westaway et al (2000). We used the freeware package CloudCompare (Girardeau-Montaut, 2014) to compute the roughness (i.e.…”