A post made some time ago describes my definition of exurban areas for the urban patterns research and the previous post refers to a paper in which I address this in greater detail. These are areas of contiguous census tracts with a minimum density of one housing unit per 15 acres around the 59 large urban areas.
After delineating the areas, I began by looking at the sizes of the exurban areas in 2010. The variation of exurban area sizes was amazing. In terms of land area, the exurban areas ranged from 171 square miles to nearly 7,800 square miles. Numbers of housing units went from 19,000 for the smallest exurban area to 1.2 million for the largest. The smallest exurban area was around the Albuquerque urban area and the largest was New York. Of course some of the variation is due to differences in the sizes of the metropolitan areas. But it is noteworthy that San Diego, not a small area, had the third smallest exurban area.
So perhaps it is more reasonable to compare the areas by looking at the ratio of their land areas and housing units to the areas and units in their urban areas. The smallest ratio for exurban land area to urban land area was about one-third, for Miami. Knoxville was the largest exurban area relative to its urban land area, nearly 8 times as large. Miami likewise had the smallest number of housing units relative to its urban area, 3 percent. And Knoxville had 1.4 times as many exurban housing units as urban units.
The contrast can be seen in the maps of the Miami and Knoxville urban and exurban areas. Also included is Portland, which had the median ratio of exurban to urban land area of about 23 percent. (The three maps are at approximately the same scale.)
Urban and exurban areas for the areas with the smallest, median, and largest ratios of exurban land area to urban land area
More information on the exurban areas in 2010 can be found in my paper, “Exurban Areas Around Large Urban Areas in the U.S. in 2010,” which can be downloaded here.
I have read a number of scholarly articles in which the authors were using census Urbanized Area data from 2000 or later in which they described those areas as consisting of territory with a population density of 1,000 or more. And that is incorrect. The density threshold for adding blocks or other small areas to an Urbanized Area (or Urban Cluster) is 500 persons per square mile. I’m not into naming and shaming and won’t. But come on! If you can’t even describe the data you are using accurately, why should anyone trust anything else you are saying?
I know where the error comes from. Starting with the 2000 census, the Census Bureau dramatically changed how they defined the notion of “urban” and Urbanized Areas (for the most part greatly improving the definition). Under the old definition, it was the case that a small area had to have a population density of at least 1,000 persons per square mile to be included in an Urbanized Area. An excellent summary of how the census definition of “urban” has evolved can be found here.
I assume that a researcher making this error had read earlier articles that described Urbanized Areas as consisting of areas with densities of 1,000 or more (either correctly, if referring to pre–2000 Urbanized Areas or incorrectly, if referring to the later areas). I expect this would be the source, not the census definition of the earlier Urbanized Areas, for if these authors were too careless and lazy to look up the definition for their current work, they likely would not have done so in the past either.
The current Urbanized Area density minimum plays a key role in the definition of urban areas for my urban patterns research. And of course I am continuing to read new articles that are published that deal with urban patterns, including those using Urbanized Area data. The first few times I read articles referring to the 1000-person-per-square-mile cutoff for 2000 or 2010 Urbanized Areas, I panicked. Did I make a mistake in understanding the definition and get it wrong? (It is a complex definition.) Each of those times I went back and re-read the formal notices on urban area criteria for 2000 and 2010 in the Federal Register. After having assured myself several times that I was correct, I no longer have to repeat this.
The 2000 and 2010 urban area criteria do make use of a population density minimum of 1,000 persons per square mile in the first stage of the delineation process. An urban area core is defined that includes small areas with population densities of 1,000 or more. Then additional areas are added with densities of 500 persons per square mile and above. The existence of an initial urban area core meeting the higher density threshold will not be an issue for Urbanized Areas.