Use of Color Composites in Landsat TM Data
Content of this page courtesy of NASA's Observatorium
When someone mentions infrared, the first thing
that generally comes to mind is the old heat lamp you use to help hatch
chickens or to convince your friends that you've spent the winter in Florida.
The energy from that lamp covers a large part of the infrared portion of
the electromagnetic spectrum. Infrared starts where visible leaves
off: in the EM spectrum, red grades into reflective infrared. This is the
infrared energy reflected from the surface of the object.
The light from that old infrared heat lamp looked
red, didn't it? Were we seeing infrared light? No, what we actually saw
was the red end of the visible spectrum, but we did feel the infrared energy.
Our eyes cannot see infrared. The heat we sensed was the other end of the
infrared spectrum.
![[Spectrum image]](obs4a.gif)
The infrared portion of the spectrum is one of
the most useful for identifying what a surface is made of because there
is both reflected energy and emitted energy. Plants reflect much more energy
in the reflected infrared than in the visible, and we can determine the
state of the health of the plant from its reflectance. The reflected infrared
energy also tells us a lot about rocks and minerals that may be present.
For example, we can often distinguish clay-bearing minerals from other
minerals (important in gold and silver exploration), although it usually
takes additional information to classify specific rocks.
The multispectral remote sensing instruments carried
on the Landsat and other satellites measure the amount of energy reflected
and emitted in several discrete portions, or bands, of the EM spectrum.
The various visible and infrared bands were chosen to measure reflected
and emitted energy in areas of the spectrum that correspond to known responses
of the target materials. These include specific characteristics of land,
vegetation, water, rocks, and temperature.
![[Infrared image of Miami, Florida]](obs2a.gif)
Three-band composites are created by using the measured reflected energy
in each of three Landsat Thematic Mapper (TM) spectral bands to control
the amount of blue, green, and red in a color output image. The way in
which the seven TM bands are mapped to the three colors in the output image
depends on what information is desired to be highlighted in the image.
For some applications, it may be desirable that landcover classes be associated
with familiar colors, e.g., grass is green. In other cases, contrasting
colors are preferred to highlight objects of interest from the background.
We give three examples of commonly used band combinations and describe
how different features appear in each.
Note: The specific bands used in three-band composites are
often identified by giving the band numbers used for red, green, and blue,
respectively. Thus, an image using band seven for red, band four for green,
and band two for blue would be designated (7,4,2). We use this same convention.
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 the atmosphere. 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 band
creates 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. |
The chart shows the various TM bands and
typical applications.
![[TM bands and typical applications]](obs3a.gif)
Band 1: 0.45 - 0.52 m (Blue)
Provides increased penetration of water bodies as
well as supporting analyses of land use, soil, and vegetation characteristics.
The shorter-wavelength cutoff is just below the peak transmittance of clear
water, while the upper-wavelength cutoff is the limit of blue chlorophyll
absorption for healthy green vegetation. Wavelengths below 0.45 m are substantially
influenced by atmospheric scattering and absorption.
Band 2: 0.52 - 0.60 m (Green)
Spanning the region between the blue and red chlorophyll
absorption bands, this band corresponds to the green reflectance of healthy
vegetation. The spectral sensitivity of bands 1 and 2 combined is similar
to Kodak's Water Penetration Aerial Color Film (SO-244).
Band 3: 0.63 - 0.69 m (Red)
This red chlorophyll absorption band of healthy green
vegetation is one of the most important bands for vegetation discrimination.
It is also useful for soil-boundary and geological boundary mapping. Band
3 may exhibit more contrast than bands 1 and 2 because of the reduced effect
of the atmosphere. The 0.69 m cutoff represents the beginning of a spectral
region from 0.68 to 0.75 m where vegetation reflectance crossovers occur
that can reduce the accuracy of vegetation studies.
Band 4: 0.76 - 0.90 m (Near infrared)
For reasons discussed above, the lower cutoff for
this band was placed above 0.75 m. This band is especially responsive to
the amount of vegetation biomass present in a scene. It is useful for crop
identification, and emphasizes soil-crop and land-water contrasts.
Band 5: 1.55 - 1.75 m (Mid-infrared)
This reflective-IR band is sensitive to turgidity
-- the amount of water in plants. Turgidity is useful in drought studies
and plant vigor studies. In addition, this band can be used to discriminate
between clouds, snow, and ice (so important in hydrologic research) as
well as being able to remove the effects of thin clouds and smoke.
Band 7: 2.08 - 2.35 m (Mid-infrared)
This important band is used to discriminate among
various rock formations. It is particularly effective in identifying zones
of hydrothermal alteration in rocks.
Band 6: 10.4 - 12.5 m (Thermal infrared)
This band measures the amount of infrared radiant
flux (heat) emitted from surfaces. The apparent temperature is a function
of the emissivities and true (kinetic) temperatures of surface objects.
Therefore, band 6 is used in locating geothermal activity, thermal inertia
mapping, vegetation classification, vegetation stress analysis, and in
measuring soil moisture.
Top Level