Chapter I Concepts and Foundations of Remote Sensing

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Chapter IConcepts and Foundations of Remote
Sensing
Geography 4260Remote Sensing
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Introduction to Remote Sensing
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• Chapter 1 covers basic concepts relevant to all
  forms of environmental remote sensing
• Electromagnetic energy,
• Energy interactions in the Earths atmosphere,
• Energy interactions with the surface features,
• Fundamentals of data acquisition and data
  interpretation,
• The Global Positioning System (GPS),
• Remote sensing systems, and
• Geographic information systems.

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Introduction to Remote Sensing
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• Your text defines remote sensing as the science
  and art of obtaining information about an object,
  area, or phenomenon through the analysis of data
  acquired by a device that is not in contact with
  the object, area, or phenomenon under
  investigation.

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Introduction to Remote Sensing
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• Why is remote sensing defined as both a science
  and an art?

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Introduction to Remote Sensing
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• Why does the definition employed by the authors
  of your text refer to an object, area, or
  phenomenon?
• In other words, what kind of objects, areas and
  phenomenon are studied by remote sensing
  techniques?

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Introduction to Remote Sensing
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• The definition of remote sensing refers to both
  data and information. Whats the difference?

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Introduction to Remote Sensing
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• Data and information represent a points along a
  spectrum ranging from single facts or numbers
  (data) through more meaningful concepts that
  contain facts in a given context (information) to
  concepts that comprise real reasoning
  (knowledge), which allows new information to be
  generated.
• Remote sensing involves gathering data and using
  that data to generate information about the
  objects being investigated.

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Introduction to Remote Sensing
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• The data used in environmental remote sensing is
  collected by electromagnetic energy sensors,
  normally aboard aircraft or satellites.
• The sensor systems mounted on these platforms
  collect either (or both) reflected energy and
  emitted energy.
• Whats the difference between reflected and
  emitted electromagnetic energy?

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Introduction to Remote Sensing
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• Figure 1.1 illustrates the generalized remote
  sensing process

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• Electromagnetic Energy

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Electromagnetic Energy
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• Electromagnetic energy is also known as
  electromagnetic radiation because it is a form of
  energy the is emitted from all objects that are
  warmer than absolute zero and then radiates
  outward in all directions.
• Familiar forms of electromagnetic radiation
  include visible light, ultraviolet light, xrays,
  and radio waves.

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Electromagnetic Energy
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• All forms of electromagnetic energy are similar
  and radiate out from their sources according to
  basic wave theory. This theory holds that energy
  travels
• As harmonic, sinusoidal waves, and
• At the speed of light (in a vacuum).
• What happens to electromagnetic waves when they
  travel through the atmosphere or through solids?

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Electromagnetic Energy
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• Different forms of electromagnetic energy have
  different wavelengths, i.e. distances between
  wave crests (or other identical points on the
  wave).

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Electromagnetic Energy
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• Wavelength is related to wave frequency because
  all electromagnetic radiation travels at the same
  speed in any given medium.

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Electromagnetic Energy
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• Because the speed of light (c) is constant, the
  relationship between wavelength (?) and frequency
  (v) is given by c v?. What does this imply?

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Electromagnetic Energy
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• c v? implies that specifying either wavelength
  or frequency is sufficient to describe a
  particular form of electromagnetic radiation.

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Electromagnetic Energy
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• In remote sensing, it is most common to describe
  particular forms of electromagnetic radiation by
  their wavelength rather than frequency.

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Electromagnetic Energy
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• Additionally, wavelength is normally specified in
  micrometers (um). One micrometer is one millionth
  of a meter.

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Electromagnetic Energy
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• Sometimes, wavelength is specified in
  nano-meters, i.e. billionths of a meter,
  particularly for wavelengths of visible and
  shorter radiation.

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Electromagnetic Energy
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• Incidentally, the term micron is an obsolete term
  formally used synonymously with micrometer.

