Visual Analysis to Assess Attraction and Organisation of Contemporary Metropolitan Systems—A Case Study of Central and Northern Italy

The landscape of scientific research is rich with experiments aimed at identifying polycentric morphologies, defining their degree of polycentricity, and the socio-economic and environmental relationships that develop within them. However, some aspects are still under-researched, such as defining a procedure for determining the extent of the metropolitan area of influence. This research aims to experiment with a graphical-analytical methodology aimed at identifying and representing the functional area of metropolises, i.e., the territorial limit beyond which a metropolis' attractiveness ceases to exert its territorial influence, and which also allows the territorial ramification of urban cores with greater attractiveness to be determined and visualized graphically. Using Visual Analytics as a conceptual basis, it is possible to combine methods and technologies that harness the potential of human understanding with the increased capabilities of electronic data processing for a more adequate understanding of the research scope. For this research, the graph-analytic mix adopted comprises the graph theory algorithms for the analytical quantification of relationships and the reference surface area of polycentric metropolises. In contrast, the subsequent visualisation of relationships and their spatial branching is based on the electronic evolution of graphical techniques based on the works of Henry Drury Harness and Charles Joseph Minard, particularly those invented to map commuter flows and migrations. The research results, focusing on Northern Italy, demonstrate a highly interconnected and polycentric system, with macro-areas whose functional boundaries seldom coincide with the administrative boundaries of the regions. The research demonstrates the existence of five territorial macro-regions encompassing the 11 regions outlined in the Italian state's legislation, containing polycentric metropolitan systems with distributional characteristics that differ from each other.