Since 1950, the Census has defined Urbanized Areas (UAs) as the contiguous, built-up areas of large urban settlements. These are areas with populations of 50,000 or more (though population of what has varied), including adjacent small areas (census tracts or blocks) that meet a minimum density threshold. The population within this area is then literally defined as urban, as opposed to the population outside, unless located in another urban area.
UAs have an advantage over Metropolitan Statistical Areas (MSAs) for some purposes because they include only the territory that is (more-or-less) built up and that can be considered unambiguously urban, making distinctions literally at the block level. MSAs, being composed of entire counties, can include widely varying amounts of land with no urban characteristics and little or no connection to the urban area. Because of this, UAs are especially appropriate for the examination of patterns of urban settlement, including urban sprawl. It makes sense, for example, to calculate densities for UAs, while it does not make sense for MSAs (as discussed here).
Because UAs include only the contiguous developed urban territory, they have very irregular boundaries and are generally much smaller than their MSAs. Here is the example of the Indianapolis UA within the counties making up the Indianapolis MSA:
While UAs are very useful for examining the patterns of urban areas at a single point in time, significant difficulties arise when looking at the changes in urban patterns over time, especially over extended periods of time. Two major problems stem from the irregular boundaries created using blocks as the smallest building blocks and from the changes in the UA definition over time.
UAs are delineated for each decennial census, and a variety of data from that census are reported for each of those areas. So it is possible to compare the population, land area, and density, and many other characteristics of a UA in 2010 with a UA in 2000. Of course, by definition, these represent different urban extents. The Census does not report the 2010 statistics for the 2000 (or earlier) UAs, nor does it go back to the 2000 Census data and report that information for the newly delineated 2010 UAs. So while you can make comparisons of the entire UAs at two points in time, you cannot directly consider the population, land area, or other characteristics of the territory that has been added to a UA between 2000 and 2010. You can say that the population of the entire UA has increased by some amount over the decade, but you cannot say how much of that increase occurred from the addition of the new territory as opposed to an increase (or decrease) in the population in the original 2000 UA.
A workaround is possible for recent censuses. You could take the block data from the 2010 census and use it to estimate values for population and other variables for the 2000 UA and vice versa. (Because block data would be required, you would only be able to use the limited set of variables reported at that level.) There would be some minor problems resulting from changes in block boundaries. And (sometimes related to the changes in block boundaries), some areas that were within the 2000 UA will not be in the 2010 UA. But these problems are relatively small and this procedure would be workable.
Making the estimates using block data will be possible only for those recent censuses for which machine-readable block data and the associated geographic boundary files for both blocks and UAs are available. No problem for 2000 and 2010. I think it might be possible for 1990. Doubtful whether one could go back further. (And don’t consider doing this without machine-readable data. My dissertation involved my doing this type of estimation, by hand, for data from multiple points in time for traffic analysis zones, census tracts, and wards (not blocks), for one urban area. That was several months of my life.)
But even if one would be content with comparing the totals for the UAs from each census over time, changes in the definition of UAs makes this highly problemmatic. From the 1950 through the 1990 Censuses, the definitions evolved each decade but were fairly consistent, allowing for reasonable comparisons. But a major change in the definition of UAs was made for the 2000 Census. The change was made to create a cleaner, more consistent definition that made use of the data processing capabilities that had become available. The new definition was a major improvement, but it creates problems for making comparisons with the earlier UAs.
While the 2000 UA definition was different from the earlier one in many ways, two changes are of greatest importance. First, the minimum population density required for an area to be added to the UA was reduced from 1,000 to 500 persons per square mile. It is obvious that this is a major change. (I have seen numbers of articles, post–2000, still referring to the 1,000 person per square mile value as the threshold for inclusion in the UA. Either the authors have not read the new standards or have not read far enough. The new definition requires that an urban area start with a core with a population density of at least 1,000 persons per square mile, but then goes on to require only the 500 persons-per-square-mile density for adding adjacent areas. And for UAs, with populations exceeding 50,000, there will hardly be an issue of their having a core meeting the higher density threshold.
The second major change from 1990 to 2000 is that the earlier UAs included the entire areas of incorporated places and Census Defined Places. The new definition uses only blocks and either block groups (in 2000) or census tracts (in 2010) and totally ignores the boundaries of these larger areas. For incorporated places that are entirely developed and urban, where all of the subareas meet the minimum density threshold, this makes no difference. But for cities that have annexed significant areas of nonurban land in advance of development, these areas were included in the UAs in 1990 and before while they are no longer included in 2000 and 2010. A comparison of the areas of UAs between 1990 and 2000 shows the effect. Many UAs grew in both population and land area over the decade. But a number of areas, while growing in population, showed significant drops in land area. This almost certainly was the result of the nondeveloped land within incorporated places being excluded in 2000.
At least the UA definition has remained largely consistent from 2000 to 2010. Even then, little changes in Census procedures can produce strange anomolies. I was comparing the extent of 2000 and 2010 UAs and was surprised to find that the land area of the San Diego UA declined dramatically despite a major population increase. I compared the areas on a map and found what happened. A significant portion of Camp Pendelton, a very large Marine base at the northern tip of the UA was included in 2000, despite the fact that much of that area was hardly urban. (It would not do to have military exercises, including artillery practice and tank operations, within a real urban area!) In 2010, only a very small portion of the base was included in the UA, presumably the areas where the Marines were housed. It appears that this change resulted from an agreement between the Department of Defense and the Census Bureau on changes in how census subdivisions like tracts and blocks could be delineated on military installations. (Don’t tell me that DOD forbad such subdivision earlier because that would provide intelligence on the location of personnel on military bases! Probably!)
In case anyone actually wants to read the official definitions of the UAs, they are published in the Federal Register for 2000 and 2010 and are available on the Census website here. A history of the UA definitions up through 1990 can be found here.