Regional Applications
Topographic Maps of the Wadden Sea from ERS-SAR Data

Jens Dannenberg*, Ralf Schmidt, Georg Heygster
Founding: MARUM, Doktorandenkolleg Nordseeküste

The tidal flats of the Wadden Sea along the coast of northwest Germany are an worldwide unique ecosystem. The monitoring of this large and inaccessible arrea (appox.7300 km²) is very difficult. Its highly dynamic tidal zone whose topography changes on timescales from a few hours (storm events) up to several months (sediment ablations and disposal) requires a freqent update of topographic maps. Conventionally generated maps based on echo sounding and geodetic leveling techniques often do not represent the actual situation because it takes several years to map the complete Wadden Sea.

With the help of remote sensing data, which allow the monitoring of large areas, it is possible to achieve a more frequent updating of the topographic maps of tidal areas between high and low water level using the 'waterline method'. For this study the images of the Synthetic Aperture Radar (SAR) sensor aboard the European Remote Sensing Satellite ERS-2 are used. This active sensor works independently of daylight and cloud coverage which is important to get a sufficient number of utilizable images of the German coast. A sample image (the coast of Schleswig-Holstein) is shown on the left. The red box marks the location of the test area shown on the right hand.

The Waterline Method: In the first step waterlines are extracted from several images acquired at different tidal levels. Because of the speckle-noise in the SAR-images advanced image processing techniques are required for an acurate detection of the waterlines. The right image shows the results of the edge detection process using a wavelet based algorithm. After geocoding the SAR images, each point of the extracted waterlines is assigned to a water level provided by a water level model to represent the height of the sea bottom. The operational model of the German Maritime and Hydrographic Agency (BSH) calculates currents and water levels based on forecasts of wind and air pressure above the North Sea. The heights of the points along each waterline form a random grid of topographic points with a wide spacing when the gradient of the sea bottom is flat and with a narrow spacing when the gradient is steep. A digital elevation model can be derived from this irregular grid by triangulation and interpolation.

This procedure will be used to evaluate a total of 76 ERS-SAR-Images taken between 1995 and 1999. The maps are derived for each year and are used to observe the sediment transport in the recent years.

Cooperations

• BSH Hamburg

• MARUM

• Doktorandenkolleg Lebensraum Nordseeküste (in German)

Related Links

• Trilateral Wadden Sea Cooperation

• ERS-SAR Sensor

Conferences

• Klocke, B., Schmidt, R., Heygster, G.: Topographische Karte des Wattenmeeres aus ERS SAR-Daten. Jahrestagung 2000 des Arbeitskreises "Geographie der Meere und Küsten" (AMK), Vechta 31. Mai-3.Juni 2000.
• Klocke, B., Dannenberg, J., Schmidt, R., Müller-Navarra, S., Heygster, G.: Topographic mapping of the german tidal flat areas using ERS-SAR images, ERS-ENVISAT Symposium, Göteborg 16.-20. Oktober 2000.

Publication

• Dannenberg, J., Klocke, B.: Erstellung topographischer Karten des Wattenmeeres, Vechtaer Studien zur Angewandten Geographie und Regionalwissenschaft, Band22, pp.44-51