Smith cites previous researchers who have shown a relationship between the amounts of impervious surface cover within a watershed and the quality of surface water within that watershed. Generally, when 10 percent to 15 percent of an area is covered by impervious surfaces, the increased sediment and chemical pollutants in runoff have a measurable effect on water quality. When 15 percent to 25 percent of a watershed is paved or impervious to drainage, increased runoff leads to reduced oxygen levels and harms stream life. When more than 25 percent of surfaces are paved, many types of macro and microorganisms in streams die from concentrated runoff and sediments. Impervious surface maps also are useful in mapping urban sprawl.
Sprawl is indicated on the maps by increases in land consumption and housing construction.
By monitoring an area over time, maps can show the progress of residential development. Smith has produced a map of the Washington-Baltimore area that quantifies how much impervious surface there is across the entire region. Baltimore and the counties that border it have at least 20 percent, and up to 40 percent, impervious surface area, indicating that pollution from runoff could be a problem. The District of Columbia and surrounding watersheds in Virginia and Maryland have levels of impervious surfaces between 20 percent and 30 percent percent. NASA launched Landsat 7 in April, 1999.