Lighting
Up the Ecosphere
Using satellite images of city
lights at night, NASA scientists are mapping the
spread of urban areas around the globe and monitoring
their impact on our planet's ecosystem.
NASA
Science News
Courtesy of NASA's Earth
Observatory.
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to this story (requires RealPlayer)
November
15, 2000 -- Watching the familiar, rural landscapes of
our youth give way to suburban sameness has become as
much a part of modern American life as portable
electronics, instant food, and time spent in front of
the television.
Nearly all of us have had the
disappointing experience of returning to what used to
be the woods near our childhood homes and finding a
new subdivision. Or we have been shocked to see that
some corporate entity has erected aluminum-sided
duplexes and an outlet mall in the middle of our
favorite vacation spot.
Like it or not, throughout this
century, the United States has undergone a steady
process of urbanization as a larger and larger
percentage of the population has moved towards the
cities.
Above: Based upon satellite
measurements of city lights, this image is a map of
the urban population density of North America. Red,
yellow and green are urban areas, and blue is peri-urban.
The city light data is laid over elevation data (black
is sea level, light gray is over 10,000 feet).
While increasing urbanization may
have some positive impacts on our environment, such as
the lower birth rates that come with a city lifestyle,
scientists are becoming more concerned about the
negative long-term effects. Unlike rural communities,
urban sprawl completely transforms the landscape and
the soil and alters the surrounding ecosystem and the
climate.
Marc Imhoff, a biologist at NASA's
Goddard Space Flight Center, is one of these concerned
scientists. For the past six years, he and a team of
researchers have been looking for ways to measure the
effects of urbanization on the biological productivity
in the U.S. and other countries around the world.
They created a method of mapping
urbanization on a countrywide scale by using satellite
images of the light cities generate at night. With the
resulting city lights maps, they are now zeroing in on
the impacts urban sprawl has on the food we eat, the
air we breathe, and the ecosystem within which we
live.
Urbanization in any country
generally begins when large-scale commerce takes root
and most new jobs are to be found in the factories and
financial centers in cities.
In the United States, urbanization
began to occur roughly around the turn of the last
century. Since then, the percentage of people living
in the United States in urban areas has risen from 39
percent to more than 73 percent, according to the U.S.
Census Bureau.
Above: Urbanization map of the
United States derived from city lights data. Urban
areas are colored red, while peri-urban areas are
colored yellow. (Image courtesy Mark Imhoff, NASA GSFC)
From decade to decade, the amount of
rural land that has been consumed by urbanization is
enormous. Between 1982 and 1992, for instance, 19,000
square miles of otherwise rural cropland and
wilderness were developed in the U.S. This would be
the equivalent of converting half of Ohio into one big
subdivision in a ten-year period, according to the
World Resources Institute.
Urbanization is not just an issue in
the United States.
Right now, researchers estimate that
worldwide movement towards cities is growing at three
times the rate of population expansion worldwide. Only
a third of the planet's population lived in urban
areas ten years ago. Now that number is up to 50
percent, and in ten more years roughly two thirds of
humanity will live in the cities, the World Resources
Institute reports.
Imhoff says that while working at
Stanford University as a post-doctorate, he initially
became intrigued by the biological implications of
urbanization after he saw how urban sprawl appeared on
the Earth's surface when viewed from orbit.
"I spent a number of years
looking at the Earth from space, and I found it
remarkable how human development of land looked a lot
like biological growth -- like mold on an
orange," he says.
Right: Cities
and suburban areas are growing rapidly, as shown by
these two images of Plano, near Dallas, Texas. (Images
courtesy USGS Earthshots)
He wanted to know how urban sprawl
was changing the landscape on a global scale, and
whether this increased development was affecting food
supplies, local ecosystems, and even the global
climate.
Early on, he realized that the best
way to do such a study would be to construct a map of
urbanization using remote-sensing data from
satellites.
"Satellite data would give us a
synoptic view of the globe from which we could get an
explicit idea of where the human-dominated surface
features are -- especially with respect to
cities," he says. "We could also merge that
information with soil maps and biosphere data from
other satellites to assess the impact of urbanization
on ecosystems."
To construct such a map Imhoff
needed a satellite instrument that would give him a
snapshot of the urbanization on an entire continent
all at once.
When he began this research, he had
access to data from a number of remote-sensing
satellites, such as Landsat 5 and NOAA's operational
satellites, which record the reflected sunlight and
heat emissions from the surface of the Earth. But
demarcating urban sprawl with the instruments on these
satellites would require the researchers to retrieve
close-up images of cities and separate each
individual, urbanized area from the surrounding farms,
parks, and wilderness. Doing so for an entire
continent would have been labor intensive and tedious.
Imhoff found a solution in an
unlikely place. He recounts a weekly astronomy club
meeting he attended in 1996.
"The people at the meeting were
talking about how light pollution was a problem,"
he recalls. "They pulled out a
satellite-generated city lights map. They were looking
at it and saying, 'Look at all that awful light
pollution.' And I'm there thinking, 'This is exactly
what I need. There's my global map of where human
beings are.'"
The images were taken by a Defense
Meteorological Satellite Program's (DMSP) Operational
Linescan System (OLS). This network of satellites was
originally designed to pick up on lunar illumination
reflecting off of clouds at night in order to aid
nighttime aircraft navigation.
