A New Proposal for the Global and Optimal Conformal Updating of the Cadastral Digital Mapping: the Italian Case

Referring to the current state of the Italian cadastral map, the paper describes an innovative analytical method to perform the general adjustment of the fiducial point network, and the optimal conformal adaptation of new surveyed cadastral parcels into an already existing digital map. Working on the surveyors' data files containing the mutual distance values between couples of fiducial points, the proposed procedure reciprocally fits, in the least squares sense, the various and corresponding fiducial polygons, as elemental component parts of the general fiducial network, by way of similarity transformation adjustment models computed by Generalised Procrustes algorithms. The proposed method furnishes more accurate results than those obtained by the conventional least squares adjustment of the individual distances measured between fiducial points; furthermore it satisfies and advantageously preserves the geometrical shape of the original surveys. In addition, the preservation of the original shape allows also the conformal mosaicking of the new surveyed cadastral entities with those ones obtained by the digitisation of the original maps. Finally, some numerical examples show the capabilities of the method proposed. In the first example, a network of fiducial points has been simulated, and the original data have been modified in such a way to reproduce different error situations. The analysis of the results has been carried out in a systematic manner. It has furnished the comparison of the obtainable levels of precision and accuracy by applying the two alternative methods: the Procrustes algorithms for the global and simultaneous adjustment of the fiducial point polygons, and a conventional approach based on the adjustment of the single polygon sides. In the second experiment, the Procrustes procedure has been applied to the general adjustment of a relevant number of real and surveyed fiducial polygons, located in the north-west side of Udine municipality. The realisation of this experiment is given to the cooperation between the University of Udine and the Udine “Agenzia del Territorio“ of the Italian Cadastre Administration.