A new methodology to manage Italian geodetic datums of the cadastral systems and of the historic maps

Cadastral maps represent the result of the highest scale survey of any region. They are not topographic maps. Their goal is to show the property system of the mapped area, so any topographic element that is important for this aspect is shown while others often omitted. Historical cadastral sheets provide valuable information about not only the natural and built environment but also about some aspects of the society of the time of the survey. Because of their high scale, their creation is extremely expensive. Even the updating of the cadastre claims considerable funds from the state budgets. That is why the coordinate system, the geodetic basis of a cadastral work is rarely changed. Re-ambulation is often made on the basis of the older version, following their geodetic and topographic 'skeleton'. The main period of the cadastral works in Europe was the first two thirds of the nineteenth century. Countries that gained their independency after this period have quite complicated cadastral systems, more or less preserving the political distribution of their lands of the time of cadastral mapping. Besides Germany, Italy provides the best example to this. Concerning Italy, the complete unification of the country was in 1870. To this time, a large part of the Apennine Peninsula was covered by cadastral surveys, mainly carried out by Piedmont, the Kingdom of Naples, and the Papal State as reported by Frazzica et al. (Geophysical Research Abstracts 11:4791, 2009). After the WWI, territorial gains from the former Habsburg Empire resulted three new cadastral systems to be incorporated (one of them 'lost' after WWII). Together with them, nowadays, Italy has 31 major ("grandi origini") and more than 800 smaller, local ("piccole origini") cadastral systems; all of them have its own projection origin (Fig. 1; Baiocchi et al. 2011). At some smaller parts of the country, the modern national map grid system was later introduced and applied as reported by Moncada (1948), Bonifacino (1953), and Giucucci (Rivista del Catasto e dei Servizi Tecnici Erariali 8(2):109-113, 1953). Fortunately, the story of the related geodetic datums was simplified prior to nowadays. Of course, the abovementioned systems had several trigonomentric networks as geodetic bases. In the first decades of the twentieth century, however, the Italian Institute of Military Geography (Istituto Geografico Militare; I.G.M) developed four geodetic networks, all on the Bessel 1841 ellipsoid; the Genova 1902 (Fig. 2), the old Monte Mario (Fig. 3), the Castanea delle Furie 1910, and the Guardia Vecchia (Fig. 4) datums, for northern, central, and southern Italy and Sardinia, respectively (Mori 1922). However, these systems were independent ones; afterward, one of them, the Genova 1902 datum, was extended to cover other parts of the country. Outside of northern Italy, the original area of this datum, the coordinates of the basepoint, and the cadastral system origins were transformed from the locally valid network to Genova 1902 datum. The present paper aims to estimate the accuracy of this extension of that system and to describe the parameters of the other systems for GIS applications, thus offering a tool for future, higher accuracy methods to fit the cadastral maps of southern Italy and Sardinia to modern grids. This can be useful also to convert some older technical maps that were referred to the same geodetic datums. © 2013 Società Italiana di Fotogrammetria e Topografia (SIFET)