What the Air Force discovered is
that on evenings when there was a new moon, the
satellites were sensitive enough to record the
illumination from city lights. Over a period of
several new moons, the data the satellites retrieved
could be pieced together to produce a global image of
city lights.
Above: A map of the night-time
city lights of the world constructed from images taken
by the Defense Meteorological Satellite Program's
Operational Linescan System
Upon receiving the satellite data,
Imhoff realized there was one big problem they'd have
to overcome.
"The raw image overestimates
urbanized areas by as much as seven or eight
times," he says.
The problem came mostly from the
effects of the relatively bright city lights on the
satellites' sensor array. The sensors on these
satellites are made up of photoelectric cells
organized into in a grid-like pattern, like pixels on
a computer monitor. When light emanating from the
Earth's surface hits one pixel on the array, it is
registered in the satellite data as a 2.7-kilometer by
2.7-kilometer square area of well-lit land surface.
Yet sometimes, bright lights trigger
the initial pixel and inadvertently set off
neighboring pixels in the array as well. If this
happens, then an area the size of a city block will
appear to be the size of three or four square blocks
on the raw satellite image.
It's somewhat similar to what
happens when a flash photo is taken of a mirror.
Though the flashbulb itself may not be more than a
couple of inches across, the light from the flash
reflecting off the mirror would likely cover an area
the size of a person's head on the photograph.
To correct for this
"blooming" effect, the Goddard team zoomed
in on the lights emanating from individual cities,
effectively isolating them from the larger,
continental image. Using computers, they then lowered
the overall brightness levels of the city image. The
blob of lights representative of the given metropolis
would begin to shrink on the outside in a manner
similar to an evaporating puddle of water.
"We
scale back on the brightness levels of the imaging
data, until the perimeter stops shrinking on the
outside and the interior lights of the city begin to
break up," Imhoff says. "At that point we
stop."
Right: The black waters of Lake
Michigan on the right bound the bright lights of
Chicago, which fade into dark rural landscapes on the
left. NASA scientists assembled a dataset of lights
detected from space to measure the extent of cities
and urban areas worldwide.
The researchers classified the
lights left on the image, after this dimming process,
as urban area. The previously lit areas on the image
that shrank back were classified as peri-urban
(low-density suburban areas or farmland). Any areas
that had no lights to begin with were labeled as
non-urban.
They compared these classifications
to the boundaries on the actual urban areas of the
city and found there was a close match.
Imhoff and his team now had a set of
numbers (threshold values), which told them to what
extent the lights from any portion of the United
States should be dimmed to get an accurate and
spatially explicit representation of urbanization.
Using the threshold values, Imhoff's group categorized
the entire continental United States into urban, peri-urban,
and non-urban areas.
To make sure the classifications
were correct on a nationwide basis, they checked each
state on their map against the 1990 U.S. Census
population statistics. Imhoff explains the Census
Bureau doesn't map urban areas. However, it does
classify urban areas as any region where there are
1000 people or more per square mile, and it takes a
tally of who lives in these areas. By merging the city
urban map with the Census data, the researchers could
calculate population density for the urban, peri-urban,
and non-urban lands. They found that the number of
people per square mile on his map measured up to the
Census's definition of an urban area (1000 people and
up per square mile).
"After the thresholding we had
an almost perfect match, which is amazing since we
didn't use any Census data to create the satellite
map," Imhoff said. "We thought this is good;
this is working."
For peri-urban areas Imhoff found
there were roughly 100 people per square mile, and in
non-urban areas, roughly 10 people per square mile.
While the Goddard team's map still
couldn't give them the exact density of the population
for these classifications, it presented them with a
picture of where the landscape had been transformed to
the point where it no longer resembled the natural
ecosystem. The researchers could be fairly certain
that any area classified as urban on their maps had at
least a few subdivisions, strip malls, and parking
lots.
Imhoff and his group could now
overlay this map of the United States with other maps
showing where the best soils are, where fragile
ecosystems exist, and where plant life is the most
robust. With such comparisons, the NASA scientists
could determine exactly how urbanization is affecting
our planet, our natural resources, and even our
climate. By repeating the entire process for other
countries, they could get an idea of what was
happening all over the world.
To find out what Imhoff learned when
he and his team compared urbanization maps of the
United States with a map of the most fertile soils in
the nation, visit NASA's Earth Observatory and read
"Reaping
What We Sow," the second part of this feature
story. A third article in the series, yet to be
published, will examine how urban sprawl might
contribute to the greenhouse effect in the Northern
Hemisphere.
Source: Earth
Observatory
Production Editor: Dr.
Tony Phillips
Curator: Bryan
Walls
Media Relations: Steve
Roy
Responsible NASA official: Ron
Koczor
Related Links
Earth
View - A
fascinating, "live" look at our home from
above. Also try the TerraServer for high
resolution satellite maps. In some cases, you can
see your back yard! This data is old by today's
standards (one year to six years old), but very
good. Mother Earth doesn't change that much in a
few years, just civilization...
Bright
Lights, Big City -- Original Earth Observatory
feature article
Goddard
Space Flight Center -- Homepage
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