The four clusters show four special types of the urban structural hierarchy of Seoul: the CBD area (D), the inner suburbs (C), the outer suburbs (B), and the periphery area (A) (see Fig. 3b). The main difference between the two clustering types is that the station areas in the fringe area (A) of the three clusters are divided into the outer suburbs (B) and the periphery area (A) for the four clus- ters. Most of the station areas of the outer suburbs are located in regional hub footholds, which were developed with higher density, more diversity, and higher intermodal connectivity than those of the periphery area (see Table 3). Cluster B can be defined as a transition zone (Kim, 2008), into which the employment and shopping activities of the CBD and inner suburbs overflow. Clusters C and D of the four clusters are quite similar to clusters B and C of the three clusters.