Surface imperviousness is a useful indicator of urban build-up. It describes the amount of concrete and other impermeable surfaces through which water cannot filter, one effect of which is urban runoff. This map shows the percent increase in surface imperviousness from 2001 to 2011. Each pixel represents the average across an area of 500 square meters.
Data source: USGS Multi-Resolution Land Characteristics Consortium (MRLC), National Land Cover Database 2001 & 2011, Retrieved from www.mrlc.gov.