FOR 326 Remote Sensing

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FOR 326 Remote Sensing Introduction
Instructor Michael Henderson Office
329-E E-mail mhender2_at_wvu.edu
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Web Addresses http//www.forestry.caf.wvu.edu/mhe
nderson Click on Remote Sensing Lectures can be
viewed below. You must have Microsoft Powerpoint
2000 or above to view them.
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Electromagnetic Spectrum
Most common to categorize wavelengths based on
their location within the electromagnetic
spectrum. Common measurement is micrometer (um)
which equals 1X10-6
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The visible spectrum of the human eye only
extends from 0.4um to 0.7um Blue 0.4 to
0.5um Green 0.5 to 0.6um Red 0.6 to
0.7um UV Below 0.4um Near IR 0.7 to
1.3um Mid IR 1.3 to 3um Thermal IR 3 to 14um
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Energy cant be created nor destroyed
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Energy Interactions With Earths Surface

• Law of Conservation of Energy
• Formula Ei Er Ea Et
• Ei Incident energy
• Er Reflected energy
• Ea Absorbed energy
• Et Transmitted energy
• Must equal to 1
• Most remote sensing devises are developed to
  collect reflected energy.

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Two Types of Reflection

• SPECULAR
• Smooth surface reflection relative to the
  incident wavelength. Surface particles are small
  relative to the wavelength.
• Light is reflected in a single direction. The
  angle on reflection is equal to the angle of
  incidence.
• Sometimes called 'mirror' reflection
• Radar instruments have a hard time identifying
  water bodies because the wavelength is much
  longer than the general character of the surface
  roughness.
• Specular reflectance helps and hinders remote
  sensing depending where the sensor is situated
  relative to the outgoing radiation.

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Two Types of Reflection

• DIFFUSE
• Surface is rough relative to the incident
  wavelength.
• Also called isotropic or lambertian reflectance.
  
• Energy is reflected (scattered) equally in all
  directions (more or less).
• Many natural surfaces act as a diffuse reflector
  to some extent.
• A perfectly diffuse reflector is termed a
  lambertian surface and the reflective brightness
  is the same when observed from any angle.

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Atmospheric Window
The white spikes seen in the graph are the
wavelengths that are capable of penetrating the
Earths atmosphere. Otherwise known as the
Atmospheric Window.
Most remote sensing instruments on air or space
platforms operate in one or more of these windows
by making their measurements with detectors tuned
to specific frequencies (wavelengths) that pass
through the atmosphere.
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Spectral Signatures
For any given material, the amount of solar
radiation that reflects, absorbs, or transmits
varies with wavelength. This important property
of matter makes it possible to identify different
substances or classes and separate them by their
spectral signatures.
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For example, at some wavelengths, sand reflects
more energy than green vegetation but at other
wavelengths it absorbs more (reflects less) than
does the vegetation. In principle, we can
recognize various kinds of surface materials and
distinguish them from each other by these
differences in reflectance
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Spectral Signatures
Why are Spectral Signatures important?
Making spectral measurements depends on the
interactions between the incident radiation and
the atomic and molecular structures of the
material. These interactions lead to a reflected
signal, which changes some as it returns through
the atmosphere. Finally, the measurement depends
on the nature of the detector system's response
in the sensor. After testing the response of many
materials, remote sensing experts can use
spectral measurements to describe an object by
its composition. In practice, we describe objects
and features on Earth's surface more as classes
than as materials per se.
We can subdivide vegetation in a variety of ways
trees, crops, grasslands, lake bloom algae, etc.
Finer subdivisions are permissible, by
classifying trees as deciduous or evergreen, or
deciduous trees into oak, maple, hickory, poplar,
etc.
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Remote Sensing Sensors
Two Types of sensors Passive and
Active Passive Uses the sun as a energy source
and measures the data. (Downward looking
satellites) Active Generates its own energy
source and measures the time to respond. (LIDAR
RADAR)
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The Cross Track mode normally uses an rotating
(spinning) or oscillating mirror (making the
sensor an optical-mechanical device) to sweep the
scene along a line traversing the ground that is
very long (kilometers miles) but also very
narrow (meters yards), or more commonly a series
of adjacent lines. This is sometimes referred to
as the whiskbroom mode from the vision of
sweeping a table side to side by a small handheld
broom.)
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The Along Track mode does not have a mirror
looking off at varying angles. Instead there is a
line of small sensitive detectors stacked side by
side, each having some tiny dimension on its
plate surface these may number several thousand.
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Gathering Data
In the past, spy satellites would be loaded with
large rolls of black and white film, which would
take pictures continuously until the canister was
spent. Then a predestine time, the canister
would be ejected over an U.S. territory to be
recovered in mid-air by a plane. This was very
risky, and many canisters werent
recovered. Nowadays the light reflected is
captured by a satellite and transformed to a
digital signal. This allows for the government
to receive data minutes after the photo is taken.
Multispectral Images normally are sold in Bands
4, 5, 6, 7 Band 4 Red Band 5 Green Band 6
Blue Band 7 Near Infrared
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Color False Color Composites
We can use color-filtered b w pictures with
color filters to produce color composites by
projected superposition. Project the blue
transparency through a blue filter, the green
through green, and the red through red.
Co-register (line up) the three projections by
superimposing several distinctive patterns that
are common within the photographed scene. Blue
features are clear areas on its (blue)
transparency is projected through the blue filter
as blue, green project through its filter as
green, and red as red. The result will be a
simulated natural color image. Other colors
present are additive mixes of two or more
primaries (e.g., yellow is a mix of red and
green orange is a mix of more red and some
green white is an equal mix of all three
primaries, and black is simply the absence of any
colored light of any wavelength). We can change
the image if one of the transparencies is a b w
infrared film, which we ordinarily use to
emphasize a property of healthy vegetation in
which light in the range of 0.7 - 1.1 µm reflects
strongly from the internal cells of plants,
giving rise to bright tones in the film. We
generate a false color composite by projecting a
green light tones transparency through a blue
filter, a red through green, and this
IR-transparency (with light tones corresponding
to vegetation) through a red filter, all onto a
screen or on color film.
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Color
Blue Filter
Green Filter
Red Filter
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Notably a different color version is developed by
placing IR through a red filter, (typical false
color rendition) in which various kinds of
vegetation display in several tones of red, pink,
or yellow (the latter two may indicate a degree
of stressed or unhealthy vegetation).
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Absorption centered at about 0.65 µm (visible
red) by chlorophyll pigment in green-leaf
chloroplasts that reside in the outer or Palisade
leaf, and to a similar extent in the blue,
removes these colors from white light, leaving
the predominant but diminished reflectance for
visible wavelengths concentrated in the green.
Thus, most vegetation has a green-leafy color.
There is also strong reflectance between 0.7 and
1.0 µm (near IR) in the spongy mesophyll cells
located in the interior or back of a leaf, within
which light reflects mainly at cell wall/air
space interfaces, much of which emerges as strong
reflection rays.
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