Job density and employment subcenters in the four U.S. metropolitan areas

Examining urban spatial structure has been always a hot issue in the field of urban geography. Identification of job centers provides a foundation to further analyze any spatial-economic relationship, and the nature of the spatial changes in a metropolitan area. Based on the assessment of the strengths and weaknesses of various approaches that have been used to define job centers, this research chooses a geographically weighted regression (GWR) and weighted median (WM) method to identify employment subcenters in Chicago, Boston, New York, and Los Angeles. Then in order to find out the spatial changes of employment subcenters in the four metropolitan areas, a simple descriptive approach -- Zipf's Law of rank frequency distribution is used to examine the spatial pattern and compare the spatial change in the selected four cities from 1990 to 2000. Finally, to achieve better geovisualization of job density, this research utilizes an intelligent dasymetric mapping (IDM) technique to visualize density surface of Chicago in a 3-D GIS environment. This study reveals obvious polycentric spatial structure in the four U.S. Cities. Despite New York, Chicago and Boston still have strong and long-established downtown as the largest employment centers, the urban structure has been changing to a unique pattern of decentralization. In contrast, Los Angeles remains the typical polycentric urban structure city and still keeps more tendency of decentralization.