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 145

Fig. 9.7 Clustering of worldwide air traffic: focus on main hubs

The central component of the third level of the graph can also be divided into
two lower levels, i.e., a fifth level is introduced (Fig. 9.7 ). This fifth level represents
the local organization of the center and the western North American area. These
are secondary areas, which are very dependent on the connections between them
and are, at the same time, deeply “embedded” in the American hub system (levels 2
and 3) that is dominant at the world level (level 1). The strong connections between
the secondary American cities and the world level arise from the characteristic
organization of American airspace, which has been hierarchical, from very early
on, in terms of three main hub categories:

￿ traditional “gateways” (Los Angeles, Chicago, Boston, and San Francisco);

￿ principal hubs (Philadelphia, Atlanta, Houston, and Dallas); and

￿ secondary hubs (Minneapolis, Denver, Phoenix, Cincinnati, St Louis, Tampa, and

Las Vegas) ( Connor , 2003 ; Weber & Williams , 2001 ).

This hierarchical organization integrates American secondary cities particularly
well into the world air network.
The position of all these cities is thus at the core of air world traffic. The
encapsulated shape of these graphs shows a highly integrated system, primarily on
the part of the United States territory.
In comparison to the North American system, the Asian and European systems
appear less integrated into the world system. However, while the European cities are