Georeferencing old maps: a polynomial-based approach for Como historical cadastres

Recent developments in digital technologies have opened new and previously unimagined possibilities for the exploitation of cartographic heritage. In particular, georeferencing converts them from pure archival documents to real geographic data. This study investigates the issue of georeferencing the historical maps which are currently preserved at the State Archive of Como. These maps, about 15000 at the scale of 1:2000, belong to different cadastral series: the Theresian Cadastre (XVIII century), the Lombardo-Veneto Cadastre (mid-XIX century) and the New Lands Cadastre (1905). Georeferenced maps should then be inserted in the Internet GIS system, developed within the Web C.A.R.T.E. project, for an interactive 2D- and 3D consultation. Due to the peculiar nature of maps, which are divided in several adjacent cadastral sheets for each municipality, a preliminary mosaicking of these sheets was performed. Using the digital cartography of current municipalities, Ground Control Points and Check Points were collimated on the historical maps. A polynomial transformation was chosen to georeference the maps. An ad hoc-built procedure based on statistical evaluation of GCPs and CPs residuals was implemented, in order to determine the optimal polynomial order to be used. Evaluation of georeferencing results was performed both qualitatively and quantitatively. The obtained accuracy is much higher, as the territories covered by the maps are smaller and more densely-built. The methodology is automated and can be proposed as a reference for georeferencing maps of comparable characteristics. Historical maps can thus be continuously navigated into a georeferenced framework and compared with current cartography. This clears the way for the usage of historical maps in a wide range of applications, such as territorial planning, urban and landscape changes analysis and archaeological research.