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• The Electromagnetic Spectrum

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The Electromagnetic Spectrum
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• The different forms of electromagnetic radiation
  range from cosmic waves (with wavelengths of less
  than one nanometer) to radio waves (with
  wavelengths measured in thousand of meters).
• There is a continuous spectrum of electromagnetic
  energy at all wavelengths between the shortest
  cosmic waves and the longest radio waves.

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The Electromagnetic Spectrum
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• This continuous spectrum is commonly divided into
  discrete segments based on human perception and
  human interaction with electromagnetic waves
  within particular ranges of wavelengths.
• For example, visible light consists of waves of
  electromagnetic energy ranging from 0.4 um to 0.7
  um. For comparison, the average human hair is
  about 50 um in diameter.

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The Electromagnetic Spectrum
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• Visible light is physically no different from
  shorter wavelengths of ultraviolet light, xrays
  or gamma rays or longer wavelengths of infrared
  light, thermal infrared energy, microwaves or
  radio waves except for its wavelengths and the
  physical interactions that are a result of the
  various wavelengths.

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The Electromagnetic Spectrum
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• The visible light portion of the electromagnetic
  spectrum is called visible light only because the
  rods and cones in our eyes are not sensitive to
  shorter and longer wavelengths of electromagnetic
  energy.
• Thermal infrared energy is differentiated from
  shorter wavelengths of near- and mid-infrared
  energy because it cant be refracted by a camera
  lens.

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The Electromagnetic Spectrum
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• Thermal infrared energy is differentiated from
  shorter wavelengths of near- and mid-infrared
  energy because it can be sensed as heat and cant
  be refracted by a camera lens.

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The Electromagnetic Spectrum
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• The point is that the divisions of the
  electromagnetic spectrum are artificial even
  though they are meaningful to us.

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The Electromagnetic Spectrum
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• The electromagnetic spectrum has no boundaries
  where the energy is fundamentally different on
  either side. Different forms of electromagnetic
  radiation grade imperceptibly into each other.

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The Electromagnetic Spectrum
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• Cosmic rays and gamma rays represent the shortest
  wavelengths of electromagnetic radiation.

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The Electromagnetic Spectrum
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• AM radio waves have the longest wavelengths, but
  the spectrum continues beyond the longest
  wavelengths that are used by radio stations.

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The Electromagnetic Spectrum
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• Visible light lies near the center of the
  spectrum. It, infrared, and the shortest radio
  waves (including microwave/radar waves) are most
  commonly used in environmental remote sensing.

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The Electromagnetic Spectrum
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• Visible light is a continuous spectrum, but is
  subdivided into named colors based on the way
  different wavelengths create visual sensations.

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The Electromagnetic Spectrum
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• The simplest subdivision of the visible spectrum
  is into red (0.6 - 0.7 um), green (0.5 - 0.6 um),
  and blue (0.4 - 0.5 um) as in RGB monitors.

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The Electromagnetic Spectrum
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• Note that this figure shows visible light
  extending beyond the usual range of 0.4 to 0.7
  um. For our purposes, these regions lie in the UV
  and IR.

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The Electromagnetic Spectrum
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• Wave theory does a good job of describing the
  behavior of electromagnetic energy, but it is
  incapable of describing all of the behaviors of
  electromagnetic energy.
• Under certain circumstances, it is more
  convenient to describe electromagnetic radiation
  as consisting of particles, i.e. photons or
  quanta of energy.

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The Electromagnetic Spectrum
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• Wave theory and quantum theory can be related to
  each other because the energy of a photon is
• Q hv
• where
• Q The energy of a photon,
• h Plancks constant, and
• v frequency.

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The Electromagnetic Spectrum
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• Given that Q hv, what happens to the energy of
  a photon as wave frequency increases (and
  wavelength decreases), e.g. ultraviolet light?
• What happens as wave length increases (e.g.
  infrared light or radio energy)?
• Q The energy of a photon,
• h Plancks constant, and
• v frequency.

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The Electromagnetic Spectrum
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• Even if you are confused by the equation,
  understand that wavelength and energy are
  inversely proportional
• Shorter wavelengths contain more energy (and are
  produced by more energetic (hotter) sources), and
• Longer wavelengths contain less energy (and we
  therefore required more photons to produce any
  physical or chemical reaction based on their
  energy content).

