Accuracy of different georeferencing strategies on historical maps of Rome

The map of the city of Rome 1:5000 scale of 1908 and the map of "Roma e l'agro Romano" that represents a wider area, realized in 1912 have been studied in earlier papers were parameters useful to perform the transformation of the used geodetic system in more modern systems were estimated. Here we want to evaluate the effect of different georeferencing strategies on these maps evaluating separately precision of the model and accuracy obtainable from the maps. The geodetic system used the ellipsoid of Bessel oriented locally in the geodetic observatory in Genoa, while for the projection was used conformal Flamsteed one. To be able to exclude that the deformations are mainly due to the different projection in comparison to the transverse of Mercatore used both for the UTM and for the national system used after 1940, called Gauss-Boaga and based on the Datum Roma40, we decided to study a transformation able to re-project the coordinates of the points with known coordinates as the vertexes of the cartographic elements. The reprojection of the maps need use of package with more complex algorithms of those available on the most diffused software to resample the raster images. After the geroreferencing step, the residual errors must be studied to assess the partial or complete remove of systematic errors. The map georeferenced using the cartographic reprojection can furnish a valid tool to detect variations as the geomorphologic variations due to natural and human causes within one century. The studied cartographies are one of the only existing documents that uniformly represents this whole territory and realized according to modern cartographic methodologies.