Ideally, the denominator should be a measure of net residential acreage to closely match the land use, but such data are not readily available.1

Data Sources

The principal data source for the numerator of the density calculation is the decennial U.S. Census of Population and Housing. As discussed in Chapter 2, two national sources of data can be used to track changes in land use—the denominator—over time, each with its pros and cons. The first is the National Resources Inventory (NRI), collected by the Natural Resources Conservation Service of the U.S. Department of Agriculture. The NRI surveys hundreds and sometimes thousands of sample points in each U.S. county and reports, among other data, the acres of land in each of a series of land-cover categories. The NRI combines two of these categories, urban and built-up areas and rural transportation land, to derive an estimate of developed land (see Box C-1 for detailed definitions). For the purposes of this study, the NRI’s landcover data were aggregated to correspond to U.S. census designations for metropolitan areas as defined in 1999.2 For the density calculation, the NRI’s urban and built-up land category was used as the denominator— referred to in this appendix as urban acres—and intercensal estimates of housing units at the metropolitan level as the numerator.

The advantage of using the NRI data is that they provide true landcover-based estimates of aggregate density, excluding land—even that within city limits—upon which development has not occurred. The NRI data also include urban and built-up land beyond census-defined “urbanized areas” where a very large share of recent development has


Net residential acres include only residential land. Nonresidential land uses are excluded, as are local streets and parks (Downs 2004). Gross residential acres, by comparison, include all land within a geographic area, regardless of its uses. Unfortunately, there is no simple or well-established way to relate these two density measures because conditions vary widely from one location to another.


The most recent year for which detailed inventory data from the NRI are available is 1997.

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