As Hamilton noted when posting the map of updated Virginia Metropolitan Statistical Areas, the MSA’s are calculated with commuting statistics. If more than 25% of workers in a locality commute to a nearby metropolitan area, that county is considered to be closely linked to it. This is a useful cutoff point, but like all cutoffs, it is completely arbitrary. I mapped five of the state’s largest MSA’s to show the degree to which their surrounding counties are dependent on them. This is important to understand since a significant part of Virginia’s “rural” areas are becoming increasingly exurban – dependent on nearby urban economies rather than agriculture or local industries. Again, 25% is the cutoff for inclusion in the MSA (see Hamilton’s map linked above). That includes all workers commuting to one of the core counties of the MSA. The “core counties” are not explicitly listed anywhere that we’ve been able to find, but there is a definition and by working backwards from the commuting numbers it’s not too hard to figure out.