Ableitung der Topographie des Wattenmeeres aus ERS-SAR Daten

Aim of this thesis is the development of an operationally applicable method to derive the topography of the Wadden Sea by evaluation of satellite image using the so-called waterline method. The Wadden Sea along the North Sea coast of Denmark, Germany and the Netherlands is amongst Europe’s last extensive habitats nearly in its natural state. A frequent monitoring is required to protect this valuable and vulnerable ecological system, which is endangered by shipping, fishery or tourism. Accurate and up-to-date topographic maps of the tidal flats are an important basis for the protection of the Wadden Sea as well as for coastal preservation. Mapping this vast and often inaccessible area is a difficult task and requires considerable logistic and financial efforts. A survey of the German part of the Wadden Sea requires approximately five to seven years using echo sounding measurements which are only able to cover navigable areas. Due to the high variability of the topography in this area caused by storms and tidal currents such maps often do not represent the actual terrain. An additional way to generate topographic maps is the waterline method. For this method a series of satellite images of the German Bight which were taken at different water levels are analysed to detect the borderline between tidal flats and adjacent water areas. After geo-coding, those waterlines are combined with the corresponding water levels to represent the topography on an irregularly spaced grid. Interpolation of these data into a regular grid yields a topographic map of the intertidal zone. In this thesis the waterline method is used to map the extensive tidal flat of Nordfriesland and Cuxhaven. The study is based on the analysis of 71 images taken by the Synthetic Aperture Radar (SAR) aboard the European Remote Sensing Satellite ERS2 in the period from 1996 to 1999. This active instrument is able to image large areas with a spatial resolution of 25 m and is independent of daylight or cloud coverage, thereby providing a good sample for the waterline method. Using the edge detection algorithm developed in this study a largely automated estimation of the waterline is possible in spite of the high level of multiplicative noise in the SAR images. Based on wavelet transforms, this algorithm uses a multi-scale approach to locate nearly all edges within the image, from which the waterline is extracted in a second step by application of a segmentation algorithm. This procedure reduces the amount of work for an human operator considerably. A modified pass point correction is used for the geo-coding of the derived waterlines. By averaging a large number of SAR images and larger parts of the road network instead of single ground control points, the accuracy of the transformation is considerably improved. Water levels obtained by the water level model running operationally at the Bundesamt für Seeschifffahrt und Hydrographie (German Maritime and Hydrographic Agency) which were corrected by comparison to tidal gauge measurements are assigned to the geo-coded waterlines. The resulting grid of topographic points are interpolated into a topographic map of the intertidal area for each of the four years using the software package TASH developed at the University of Hannover, Germany.