Tag Archives: MSA

Defining exurban areas

For the urban patterns research, in addition to delineating the urban areas for each year, I wanted to delineate exurban areas beyond the urban areas that could reasonably be considered to be parts of the metropolitan area related to the urban core. Unlike the census Urbanized Areas, however, there is no accepted standard definition for exurban areas. Fortunately, a thorough review of past studies of exurban areas and how they were defined has been provided by Berube and others (Finding Exurbia, Brookings, 2006).

A minimum population or housing unit density–obviously much lower than the urban density threshold–was the most common criterion used in defining exurban areas. Other factors were also considered, especially commuting to the urban area. Data are not available over the entire period of the urban patterns dataset to allow the use of commuting. However, the maximum extent of the exurban area would be limited to the area of the Combined Statistical Area (CSA) or Metropolitan Statistical Area (MSA), which at a minimum guarantees interaction with the urban area for 2010 for the counties as a whole, if not individual tracts.

I decided to define exurban areas as the sets of contiguous tracts that were adjacent to the urban areas and had housing unit densities greater than some value. The minimum density levels used to define exurban areas in various studies varied widely, from 40 acres per housing unit down to about 10 acres per unit. (For studies using the lowest densities, the extent of the exurban areas was most often limited by the commuting criterion rather than density.) I approached the problem by mapping the tracts meeting different minima in 2010 to make a judgment as to what looked reasonable.

The very low minimum density thresholds of 30 or 40 acres per unit frequently resulted in all or most of the CSA or MSA being considered exurban, with the tracts meeting these levels extending far beyond those areas, especially in the eastern U.S. On the other hand, a density minimum of 10 acres per unit produced much smaller exurban areas than seemed reasonable and consistent with personal observation.

The choice came down to thresholds of either 15 acres per unit or 20 acres per unit. The resulting exurban areas generally looked appropriate for most areas. The final choice of 15 acres per unit came down to a number of specific situations where the lower density level produced areas that seemed too large. I’ll give two examples: The exurban area for Indianapolis in 2010 would have extended south at least halfway to Louisville, through area I would never consider exurban. And the Portland exurban area would have encompassed a large portion of the Willamette valley.

A further check reinforced my decision on the minimum density for exurban areas of 15 acres per unit, which is one-fifth of the urban density theshold. For CSAs or MSAs adjacent to other CSAs or MSAs, it was not uncommon for both exurban areas to extend to the common boundary. But for areas not adjacent to others, the extent of contiguous exurban density tracts was generally either confined within the boundaries of the CSA or MSA or extended beyond the boundary at only one or two points, with a string of exurban density tracts along a highway. (This is much like census Urbanized Areas, which frequently have such tendrils of urban development extending outward.) So the density threshold for exurban areas seems consistent with the areas of significant metropolitan interaction as indicated by the CSA and MSA boundaries.

This process of defining the exurban areas is treated in greater detail in the paper, “Defining Exurban Areas for the Analysis of Urban Patterns Over Time” which can be downloaded here.

On the choice of Combined Statistical Areas

Last year, I wrote a post discussing why I chose to use the larger Combined Statistical Areas (CSAs) for my urban patterns research rather than the commonly used Metropolitan Statistical Areas (MSAs). I followed this up with a second post giving examples of how the sharing of transportation infrastructure–commuter rail and airports–could be an indicator of the integration of areas that should be considered together as a single, larger metropolitan area.

This decision to use the CSAs is of such fundamental importance to my research that I felt it deserved more extended, formal treatment. I prepared the paper “On the Choice of Combined Statistical Areas” that provides greater background, covers the topics addressed in those blog posts in more detail, and addresses some other implications of the the choice of CSAs over MSAs. It also shows how the CSAs are comparable in extent to MSAs as they had been defined earlier for the 2000 census. This last topic was also addressed in an earlier post.

The paper is posted on the Research page of the website and can also be downloaded here.

Defining exurban areas

For the urban patterns research, in addition to delineating the urban areas for each year, I wanted to delineate exurban areas beyond the urban areas that could reasonably be considered to be parts of the metropolitan area related to the urban core. Unlike the census Urbanized Areas, however, there is no accepted standard definition for exurban areas. Fortunately, a thorough review of past studies of exurban areas and how they were defined has been provided by Berube and others (Finding Exurbia, Brookings, 2006).

