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.