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The Electromagnetic Spectrum
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• One important consequence of the fact that longer
  wavelength photons are less energetic than
  photons with shorter wavelength is that remote
  sensor systems that acquire data at these
  wavelengths must either
• Be exposed to radiation for a longer period of
  time to capture sufficient data, or
• Have larger detector elements (and consequently,
  lower image resolution).

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The Electromagnetic Spectrum
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• However, before considering concepts related to
  image acquisition and image resolution we need to
  understand the sources and nature of
  electromagnetic radiation in more detail.

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• Sources of Electromagnetic Radiation

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The Electromagnetic Spectrum
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• The sun is the most obvious source of the
  electromagnetic radiation used in most remote
  sensor systems.
• However, all matter at temperatures above
  absolute zero continuously emits electromagnetic
  radiation. The earth, the other planets,
  interstellar hydrogen clouds and you and I also
  emit radiation that can be remotely sensed.

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The Electromagnetic Spectrum
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• The Stefan-Boltzmann law describes the
  relationship between the temperature of an object
  and the rate at which it radiates electromagnetic
  radiation
• M sT4
• where
• M total radiant exitance in watts/m-2
• s Stefan-Boltzmann constant
• T Absolute temperate in degrees Kelvin

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The Electromagnetic Spectrum
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• The important relationships revealed by the
  Stefan-Boltzmann law are
• Hotter objects emit radiation more rapidly than
  cooler objects, and
• Even small increase in temperature produce much
  higher radiation rates because the rate is
  dependent on the 4th power of the temperature
  (i.e. rates increase at an ever increasing rate
  at higher temperatures).

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The Electromagnetic Spectrum
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• It is important to understand that the
• Stefan-Boltzmann law is strictly applicable only
  to hypothetical black bodies.
• A black body absorbs all radiation that is
  incident on it and reemits all of the radiation
  it absorbs. Some real objects approach the
  behavior of black bodies, but many objects
  reflect, scatter or transmit some of the energy
  they receive from other sources.

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The Electromagnetic Spectrum
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• The sun and other stars approach the ideal
  behavior of black bodies, but the Earths
  atmosphere and surface absorb, reflect and
  scatter a considerable amount of the energy they
  receive from the sun.
• Nevertheless, the Earths atmosphere and its
  surface both radiate energy more rapidly at
  higher temperature as would be expected if they
  were black bodies.

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The Electromagnetic Spectrum
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• In addition to influencing radiation rates,
  temperature also influences the wavelengths of
  the emitted electromagnetic radiation.

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The Electromagnetic Spectrum
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• The sun, with a radiant temperature of about
  6000K, emits shorter wavelength than the Earth
  whose radiant temperature is closer to 300K.

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The Electromagnetic Spectrum
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• What is the peak wavelength of solar radiation?

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The Electromagnetic Spectrum
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• What type of radiation does the sun emit at its
  peak wavelength of radiation? (Note that the
  wavelength scales use different units).

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The Electromagnetic Spectrum
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• What other wavelengths make up the bulk of the
  suns radiant output?

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The Electromagnetic Spectrum
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• Because of its lower temperature, Earth radiation
  peaks at about 9.7 um (for simplification, this
  is approximately equal to 10-4 meters).

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The Electromagnetic Spectrum
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• What name do we apply to electromagnetic
  radiation at 9.7 um?

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The Electromagnetic Spectrum
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• Specifically, this part of the infrared spectrum
  is referred to as thermal infrared. We sense
  thermal infrared energy as heat.

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The Electromagnetic Spectrum
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• Thermal infrared energy can be neither seen nor
  photographed.
• However, it can be sensed by electronic detectors
  known as radiometers. Therefore, emitted
  terrestrial radiation can be used to acquire
  remote sensing data. More commonly, though,
  reflected radiation from the sun (or another
  source) is used to generate remote sensing data
  for objects on the Earths surface.