A minimum population or housing unit density–obviously much lower than the urban density threshold–was the most common criterion used in defining exurban areas. Other factors were also considered, especially commuting to the urban area. Data are not available over the entire period of the urban patterns dataset to allow the use of commuting. However, the maximum extent of the exurban area would be limited to the area of the Combined Statistical Area (CSA) or Metropolitan Statistical Area (MSA), which at a minimum guarantees interaction with the urban area for 2010 for the counties as a whole, if not for individual tracts.

I decided to define exurban areas as the sets of contiguous tracts that were adjacent to the urban areas and had housing unit densities greater than some value. The minimum density levels used to define exurban areas in various studies varied widely, from 40 acres per housing unit down to about 10 acres per unit. (For studies using the lowest densities, the extent of the exurban areas was most often limited by the commuting criterion rather than density.) I approached the problem by mapping the tracts meeting different minima in 2010 to make a judgment as to what looked reasonable.

The very low minimum density thresholds of 30 or 40 acres per unit frequently resulted in all or most of the CSA or MSA being considered exurban, with the tracts meeting these levels extending far beyond those areas, especially in the eastern U.S. On the other hand, a density minimum of 10 acres per unit produced much smaller exurban areas than seemed reasonable and consistent with personal observation.

The choice came down to thresholds of either 15 acres per unit or 20 acres per unit. The resulting exurban areas generally looked appropriate for most areas. The final choice of 15 acres per unit came down to a number of specific situations where the lower density level produced areas that seemed too large. I’ll give two examples: The exurban area for Indianapolis in 2010 would have extended south at least halfway to Louisville, through area I would never consider exurban. And the Portland exurban area would have encompassed a large portion of the Willamette valley.

A further check reinforced my decision on the minimum density for exurban areas of 15 acres per unit, which is one-fifth of the urban density theshold. For CSAs or MSAs adjacent to other CSAs or MSAs, it was not uncommon for both exurban areas to extend to the common boundary. But for areas not adjacent to others, the extent of contiguous exurban density tracts was generally either confined within the boundaries of the CSA or MSA or extended beyond the boundary at only one or two points, with a string of exurban density tracts along a highway. (This is much like census Urbanized Areas, which frequently have such tendrils of urban development extending outward.) So the density threshold for exurban areas seems consistent with the areas of significant metropolitan interaction as indicated by the CSA and MSA boundaries.

Data for studying urban patterns over time

I want to study urban spatial structure over an extended period of time. Here are my data requirements: Data for population or housing units that can show the level of urban development. Data for small areas that enable the definition of the extent of urban development and the examination of distributions within the urban areas. Data for multiple points in time–as many as possible. Data for the same small areas at each point in time, to allow examination of changes in those areas over time.

My dataset begins with a unique resource, the Neighborhood Change Database created by the Urban Institute and Geolytics. This dataset includes census tract data from the 1970 through 2000 censuses, with the data for the years from 1970 through 1990 normalized for the 2000 census tract boundaries. So that’s 4 points in time. The block data from the 2010 census for population and housing units can be aggregated to the year 2000 tract boundaries, giving another year.

While many studies use population and population densities to study urban patterns, I have chosen to use housing units (as have others). They are more fixed and I think better represent the pattern of urban development. (The Census Bureau uses a minimum population density threshold to define urban areas. It is literally possible for an area to go from rural to urban from one census to the next without any new housing being developed. All it would take is an increase in population, for example, some babies being born.)

Using housing units also provides the opportunity to extend the data back in time. The census and the Neighborhood Change Database include the distribution of housing units by the year in which they were built. One can use this information for 1970 to estimate numbers of housing units that existed in earlier years. There are errors, as this approach cannot take into account changes to the stock of the older units that have occurred in the interim. I did an analysis that considered the extent of the error and concluded that it was reasonable to estimate housing units for the tracts back two decades, to 1950 but not further. This is discussed in a note Year-built Estimates Analysis on the Research page.

A remaining question involved which areas to examine and what would be their extent? As I noted in an earlier post, I believe Combined Statistical Areas (CSAs) better represent the extent of metropolitan areas than Metropolitan Statistical Areas (MSAs). I am choosing to examine urban patterns within the 59 CSAs (or MSAs, for areas not included in a CSA) that had populations over 1,000,000 in 2010.

Documentation of the urban patterns dataset is provided in a note Urban Patterns Dataset Description on the Research page.

