Remote sensing has been used to map river bathymetry for several decades. Non-contact methods are necessary in several cases: inaccessible rivers, large-scale depth mapping, very shallow rivers. The remote sensing techniques used for river bathymetry are reviewed. Frequently, these techniques have been developed for marine environment and have then been transposed to riverine environments. These techniques can be divided into two types: active remote sensing, such as ground penetrating radar and bathymetric lidar; or passive remote sensing, such as through-water photogrammetry and radiometric models. This last technique — which consists of finding a logarithmic relationship between river depth and image values — appears to be the most used. Fewer references exist for the other techniques, but lidar is an emerging technique. For each depth measurement method, we detail the physical principles and then a review of the results obtained in the field. This review shows a lack of data for very shallow rivers, where a very high spatial resolution is needed. Moreover, the cost related to aerial image acquisition is often huge. Hence we propose an application of two techniques, radiometric models and through-water photogrammetry, with very- high-resolution passive optical imagery, light platforms, and off-the-shelf cameras. We show that, in the case of the radiometric models, measurement is possible with a spatial filtering of about 1 m and a homogeneous river bottom. In contrast, with through-water photogrammetry, fine ground resolution and bottom textures are necessary.

Archival aerial imagery is a worldwide resource for documenting past 3-D change at very high-resolution. However, external information is normally required so that accurate 3-D models can be computed from archival aerial imagery. In this research, we propose and test a new method which joins multi-epoch images in a single block in the first steps of the SfM processing. It allows for computing coherent multi-temporal digital elevation models (DEMs) using just image information. This method is based on the invariance properties of the feature detection procedures that are at the root of the structure from motion (SfM) algorithms. On a test site covering 170 km 2 , we applied SfM algorithms to a single image block consisting of all images captured at four different epochs and spanning a forty year period. We compared this approach to the more classical methods which imply a separation of epochs in different processing blocks. We tested different densities of ground control points derived simply and cheaply from a recent orthophoto and DEM, different ways of image preprocessing and different autocalibration procedures. By determining which choice most affected the final result through this extensive testing procedure, we evaluated the potential of the proposed method for detecting 3-D change. Our study showed that the proposed method resolves the problem of registration between epochs, so allowing the production of informative DEMs of difference using almost exclusively image information and limited photogrammetric expertise and human intervention. As the proposed method can be automatically applied using just image information, our results pave the way to more systematic processing of archival aerial imagery with very large spatio-temporal windows, which should greatly help document of past 3-D change.

Abstract. In the present study SAR interferometric techniques (stacking of conventional interferograms and Permanent Scatterers), using images from satellites ERS-1 and 2, have been applied to the region of Thessaloniki (northern Greece). The period covered by the images is 1992-2000. Both techniques gave good quantitative and qualitative results. The interferometric products were used to study ground surface deformation phenomena that could be related to the local tectonic context, the exploitation of underground water and sediments compaction.