A major advance in satellite-based land surface mapping
has led to the creation of more accurate and detailed maps of
our cities. These maps provide urban planners with a better
understanding of city growth and how rainfall runoff over
paved surfaces impact regional water quality.

Maps taken from space are invaluable to city planners and
state agencies monitoring water quality in urban areas, and
are replacing the more expensive and time-consuming
traditional aerial photography.

These space-based maps of buildings and paved surfaces, such
as roads and parking lots, which are impervious to water, can
indicate where large storm water runoffs occur. Concentrated
amounts of runoffs lead to erosion and elevated amounts of
soil and chemical discharge into rivers, streams and ground

Scott Goetz, Project Manager of the NASA-sponsored Mid-
Atlantic Regional Earth Science Applications Center (RESAC) at
the University of Maryland, presented these highly detailed
surface maps today at the American Geophysical Union spring
meeting, Boston, MA.

Andrew Smith, a faculty research assistant working with Goetz
at the Mid-Atlantic RESAC, developed a faster and less
expensive capability, utilizing Landsat 7 and Space Imaging’s
Ikonos satellite data, to generate accurate maps of paved
surfaces. “It’s a major advance in monitoring capability
because aerial photo mapping can’t keep up with the pace of
change,” Goetz said. “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.”

Urban sprawl results in more paved surfaces and less area for
water to drain into soils.
Reduced drainage areas bring more water into fewer drainage
systems at a faster rate, eroding the banks of streams and
rivers, and adding more sediment into the water. “If you
increase an impervious surface near a stream by creating a
paved parking lot, for example, you directly affect the
quality of life in the stream because of the runoff that
surface will generate,” Goetz said.

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

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. Currently, the RESAC team is working with
planning departments to add the data from the maps into future
urban planning models.

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. Areas between and
beyond the Baltimore-Washington corridor are more “green” with
levels that range from 0 percent to 20 percent impervious
surface areas.

The RESAC team has provided maps to the Chesapeake Bay
Program, Maryland’s Departments of Planning and Natural
Resources and Montgomery County Department of Environment,
among others, to monitor water quality and changes in
residential land use. These organizations incorporate the
impervious surface area data into models that predict water
quality, future land use and the potential effectiveness of
various “smart growth” policies.

NASA launched Landsat 7 in April, 1999. Images are archived,
processed, and distributed by the U.S. Geological Survey,
which is also responsible for day-to-day operations of the
satellite. This research is being conducted as part of NASA’s
Earth Science Enterprise, a long-term study of how natural and
human-induced change affects our global environment.

Additional information and images can be found on the Internet