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The Electromagnetic Spectrum
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• Shorter wavelengths of infrared energy behave
  much like visible light in that they can be
  reflected from the Earths surface, refracted by
  camera lenses, and detected with photographic
  film.
• The sun is a strong source of these near- and
  mid-infrared wavelengths and they are used for
  both infrared photography and infrared imaging
  with digital cameras.

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The Electromagnetic Spectrum
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• A short review of concepts
• Objects warmer than absolute zero emit
  electromagnetic radiation,
• Hotter objects emit more radiation with higher
  energy photons,
• Hotter objects photons with shorter wavelengths,
• The suns energy peaks in the green portion of
  the visible light spectrum, but it emits
  significant ultraviolet energy and considerable
  infrared energy,
• The Earth emits thermal infrared energy.

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The Electromagnetic Spectrum
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• Energy Interactions in the Atmosphere

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The Electromagnetic Spectrum
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• The electromagnetic radiation used to collect
  remote sensing data passes through the atmosphere
  en route from its source to the sensor system.
• The distance that energy passes through the
  atmosphere is its path length, and it includes
  both the part of the path from the energy source
  to the target and the part from the target to the
  sensor.

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The Electromagnetic Spectrum
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• Because electromagnetic radiation can interact
  with the gases and particulate matter (e.g. dust)
  found in the atmosphere, not all of the radiation
  leaving the radiant source (e.g. the sun) passes
  through the atmosphere and reaches the target and
  not all of the energy leaving the target reaches
  the sensor.
• Scattering and absorption both impact radiation
  traveling through the atmosphere.

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The Electromagnetic Spectrum
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• Scattering

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The Electromagnetic Spectrum
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• Scattering is the unpredictable redirection of
  electromagnetic radiation by gases molecules and
  other particles in the atmosphere.

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The Electromagnetic Spectrum
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• Scattering is classified as one of three types
• Rayleigh scattering occurs when photons interact
  with particles that are much smaller than their
  wavelength (e.g. atmospheric gases),
• Mie scattering occurs when photons interact with
  particles that are about the same diameter as
  their wavelength (e.g. water vapor, small water
  droplets, and dust), and
• Nonselective scatter occurs when the particles
  are significantly larger than the wavelength of
  energy involved (e.g. larger water droplets).

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The Electromagnetic Spectrum
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• Rayleigh Scattering

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The Electromagnetic Spectrum
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• The effect of Rayleigh scattering is much more
  pronounced at shorter wavelength than at longer
  wavelengths.
• What are the shortest wavelengths of visible
  light?

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The Electromagnetic Spectrum
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• What wavelengths are scattered even more easily
  than blue light (0.4 0.5 um)?
• What visible wavelengths are least effected by
  Rayleigh scattering?

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The Electromagnetic Spectrum
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• How does Rayleigh scattering explain the fact
  that the sky is blue if there are few particles
  larger than atmospheric gases present in the
  atmosphere?

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The Electromagnetic Spectrum
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• In addition to creating blue skies, Rayleigh
  scattering is responsible for the bluish cast to
  high altitude photographs.

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The Electromagnetic Spectrum
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• When the path length is very long, Rayleigh
  scattering (and the absorption of photons by
  atmospheric gases and dust) can prevent most
  short wavelength photons from reaching the
  sensor, allowing only the longest wavelengths
  through.

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The Electromagnetic Spectrum
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• Mie Scattering

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The Electromagnetic Spectrum
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• Mie scattering is mostly produced by water vapor,
  small water droplets and minute dust particles in
  the atmosphere. It tends to scatter longer
  wavelengths of energy than are scattered by
  Rayleigh scattering, and it is significant when
  the humidity is very high or when there are
  abundant small water and dust particles in the
  atmosphere.
• However, Rayleigh scattering tends to predominate
  even when Mie scattering is important.

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The Electromagnetic Spectrum
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• Like Rayleigh scattering, Mie scattering is
  selective, scattering shorter wavelengths of
  electromagnetic radiation more easily than longer
  wavelengths.
• However, Mie scattering is not as strongly
  selective as is Rayleigh scattering.

