Urban areas in the United States are defined by the U.S. Census Bureau as contiguous census block groups with a population density of at least 1,000 per square mile (about 400 per square km). Urban areas are delineated without regard to political boundaries. Urban areas with a population of at least 50,000 serve as the core of a metropolitan statistical area. Urban areas are among the most accurate measure of a city's true size, due to the fact that they are all measured from the same criteria.
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