A digital elevation model of the inter-tidal areas of the Wash, England, produced by the waterline method

Figure 1 (cover) shows a digital elevation model (DEM) of the inter-tidal areas within the Wash, England, produced by the waterline method using ERS-1 SAR images and hydrodynamic modelling. The waterline method (Cracknell et al. 1987, Koopmans and Wang 1995, Mason et al. 1995, Ramsey 1995) involves ® nding the georegistered positions of the shoreline (the land± sea boundary) from a remotely sensed image using image processing techniques, then superimposing the heights of the shoreline relative to mean sea level on the corresponding positions. These heights are predicted using a hydrodynamic tide-surge model run for this area with the atmospheric conditions pertaining at the time of image acquisition. From multiple images obtained over a range of tide and surge elevations, it is possible to build up a set of heighted shorelines within the inter-tidal zone, and from this a gridded DEM may be interpolated. Such a DEM is useful for developing improved tide-surge models, and changes in the DEM over time allow measurement of sediment mass transfers in the inter-tidal zone due to storm or seasonal changes. Figure 2 shows the shorelines used to produce the DEM. These were extracted from thirteen ERS-1 SAR images obtained mainly during the winter months of 1992 ± 1994, using the semi-automatic shoreline delineator described in Mason and Davenport (1996). They were heighted using shoreline elevations generated by a hydrodynamic tide-surge model of the English east coast similar to that of Flather (1994). Model heights were corrected using local tide gauge information as described in Davenport et al. (1996 ). The lowest shoreline present had a mean elevation of O 2 8m ODN, whilst the highest shoreline had a mean elevation of 2 9m ODN. A raster DEM was interpolated from the heighted shorelines using universal