Comparing Multilevel Clustering Methods on Weighted Graphs: The Case of Worldwide Air Passenger Traffic 2000–2004 - Methods for Multilevel Analysis and Visualisation of Geographical Networks - page 144

Fig. 9.6 Clustering of worldwide air traffic using the strength index

The cities forming the “principal worldwide hubs” group are American and
European (Fig. 9.7 ). Examining the second, finer level of this area of the graph
reveals European hubs such as Paris, London and Frankfurt, as well as the principal
American hubs such as New York, Los Angeles, Houston and Miami. These
American hubs are dissociated for two principal reasons:

￿ American hubs are the continental entrances for connections arriving from

Europe;

￿ The majority of American hubs are chosen by large American companies, such

as Delta Airlines in Miami or American Airlines in New York ( Vo w l e s , 2000 ).

An American sub-network is dissociated in the center, which can be specified
at a third level. In fact, the American territorial extent creates a more complex air
organization than does Europe. Indeed, air is the fastest and most effective means of
transportation in the United States. This level mainly consists of American cities, but
three European cities appear to be particularly connected to these cities: Amsterdam,
Dublin and Manchester. This European presence can be explained by the bilateral
agreements and alliances between companies, in particular Continental Airlines,
which is present at all three airports.