Our maps of counties and cities capture new development and can be repeated much more quickly than the tedious and expensive traditional photo interpretation work. Smith cites previous researchers who have shown a direct relationship between the amounts of impervious surface cover within an area and the quality of its surface water. Generally, when 10 percent to 15 percent of a given land area is covered by impervious surfaces, the increased sediment and chemical pollutants in any runoff leads to reduced oxygen levels and damage to stream life. When even more than 25 percent of surfaces are paved, many types of macro and micro-organisms in streams die from the concentrated water flows. Crazy pave 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 would most definitely 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. NASA launched Landsat 7 in April, 1999.