Problems with the urban and metropolitan area definitions

The previous post described how the changes to the metropolitan area definition resulted in the splitting of numbers of large Metropolitan Statistical Areas (MSAs) as they were delineated for the 2000 census into 2 or more MSAs in 2003. This raised the obvious question, what was it about the new definition that produced these changes? The answer proved to be complex and bizarre, and the situation was only made worse by a horrible decision made by the Census Bureau with the 2010 Urbanized Area (UA) definition.

A major change made in the MSA definition first used in 2003 was to begin the delineation with the Urbanized Areas, including all counties with substantial portions of a UA in the MSA. After that, commuting to those central counties was used to add outlying counties to the MSA. The definition included provisions for merging adjacent MSAs using the same commuting criterion, but this made it highly unlikely that adjacent large MSAs could ever be merged. So as a result, the general extent of MSAs was determined by the extent of the UAs. If an area of contiguous urban settlement were split into 2 or more UAs, this would likely produce multiple MSAs.

So now we have to go to the UA definition. The new UA definition for the 2000 census provided for the splitting of large urban agglomerations into multiple UAs using the MSA (and CMSA and PMSA) boundaries as delineated for the census. So the extent of the MSAs depended on the UAs, and the extent of the UAs depended on the MSAs! It is a circular definition!

So what happens with the UAs for 2010? The Census proposed to maintain the status quo, keep the largest UAs with populations over a million the same, but not split smaller urban agglomerations, so contiguous UAs would be merged. This generated opposition from those in areas that would be merged, as this could affect the receipt of federal funding. The. Census response was to surrender and make the decision that the set of UAs that were delineated in 2000 would be frozen! Each 2000 UA would continue to be a UA in 2010! And since the MSAs continued to be based on the UAs, they would be largely frozen as well.

Since the beginning of the UA and MSA definitions in the mid-twentieth century, the areas had been allowed to evolve, with areas being combined as formerly separate areas grew together and were more reasonably considered a single entity. For example, Dallas and Fort Worth started as separate UAs and MSAs but for decades have been considered to be a combined area. What the Census Bureau did with the 2010 UA definition was to say that the delineation of urban and metropolitan America would be frozen as it was in 2000 and would not be allowed to further evolve. This was a truly horrible decision. And the Census Bureau knew it, as was clear from the misleading obfuscation of what they were doing in the Federal Register notice for the 2010 UA definition.

Far more detail is provided in a research note discussing the problems with the urban and metropolitan area definitions that is posted on the Research page and can also be downloaded here.

The effect of the changed definition of Metropolitan Statistical Areas

In an earlier post I explained why I chose to use the larger Combined Statistical Areas (CSAs) for my urban patterns research rather than the more common and familiar Metropolitan Statistical Areas (MSAs). I felt that in some cases the MSAs did not encompass what I felt was the whole metropolitan area. Exhibit 1 was the New York MSA, which did not include any of the Connecticut suburbs.

Before this, I had no occasion to systematically look at the extent of all of the large MSAs. But my recollection was that the MSAs were not always this limited. For example, the New York MSA used to include areas in Connecticut, and Raleigh and Durham had been a single MSA. I decided to start digging to find what had happened. It turns out that the Office of Management and Budget (OMB) made major changes to the MSA definition in 2000, which was first used to delineate new MSAs in 2003. This is also when the CSAs were introduced.

I decided to do a systematic comparison of the last MSAs delineated under the old definition, which were used in reporting the 2000 census, and the areas delineated in 2003 using the new standards. I looked at the 49 MSAs (and CMSAs, which were nothing more or less than MSAs for which subdivisions had been delineated) with populations over a million in the 2000 census. For a majority of the 2000 MSAs, the 2003 MSAs produced with the new definition were similar, varying only in the outlying counties included. But 18 of the 2000 MSAs were split into 2 or more MSAs in 2003, in one instance, into 6 different MSAs. These included New York and Raleigh-Durham.

For those areas where CSAs had been delineated in 2003, I compared their extent to the 2000 MSAs. In nearly all cases, the CSAs were quite comparable to the 2000 MSAs. They included the multiple new MSAs produced by the splitting of the older areas. No wonder I found the CSAs more reasonable than the MSAs. OMB thought those larger areas better represented the extent of metropolitan areas up through 2000!

A research note providing the complete results of these comparisons is posted on the Research page and can also be downloaded here.

Problems using Urbanized Areas for comparisons over time

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:

Indianapolis Urbanized Area (in red) within the Indianapolis Metropolitan Statistical Area

Indianapolis Urbanized Area (in red) within the Indianapolis Metropolitan Statistical Area

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.