This exercise will give background information for the following:
- The electromagnetic spectrum
- The spectral signature of objects on the
ground, such as vegetation or soil
- Landsat False Color Composite interpretation
The Electromagnetic Spectrum
 Our
eyes are “remote sensors”, they help us collect information
about the physical world without coming in direct contact with
it. Our eyes, however, are limited. They allow us only to detect
a small part of the electromagnetic information that is available.
By using satellites we are able to see more parts of the spectrum
to collect information about the physical world that we could
not detect with our eyes.
 For
example, consider this photograph. It provides information in
the visible spectrum, just like our eyes would detect. What new
information could we expect to see if we displayed this same
picture as a color infrared photograph?
Sensing Outside the Visible Range
If vegetation appears as red in a color infrared photograph,
why are the daffodils not red but white?
True Color Photograph |
Color Infrared Photograph |
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The answer is that the daffodils are not
vegetation. They are made of plastic. The addition of data
from the near-infrared region of the spectrum allows us
to gain information we might not have had otherwise.
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Spectral Signatures
The sensors in satellites, like our eyes, detect the amounts
of electromagnetic energy reflected by (or emitted from) objects
on the earth’s surface. The graph below shows different
reflectance values for soil and vegetation for various parts
of the electromagnetic spectrum. The unique curve associated
with an object is called its spectral signature.
Notice that for vegetation the percent reflectance in the green
region is not as high as in the near-infrared. This is why we
use false color or infrared images instead of a true color images
when sensing vegetation. |
