Ancient map comparisons and georeferencing techniques: a case study from the Po River Delta (Italy).

Two coeval 16th century maps of the Po river delta area (Northern Adriatic Sea), both signed by Ottavio Fabri, were studied to understand the differences in their contents, to test their georeferencing and to accomplish the first evaluation of existing errors. As they are pre-geodetic documents, many problems were involved in performing the analysis. The chief problems were the inability to record the true, original author errors and the impossibility to restore the shape of the eroded landscape and morphological tracts, due to the current non-existence of recognizable landmarks for a large part of the area. To overcome these problems, an analysis approach is suggested, which consists in: i) attempting to recognize the original survey techniques and their restrictions; ii) evaluating the true differences between the ancient map and the preserved environmental context (frequently, the mapping errors do not match the residuals resulting from the relocation of the landmarks after the application of standard geometrical transformations). In this paper, different methods were used to test the reliability of the georeferencing methods, so as to understand their effectiveness in highlighting the kinds of errors recorded in the maps. The georeferencing algorithms available in software packages commonly used today work as a “black-box tool,”without providing explicit information for an analysis concerning the method adopted to construct the original map. In such a manner the georeferencing global error resulting from the processing does not directly relate with the error due to the original author survey. An improvement in the research and a possible way to overcome this problem could be by finding a tool capable of understanding and reproducing the working method applied by the ancient cartographer, based on the recognition of the original topographic reference baselines and possibly the related sightings used. The comparison between these original map parameters and the corresponding ones exhibited in the current maps will probably allow us to define the actual errors recorded in the ancient map (that is, not induced by digital processing). Furthermore, the inner reference systems and parameters existing in the original maps are basic elements for studying the real ancient errors. These inner reference systems should be physically carried over onto the modern maps and related reference systems, so as to overcome some unsolved problems, such as our knowledge of the ancient absolute magnetic declination. These are some potential ways to try to extrapolate useful information also regarding the peripheral areas of the ancient maps, not preserved today due to natural erosion phenomena, with a view to reaching an error-model capable of overcoming the dearth of landmarks. Until now, this kind of problem appears to be unsolved, making hard the comparison between time-crossing map configurations of peculiar geomorphological tracts and the evaluation of the landscape change rates in this important area.