Impervious surfaces, including paved areas such as roads, parking lots, and sidewalks as well as rooftops, prevent water from draining naturally into the ground. The results are many, including an increased burden on stormwater infrastructure, erosion, reduced water quality, 'heat island effects', and both ground-level and water pollution. To learn more about the impacts of concentrated impervious areas see the EPA fact sheet linked below.

This measurement is derived from the National Land Cover Database (NLCD) 30-meter national dataset. It represents the percent of total land within each block group that is impervious and not covered by tree canopy.

NOTE: Prior to June 2020, the Compass relied on National Agricultural Imagery Program (NAIP) data sourced from the EPA EnviroAtlas. This information reported only on the urbanized area of Durham County, and gave metric estimates which were slightly larger than the current compass values (an 8.1% larger value than the NLCD estimates on average). We switched to NLCD data in order to both extend the time series reported here - showing information for both earlier and more recent years - and to extend the area of coverage to the entire county. Comparisons to all areas of the contiguous US are also made possible by choosing NLCD for these summaries.

Source: National Land Cover Database (NLCD), accessed via the Multi-Resolution Land Characteristics Consortium.