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The Electromagnetic Spectrum
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• Nonselective Scattering

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The Electromagnetic Spectrum
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• Nonselective scattering occurs when radiation
  interacts with particles that are significantly
  larger than the wavelengths of energy involved.
  In this type of scattering, all wavelengths of
  energy are about equally affected.
• Water droplets and ice crystals in clouds and
  larger dust particles are mostly responsible for
  nonselective scatter.

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The Electromagnetic Spectrum
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• Because scattering is nonselective, fog and
  clouds appear white (a color resulting from the
  presence of equal amounts of red, green and blue
  photons).

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The Electromagnetic Spectrum
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• Nonselective scattering is also responsible for
  much of the haze in the atmosphere when
  atmospheric aerosols are abundant.

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The Electromagnetic Spectrum
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• Regardless of the type of scattering, photons
  from a remote sensing target can be scattered
  away from the sensor system and photons from
  other directions can be scattered into the sensor
  system.
• In either case, scattering degrades the quality
  of the data recorded by the sensor system whether
  it is a film camera or an electronic detector
  system such as a digital camera.

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The Electromagnetic Spectrum
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• Many of the preceding slides on scattering assume
  were talking about visible light. This diagram
  illustrates effects for other wavelengths of
  electromagnetic radiation.

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The Electromagnetic Spectrum
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• Absorption

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The Electromagnetic Spectrum
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• The emission of electromagnetic radiation occurs
  when an electron jumps from a higher to a lower
  energy level within an atom.
• Absorption is the reverse process When a photon
  is absorbed by an atom, it causes an electron to
  jump to a higher energy level.
• The process can be illustrated with a Java applet.

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The Electromagnetic Spectrum
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• Scattering simply changes the directions that
  photons travel through the atmosphere.
  Atmospheric absorption, however, cause a photon
  to cease to exist and all of its energy is
  captured by the absorbing atom.
• Effective atmospheric absorbers of
  electromagnetic radiation include water vapor,
  carbon dioxide, and ozone among other gases. Each
  of these gases tends to absorb photons of
  specific wavelengths.

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The Electromagnetic Spectrum
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• Figure 1.5a shows the wavelength distributions of
  the suns and Earths emitted electromagnetic
  radiation. The sun emits most of its energy in
  the UV, visible and IR portions of the
  electromagnetic spectrum. This is the
  distribution of wavelengths arriving at the outer
  edge of the atmosphere.

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The Electromagnetic Spectrum
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• Because the Earth is so much cooler than the sun,
  it emits electromagnetic radiation both at longer
  wavelengths (primarily thermal infrared
  radiation) and at much lower radiation rates.

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The Electromagnetic Spectrum
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• About half of the incoming solar radiation
  arriving at the outer edge of the atmosphere is
  absorbed by atmospheric gases and more than half
  of the energy emitted by the Earth is absorbed by
  the atmosphere.

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The Electromagnetic Spectrum
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• The atmosphere, however, is a selective absorber
  of electromagnetic radiation. Most of the visible
  light and shorter wavelengths of infrared energy
  arriving at the outer edge of the atmosphere pass
  through the atmosphere much as window glass
  transmits visible light.

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The Electromagnetic Spectrum
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• Some ultraviolet radiation and longer wavelength
  of infrared radiation are also transmitted, but
  other particular ranges of wavelengths within
  these broad bands are absorbed in the atmosphere.

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The Electromagnetic Spectrum
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• If the absorbed photon is not reemitted from the
  absorbing gas molecule, it raises the
  temperatures of the molecule. Most of the photons
  absorbed by the atmosphere result in temperature
  changes.

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The Electromagnetic Spectrum
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• Those wavelengths of energy that pass easily
  through the atmosphere represent atmospheric
  windows. These include the longer wavelength of
  ultraviolet, all of the visible wavelengths, and
  various wavelength bands within the infrared.

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The Electromagnetic Spectrum
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• Those wavelengths that are easily absorbed in the
  atmosphere (e.g. 2 um and 5-6 um) are not useful
  in remote sensing because the atmosphere acts
  like a window shade at these wavelengths,
  effectively preventing the energy from reach the
  surface or being detected at any distance.

