Landsat MoreThanPrettyPicture 2013Apr11
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Transcript of Landsat MoreThanPrettyPicture 2013Apr11
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More than a Pretty Pic ture:
How Landsat Images Are Made
Malaspina Glacier,
Alaska
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The wholeelectromagnetic (EM) spectrum consists
of the longest wavelengths (radio), shortest ones
(gamma rays), and everything in between.
People have grouped EM waves into these categories
in order to talk about them.
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Visible light, the light we see with our eyes alone, is
a very small partof the whole spectrum of radiant
energy in the universe.
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We measure radiant energy in wavelengths, from
crest to crest.
Wavelength (a)
is longer
than wavelength (b)
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Colors have different wavelengths!
We see colors as different becausethey have
different wavelengths.
Red has the longestwavelengths of visible light, and
blue/purple has the shortestwavelengths of visible light.
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NASA illustration by Robert Simmon. Astronaut photograph ISS013-E-8948
About 25 percent of the Suns energy is absorbed by the
atmosphere; about 50 percent is absorbed by the Earths
surface; and about 30 percent is reflected back to space.
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In this picture,
IR is Infrared light
R is red light
G is green light
B is blue light
Consider a tree and its leaves.
Red, green, blue, and infrared light from the sun hit the
tree and its leaves.
Infrared and green light are reflected from the tree.
Red and blue light are absorbed by the tree.
Image Credit: Canada Centre for Remote Sensing
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We seethe tree as green,
because wavelengths of light we call green
are reflected to our eyes by the tree.
Image Credit: Canada Centre for Remote Sensing
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Wavelengths we see as green are about 525-550
nanometers (nm) in length. Wavelengths we see
as red are 630-800 nm in length.
The red petals of this poppy
flower reflect strongly at
wave-lengths of 700 nm.
Photo: Jeannette Allen
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Every kind of surface reflects light differently, absorbing
and reflecting it weakly or strongly in different wavelengths.
Photos: Jeannette Allen
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Butter reflects weakly in blue and strongly in yellow to red.
Tomato reflects weakly in blue and strongly in red.
redblue yellow
Every kind of surface has its own spectral signature,
somewhat like a fingerprint.
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Notice that water and vegetation reflect somewhat similarlyin the visible wavelengths (about 0.4 to 0.7 nm) but are
almost always separable in the infrared.
This graph shows the spectral signatures
of vegetation and water.
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(This graph
uses
micrometers
rather than
nanometers.)
More spectral signatures.Notice how different kinds of surfaces reflect strongly
or weakly at different wavelengths.
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More spectral signatures
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A farmer using remote sensing can tell which
sugar beet fields are healthy and which are not,
if she/he knows their spectral signatures.
If s/he were designing a sensor solely to measure the health of his sugar
beets, what wavelength range would he want the sensor to detect?
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People measure the spectral signatures of
different surfaces on the ground. Then when they
look at the spectral signature of a surface in a
satellite image, they can tell what kind of surfacethe satellite was looking at.
Researcher with hand-held spectrometer
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Lines (Rows)
Pixel
s
(Colu
mns)
Landsat scenes are made up of these
30-meter squares, or pixels.
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Landsat 7 observes the Earth in 7 ranges (or
bands) of the electromagnetic spectrum.
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Lets look at how it works for just 1 band of the 7. In this
illustration, each square represents a 30m x 30m piece of
land surface.
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The Landsat instrument records the amount of reflected light
in each band for each 30 m pixel, on a scale of 0 to 255. A
numerical value of 0 represents no reflected light and a
numerical value of 255 represents maximum reflected light.
0: See the corres-
ponding black pixel in
gray-scale array.
255: See the
corresponding white
pixel in gray-scale
array.
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7 bands of data looked at side by side
in shades of gray
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Now how do we make color
images of all that grayscaledata
so we can work with it more easily?
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We have to assignColorsto represent
Landsat bands(using computer software).
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People can chose red, green, or blue to
represent any of the wavelength ranges they like.
One can make lots of color combinations. (This is Beverly, MA.)
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5 6 74321
3,2,1
Visible Infrared
Green Data is shown as GreenBlue Data is shown as BlueRed Data is shown as Red
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5 6 74321
4,3,2
Visible Infrared
NIR Data is shown as RedRed Data is shown as GreenGreen Data is shown as Blue
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Visible wavelengths
Infrared wavelengths
Making images with different band combinations,
we see more than we could otherwise.
Same scene, different wavelengths
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To view a short movie about how wavelength
ranges (bands) are combined to make an
image, go to:http://landsat.gsfc.nasa.gov/education/resources/L7_sanfran_onionskin.mov
http://landsat.gsfc.nasa.gov/education/resources/L7_sanfran_onionskin.movhttp://landsat.gsfc.nasa.gov/education/resources/L7_sanfran_onionskin.movhttp://landsat.gsfc.nasa.gov/education/resources/L7_sanfran_onionskin.mov -
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Some common band
combinations
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True-Color Composite (3,2,1)True-color composite images approximate the range of vision for the
human eye, and hence these images appear to be close to what we
would expect to see in a normal photograph. True-color images tend to
be low in contrast and somewhat hazy in appearance. This is because
blue light is more susceptible than other bandwidths to scattering by theatmosphere. Broad-based analysis of underwater features and
landcover are representative applications for true-color composites.
