at least one city of 25,000 or more inhabitants were termed the subdominant sectors, and the remaining sectors were termed the local sectors.
In each case, urban population was measured for counties and these were assigned to one or more of the sectors. Convergence of sector boundaries eliminated this process near the city and sector differences were only recorded for areas more than 25 miles out from the centre.
Figure 4.4 shows the stages in sector delimination for one of the cities, Memphis (Tennessee): route sectors are unshaded, subdominant sectors are in black, and local sectors are stippled.