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The Electromagnetic Spectrum
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• The concept of atmospheric windows is equally
  applicable to solar and terrestrial radiation. In
  fact, the atmosphere transmits thermal infrared
  energy in two wavelength bands, 3-5 um and 8-14
  um.

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The Electromagnetic Spectrum
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• Most of the terrestrial radiation between 5 and 8
  um is absorbed in the atmosphere, making is
  unusable for remote sensing. Therefore, thermal
  scanners use detectors that are sensitive to
  electromagnetic radiation between 3 and 5 um or
  between 8 and 14 um.

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The Electromagnetic Spectrum
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• Obviously, a remote sensor system has to be able
  to detect electromagnetic energy at wavelengths
  that are capable of passing through the
  atmosphere fairly easily, particularly if the
  path length of the energy through the atmosphere
  is long.

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The Electromagnetic Spectrum
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• Energy Interactions with Earth Surface Features

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• The purpose of most remote sensor systems is to
  gather data about features on the Earths surface
  (or in the Earths atmosphere).
• It is the interactions that occur between
  electromagnetic radiation and these features that
  allows a remote sensor system to distinguish
  between features and to gather additional
  information about features.

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The Electromagnetic Spectrum
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• Three energy interactions are possible when a
  remote sensing target is illuminated by sunlight,
  terrestrial radiation, microwave energy or some
  other source of radiation. The incident radiation
  can be
• Reflected,
• Absorbed, or
• Transmitted.

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The Electromagnetic Spectrum
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• Because the energy can only be reflected,
  absorbed or transmitted, the sum of the
  reflected, absorbed and transmitted radiation is
  exactly equal to the incident radiation. This
  explains the rather confusing, energy balance
  equation in your text
• EI(?) ER(?) EA(?) ET(?)
• (The inclusion of (?) restricts the relationship
  to particular wavelengths).

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• We can use the fact that different Earth surface
  features reflect, absorb and transmit different
  amounts of electromagnetic radiation to
  distinguish different features through remote
  sensing techniques.
• What?

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The Electromagnetic Spectrum
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• Because reflection, absorption and transmission
  are also wavelength dependent, two features that
  look similar over one wavelength band may be
  easily distinguishable at other wavelengths.

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The Electromagnetic Spectrum
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• For example, camouflaged vehicles are hard to
  distinguish from their surrounding in
  black-and-white or color photographs, but are
  easily identified in infrared photos because they
  reflect very little infrared energy while
  vegetation is highly reflective in the infrared
  portion of the spectrum.

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• Remotes sensing systems can generally be
  classified as either
• Active remote sensing systems, or
• Passive remote sensing systems.
• Active systems generate their own electromagnetic
  radiation while passive systems rely on emitted,
  reflected or scattered solar or terrestrial
  radiation.
• Examples?

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The Electromagnetic Spectrum
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• Because systems relying on reflected energy are
  very common, it is often convenient to express
  the energy balance equation as
• ER(?) EI(?) EA(?) ET(?)
• In other words, the total reflected energy at any
  wavelength is the incident energy minus both the
  energy absorbed by Earth surface features and the
  energy transmitted by those features.

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• The way in which energy is reflected from Earth
  surface features is important to understanding
  the sensor systems and to interpreting the data
  collected by these systems.
• The surface roughness of a feature is the primary
  determinant of how it will reflect energy.

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• Types of reflectors range along a continuum from
• Specular reflectors which are smooth, flat
  surfaces that produce mirrorlike reflections, and
• Diffuse (Lambertian) reflectors which are rough
  surfaces that reflect energy equally in all
  directions.
• Many reflectors that are neither perfectly
  specular or perfectly diffuse, but have
  properties that lie between these two extremes.

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The Electromagnetic Spectrum
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• A perfect specular reflector reflects all of the
  incident radiation at an angle that is equal to
  the angle of incidence. Many natural and
  artificial surfaces are near perfect specular
  reflectors.

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The Electromagnetic Spectrum
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• Lakes, rivers and other water bodies often
  produce specular reflections of bright skies or
  the sun, but any smooth surface can generate a
  specular reflection.