Near Infrared Composite (4,3,2)
Adding a near infrared (NIR) band and dropping the visible blue bandcreates a near infrared composite image. Vegetation in the NIR band is
highly reflective due to chlorophyll, and an NIR composite vividly
shows vegetation in various shades of red. Water appears dark, almost
black, due to the absorption of energy in the visible red and NIR bands.
Shortwave Infrared Composite (7,4,3 or 7,4,2)A shortwave infrared composite image is one that contains at least one
shortwave infrared (SWIR) band. Reflectance in the SWIR region is due
primarily to moisture content. SWIR bands are especially suited for
camouflage detection, change detection, disturbed soils, soil type, and
vegetation stress.
Mount Rainier
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Another infrared composite
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Another infrared composite
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A farmer using remote sensing can tell which
sugar beet fields are healthy and which are not,
if she/he knows their spectral signatures.
If s/he were designing a sensor solely to measure the health of his sugar
beets, what wavelength range would he want the sensor to detect?
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Color Composites in Landsat TM Data
True-Color Composite (3,2,1)
approximates range of vision for human eye
images appear as we would expect to see in
photograph. low in contrast, somewhat hazy - blue light more
susceptible to scattering by atmosphere.
Broad-based analysis of underwater features and
landcover are applications for true-colorcomposites.
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Band 3 (Visible red) = red
Band 2 (Visible green) = green
Band 1 (Visible blue-green) = blue
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Color Composites in Landsat TM Data
Near Infrared Composite (4,3,2)
Add near infrared (NIR) band and drop visibleblue band.
Vegetation in the NIR band is highly reflective NIR composite vividly shows vegetation invarious shades of red.
Water appears dark, almost black, due to theabsorption of energy in the visible red and NIRbands.
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Color Composites in Landsat TM Data
Shortwave Infrared Composite (7,4,3 or7,4,2)
contains at least one shortwave infrared(SWIR) band.
Reflectance in the SWIR region is dueprimarily to moisture content.
SWIR bands are especially suited for changedetection, disturbed soils, soil type, andvegetation stress.
B d 7 (SWIR) d
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Band 7 (SWIR) = red
Band 4 (NIR) = green
Band 3 (red) = blue
Or...
Band 7 (SWIR) = red
Band 4 (NIR) = green
Band 2 (green) = blue
vegetation- shades
of green.
Bare soils
and
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Station Fire - August 30, 2009 - 140,000 acres burned in Angeles National Forest - Mt.
Wilson Observatory threatened. Blue and white haze is smoke, pinkish areas have been
burned, active fires appear bright red.
Landsat-7ETM+
Stripin
g
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Landsat 5 image - Esperanza Fire in San Bernardino National Forest, Oct. 26, 2006. The
fire caused a huge smoke plume; reaching ~3.0 miles high. Large image is a natural-
color composite, small inset is false-color composite. The active fire fronts in the false-
color image appear bright yellow.
T f l l L d t 5 i (1984 d 2009) ]d k l id f it t t d
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Two false-color Landsat 5 images (1984 and 2009). ]dark purple grid of city streets and
green of irrigated vegetation.
Images were created using reflected light from the shortwave infrared, near-infrared and
green portions of the electromagnetic spectrum (Landsat 5 TM bands 7,4,2)
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Venice - Landsat 7 - 2001. pan-sharpened false-color composite of ETM+
bands 7, 5, and 3. Venice can be found on Landsat WRS-2Path 192 Row
28.
Cli t Ch
http://landsat.gsfc.nasa.gov/about/wrs.htmlhttp://landsat.gsfc.nasa.gov/about/wrs.htmlhttp://landsat.gsfc.nasa.gov/about/wrs.htmlhttp://landsat.gsfc.nasa.gov/about/wrs.html -
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Climate Change - Lake Chad borders Chad, Niger, Nigeria, andCameroon, as well as on a natural border, the Sahel: a grassland which divides
the Sahara Desert to the north and the more humid savannah to the south.
Because Lake Chad is a shallow lake, depths of 16 - 26 feet, its surface area
fluctuates markedly with changes in climate.
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The newest Landsat satellite, Landsat 8, measuresthe amounts of reflected light on a finer scale than
Landsat 7s scale of 0-255.
Landsat 8 measures light on a scale of 0-1024.
With Landsat 8 we can see many more shades of
the light we are measuring, and we can study
greater nuance in our scenes of interest.
A little more to know:
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Landsat 8 also uses more bands of light
than Landsat 7, and the band numbers
are a little different.
To learn the differences between Landsat 7
and Landsat 8 spectral bands, go to this URL:
http://landsat.usgs.gov/L8_band_combos.php