But more importantly, they found that the core region, called the focus of attention, has three roles - not two as proposed by previous researchers.

First, three of the counties in metro Atlanta’s five-county core region—Clayton, Cobb and Gwinnett—elected to start their own bus lines rather than merge with MARTA.

The PCF region and core diameter given are the average of three measurements along each axis, and the ellipticity is the ratio of the maximum to the minimum measurement.