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• A perfectly diffuse reflector would scatter all
  of the incident radiation equally in all
  directions. Perfectly diffuse reflectors are
  rare, but many surfaces approach the ideal.

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• Different types of surfaces reflect or scatter
  electromagnetic radiation differently. Note that
  a smooth specular reflector of radar energy
  scatters very little energy back toward the
  sensor system.

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The Electromagnetic Spectrum
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• More accurately, a single specular surface
  reflects little energy back to the sensor system.
  Dihedral reflectors and trihedral reflectors
  return very strong signals.

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• Many natural features such as cliffs and many
  engineered structures contain dihedral and
  trihedral reflectors that generate strong radar
  echos.

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• Specular reflections from lake surfaces are
  common in aerial photographs when lake surfaces
  reflect or scatter solar radiation into the
  sensor system.
• Similar specular reflections from smooth surfaces
  occur in imagery produced in other wavelengths.

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• Although it should be clear that surface
  roughness controls the type of reflection, it may
  be less clear that it is surface roughness
  relative to the wavelength of energy.
• A smooth, dry beach is a diffuse reflector of
  visible light but the same beach can be a
  specular reflector for longer wavelengths of
  electromagnetic radiation.

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Visible light wavelengths are much shorter (0.4
to 0.7 um) than the range of sand grain sizes
(62.5 to 2000 um).

• Therefore, sand acts as a diffuse reflector of
  all wavelengths of visible light.

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• However, microwave radiation ranges from 0.1 to
  100 mm, wavelengths that are generally much
  longer than the diameters of sand grains (0.0625
  to 2.0 mm) that make up the surface of a beach.
• Smooth sand acts as a specular reflector for
  microwave radiation because these wavelengths of
  energy are about the same size or longer than the
  diameters of the sand grains.

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• Because specular reflectors act like mirrors,
  very little energy is scattered back toward the
  sensor system by a smooth surface unless the
  geometry is such that the sensor system is
  located within the reflected energy beam.
• Thats why smooth lake surfaces often appear dark
  in visible images and somewhat rougher surfaces
  like smooth sand appear dark in radar images.

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• In short, if the wavelengths of energy are short
  compared to the size of surface irregularities,
  then the surface is a diffuse reflector of those
  wavelengths.
• On the other hand, if the wavelengths of energy
  are about the same size or longer than the
  surface irregularities, the surface will behave
  as a specular reflector.

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• Visually, distinguishing specular reflectors from
  diffuse reflectors is very easy
• Diffuse reflectors contain information about the
  color of the reflector, while
• Specular reflectors do not have a color of their
  own, but reflect the colors of objects that are
  seen in the reflector.
• Therefore, remote sensing relies on diffuse
  reflection to record data about a target.

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• Spectral Reflectance

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• The spectral reflectance of an object at a
  particular wavelength (??) is the proportion of
  incident light at that wavelength that is
  reflected
• ?? ER(?) / EI(?)
• Normally, spectral reflectance is expressed as a
  percentage and ranges from 0 to 100.

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• A spectral reflectance curve is a graph of
  spectral reflectance over a range of wavelengths
  for a particular object or group of objects.

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• Spectral reflectance curves are often referred to
  as spectral signatures.
• However, the authors of your textbook prefer the
  term spectral reflectance because they believe
  it implies less rigidity.

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• Although spectral reflectance curves are often
  shown as single lines, most classes of objects
  exhibit some variability in reflectance from one
  member of the class to another or even within
  individual parts of a class member.

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• Spectral reflectance curves for distinct but
  similar objects can be compared to determine the
  specific wavelengths that will allow
  discrimination between the objects.

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• As noted in your text, distinguishing individual
  coniferous trees in a mixed deciduous and
  coniferous forest with normal black-and-white
  film is almost impossible, but becomes almost
  trivial on black-and-white infrared sensitive
  film.

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• Spectral reflectance curves have been derived for
  a wide variety of surface features, but not all
  features can be distinguished solely by their
  spectral reflectance.

