Reconstruction of Coastline Changes by the Comparisons of Historical Maps at the Pomeranian Bay, Southern Baltic Sea

Coastline changes are becoming an increasingly important topic along with continuously rising sea level and an increase in extreme storm events. This holds at the southern Baltic Sea coast where eustatic change, glacio-isostatic land subsidence, and strong storms events cause at most parts a continuous coastal retreat. The coastline changes at the time scale of decadal to centennial are the long-term accumulative effect of climate forces, meanwhile anthropogenic influences have to be taken into consideration as well. From a set of historical maps covering almost 300 years, in particular the “Messtischblatt” maps (starting with 1829 AD) provide the condition to be geo-referenced for quantitative comparisons with modern Digital Elevation Models. The accuracy of these maps is quantified by using the Root Mean Square Error of spatial differences of fixed points between the modern aerial photographs and historical maps. A first-order polynomial transformation is chosen to geo-reference the maps. The comparisons between historical maps and the modern coastline derived from a Digital Elevation Model indicate that the coast can be subdivided into four zones (types) in terms of the trend of coastline changes: A continuously retreating (A-) or advancing coastline (A+); B relatively stable coastline (coastline changes are within the accuracy error bars); C anthropogenically influenced coastline changes; D randomly changing coastline. This classification is found to coincide with the mean coastline geodetic orientations and the gradient of alongshore sediment transport capacity. This remarkable coincidence confirms the reliability of the reconstructed coastline changes.