Accuracy of 1908 high to medium scale cartography of Rome and its surroundings and related georeferencing problems

Abstract

Preliminary attempts to georeference maps of early twentieth century made by the Military Geographic Institute (IGM, the Italian geodetic agency) for the city of Rome and its surroundings, reported residual errors larger than errors observed on similar maps. Previous studies carried out on one or two century older maps of the same area, showed similar or even smaller errors (Baiocchi and Lelo 2005).

Six sheets of the “City of Rome and its surroundings” map in scale 1:5 000 dated 1908 have been studied. The identified errors can be referred to the different system of geodetic projection and geodetic datum or to the derivation of some details from maps at smaller scale, but in this case historic documents seem to suggest a different explanation.

Parameters useful to perform the transformation of the geodetic systems used in historical maps to modern systems are not known; for this reason until now the various attempts of georeferencing maps of this type were based on collimation of points recognizable on modern cartographies such as corners of historical buildings. This method has often given unsatisfactory results; therefore it was decided to proceed by determining the parameters for the transformation of geodetic datum.

The history of geodetic systems used in Italy at the beginning of the 20th century is complex and, in the past, this has led some researcher to misinterpretations. For this reason a full explanation of geodetic systems used in Italy in this period is reported below. Since the parameters of the projection used for the maps in our case study are not known for sure, the reprojection was considered the only way for a correct georeferencing.