Very high spatial resolution imagery for channel bathymetry and topography from an unmanned mapping controlled platform
Résumé
The increasing availability of aerial photography and satellite imagery offers new possibilities for characterizing river morphology. The precision of new very high resolution (VHR) images allows smaller scale objects within river corridors to be studied. High survey frequencies provide increased opportunities for the monitoring of river restoration.Following this evolution in platform technology, a small radio-controlled motorized vehicle flying at low altitude was used to study both channel water depth and gravel bar geometry. The VHR imagery provided by this equipment allowed both channel bathymetry and a high accuracy photogrammetric digital elevation model (DEM) to be realized. Using case studies from the Ain and the Drôme Rivers in France, the accuracy of the results is presented and the various challenges associated with the new platform are discussed. One significant issue is that due to the low elevation of the survey the coverage of a target area is usually based on several photographs, which leads to variations in illumination conditions that stem from atmospheric changes. The images were processed to minimize this source of error but a number of issues have yet to be resolved.Bathymetric models with R2 values between 0·59 and 0·90 were created in spite of the lack of channel bed homogeneity at the various sites. The gravel bar was also effectively mapped, and the photogrammetrically predicted DEM provided a 5-10 cm pixel resolution with a vertical precision between 2 and 40 cm according to the position within the image. This paper shows that, despite unstable image acquisition, unmanned radio-controlled platforms provide significant advantages for the study of river processes, offering a flexible very high resolution data source for both channel bathymetry and gravel bar topography. Copyright ©2007 John Wiley & Sons, Ltd.