Value of ancient cartography for geoenvironmental purposes. A case study from the Po River delta coast (Italy).

A set of four large-scale pregeodetic maps was studied. They depict the Po River delta ancestor in the late-16th century AD (before the year 1604), an extremely important area for the geoenvironmental and historical evolution of Northern Italy at the beginning of the Little Ice Age. The maps are very detailed and complex. This characteristic involves some problems relating to accuracy and comparison with present-day cartography. This first attempt at map georeferencing is required in order to make possible original coastline location in areas that do not exist today because of sea erosion. Nevertheless, a further attempt is already being made for achieve a better understanding of the maps. Inner details and manifest errors were highlighted so as to better appraise the reliability of the maps and the authors’ survey methodologies. Furthermore, a particular and highly peculiar geomorphological object (i.e. an offshore megabank) was analyzed and rejected as a completely untrue ancient landscape tract. Hence, the really interdisciplinary character of this kind of studies must be ever taken into consideration and critical map analysis should not merely be seen as a useless and time-consuming analytical tool since the fortuitous preservation of ancient documents can dispel wayward interpretations in geoenvironmental reconstruction. The results of the study portray the many problems relating to the local coastal evolution, more interesting than had been believed up to now. The existence of four different maps representing the same geographical area in the same short (decadal) time span is a fortuitous and very rare historical case, as well as being a unique opportunity. The maps reproduce the same objects with an astonishingly high degree of accuracy even if there is no robust geographical frame capable of providing a definite points location. The most problematic factor is the impossibility of a reliable restitution of the original eroded coastline. Hence the need to apply a more appropriate and more accurate georeferencing method becomes quite evident. In spite of this drawback, the multiple comparison allows us to state that the Aleotti map (1D) cannot be assumed to be a reliable data source for inferences concerning the 16th century coastal and marine dynamics nor the fluvial sediment transport rates notwithstanding its undoubted scenographic value. Only mere chance was responsible for the broad diffusion and knowledge of this historical map across the European cultural environment of the 17th -18th centuries. If the megabank was not a real natural object, then it is possible that it could have originated from a bad copy job from the maps of other authors (Fabri?) or even from an attempt by Aleotti to suggest a personal interpretative model for the local coastal erosion occurring in the last decade of the 16th century, resulting from a merging of various sources of information, modified by an author’s experience acquired in the southern delta reaches. In any case, that information source appears to be rather dated. Hence, at times the casual nesting of a set of compelling evidence and suggestions offered up by the ancient primary data sources can be a very dangerous analytical tool unless it is supported by an adequate critique. Nevertheless, the question as to what the real truth of the information content offered by an ancient map corresponds remains unanswered: that is, if it is the one proposed by the original document or, conversely, the one suggested by the georeferenced derived-sample.