A procedure using a geographic information system was developed to define urban land use representative of the 1990's by overlaying U.S. Bureau of the Census 1990 population density at the block group level on 1970's digital land-use data from 1:250,000-and 1: 100,000scale maps. Any area having a population density of 1,000 or more people per square mile is re-classified as "urban" land use in the derivative product. The procedure was applied to 20 study units of the National Water-Quality Assessment program to provide what are considered reasonable indications of urbanization that has occurred since the 1970's.
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USGS Numbered Series
Refining 1970's Land-Use Data With 1990 Population Data to Indicate New Residential Development