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 UA, between their core areas and the area where the two UAs meet. The Census Bureau considers two UAs separated by less than 5 miles of water or undevelopable territory and connected by less than 5 road miles to be contiguous if the areas adjacent to the undevelopable territory and the road connection have an average population density of at least 1,000 people per square mile.
 * The two UAs share a common boundary line—not simply a point—that is at least 1 mile in length.
 * The census blocks on both sides of this common boundary have an average density of at least 1,000 people per square mile.

When two UAs touch, but do not meet all the criteria specified above, they remain separate. The separation line always follows census block boundaries, and often is a prominent physical feature or the boundary of a legal entity.

The central place(s) of a UA identifies the most populous center(s) or core of that UA. This designation permits data users to compare statistics for the dominant center with those for the remaining part of the UA—the urban fringe. Most UA central places are also the central cities of MAs. In rarer instances, where the UA does not contain an MA central city or the UA is located outside of an MA, the central place(s) of the UA is (are) determined by total population.

To determine central place status, the Census Bureau selects the most populous incorporated place that contains at least 2,500 people. It may select up to two additional incorporated places as central places, provided that each additional place has a population of at least 15,000 and that each has at least one-third the population of the most populous incorporated place. If the UA does not contain any incorporated place of 2,500 or more, the largest CDP is selected as the central place, provided that more than 50 percent of its population is located outside the boundaries of a military installation. Urban and Rural Classifications12-13