## 4 Discussion
### Geometry of Acquisition  
In general, using different geometries of acquisition, different angles of incidence, and ascending and descending orbit directions, we found that the reproduction of surface structures indicated or amplified by the contrast of sediment and water surfaces is relatively insensitive to geometry of acquisition when making use of visual image interpretation. The same holds for habitats with an extensive three-dimensional surface roughness, such as mussel beds, mud fields, and fields of shell detritus which can be visually identified by their specific patterns and textures under the differing geometries we used.  
However, we found some variations in the characteristics of the TerraSAR-X images are due to varying incidence angles of the geometry of acquisition. In near range, that is at small incidence angles <24\({}^{\circ}\) (relative orbits 131, 139), the images show sharp contrasts and widespread high backscatter. Therefore, strongly scattering structures are not well demarcated from each other: Mussel beds, humpy mud fields with a dense network of gullies, sediment surfaces roughened by sandworm (_Arenicola marina_) heaps, and steep sandy slopes (depending on exposition in relation to sensor, orbit direction) are displayed similarly brightly which makes the differentiation of these surfaces more difficult. Furthermore, in mussel beds, internal structures are less recognizable. However, when surrounded by smooth surfaces, e.g., smooth water cover, these scatterers stand out sharply. Any roughness of the water surface, on the other hand, is also highlighted and eddies and currents can clearly be seen when biofilms or other surface-active agents are present. As backscatter values of the flooded areas can be quite high, they often exceed those of smooth intertidal surfaces.  
With incidence angles of 30-40\({}^{\circ}\) (rel. orbits 40, 63), the water surface becomes more uniform and scatters less, the images are less sharp in contrast and more differentiated in the backscatter values. Mussel beds and other structures with high backscatter are better distinguished from each other and from rougher surroundings.  
Increasing incidence angles of 40-47\({}^{\circ}\) (rel. orbits 116, 154), amplify further differentiation of backscatter intensities. Mussel beds, for example, stand out more clearly from their surroundings, from other rougher surfaces, or from steep edges with high backscatter, which is also due to the fact that the internal structures are better recognizable. Fine linear structures of the sediment surfaces are clearly visible.  
All of this is reinforced with incidence angles above 50\({}^{\circ}\) (rel. orbits 25, 78). Mussel beds are clearly recognizable. However, gradual transitions are now displayed very fluently and demarcations are therefore less obvious. Under good environmental conditions, i.e., with well drained flats, fine surface structures are clearly visible (e.g., linear structures).  
In summary, for most intertidal surface types acquisitions at incidence angles between 30-47\({}^{\circ}\) therefore are most suitable. For specific questions smaller or higher incidences can be useful.