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• The variability of spectral reflectance is the
  primary reason the authors of your text avoid the
  term spectral signature.

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• The spectral reflectance curves for dry, bare
  soil, green vegetation, and clear lake water
  illustrate important characteristics of many
  other materials.

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• The chlorophyll in healthy vegetation absorbs
  blue and red wavelengths more efficiently than
  yellow and green wavelengths. Therefore, many
  types of vegetation appears light green at
  visible wavelengths.

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• However, vegetation is a much more efficient
  reflector in the infrared portion of the spectrum
  than it is in the visible portion. We dont see
  vegetation as infrared only because our eyes
  arent sensitive to those wavelengths.

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• This pattern of higher reflectance at some
  visible wavelengths and lower reflectance at
  others is largely a result of absorption by plant
  pigments including chlorophyll.

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• Plant stress due to drought, excess water, or
  disease can reduce the chlorophyll production and
  result in higher reflectance in red and green
  producing yellow (red green) or brown colors.
  Stress can also reduce infrared reflectance.

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• Absorption of infrared wavelength near 1.4, 1.9
  and 2.7 um is primarily a result of absorption by
  water contained in plant tissues. These dips in
  reflection are known as water absorption bands.

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• Soil water also absorbs energy, reducing
  reflectance. Therefore, wet soils are normally
  darker than the same soils when they are dry.

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• Soil moisture is influenced by topography, but it
  is also often closely related to soil texture
  because texture influences drainage
• Course soils have large pore spaces, allowing
  water to drain from them quickly, while
• Finer soils tend to retain water, and are
  usually darker as a result.

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• Many other factors also contribute to the
  reflectance of soils, including their iron oxide
  and organic content. For dry soils, reflectance
  generally increase with wavelength.

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• Water, whether in lakes or streams or contained
  in vegetation or soils is such an effective
  absorber of infrared radiation that high water
  content generally greatly reduces reflectance in
  these wavelengths.

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• Therefore, water bodies, wetlands and waterlogged
  soils are easily identified in infrared imagery
  by their much darker color than otherwise similar
  surroundings.

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• Suspended sediment and the bottoms of shallow
  water bodies can greatly increase reflectance
  from water areas. Specular reflections also
  change the appearance of water bodies at certain
  angles.

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• Spectral Response Patterns

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• Because spectral reflectance differs for
  different types of materials, remote sensing
  imagery can be used to distinguish different
  features from each others.

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• Spectral reflectance patterns are often referred
  to as spectral signatures. However, this term
  implies a degree of certainty that is often
  unattainable. Therefore, the text refers to
  spectral response patterns.

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• The terminology used is relatively unimportant.
  The important point to remember is that the
  pattern of reflectance is not precise or always
  unique.

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• Sometimes it is difficult or impossible to
  distinguish similar objects through differences
  in their spectral reflectance alone. In that
  case, other clues to the identities of the object
  are needed.
• On the other hand, spatial and temporal variation
  in reflectance can be used to identify features
  that would be more difficult to distinguish
  without these variations.

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• Atmospheric Influences on
• Spectral Response Patterns

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• Although many features have fairly predictable
  spectral response patterns, the energy recorded
  by a sensor system also depends on the energys
  interactions with the atmosphere in ways that are
  entirely independent of the features that are the
  subject of a remote sensing project.

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• The atmosphere affects the response of the sensor
  system and the data it records in two ways
• It absorbs and scatters energy that would
  otherwise illuminate the target and energy that
  would otherwise travel from the target to the
  sensor system, and
• It scatters energy extraneous into the sensor
  system that contains no information about the
  target because it never interacted with the
  target.

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• The total amount of energy reaching the target is
  reduced by the first effect and increase by the
  second, but both effects degrade the quality of
  the data recorded by the sensor system.

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• Data Acquisition and Interpretation

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• The primary goal of a environmental remote sensor
  system is to record data about features on the
  Earths surface. Several types of recording
  devices are used, but the data are recorded in
  one of two ways
• Photographically, or
• Electronically.

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• Photography uses photons to produce chemical
  reactions within a light-sensitive emulsion on a
  photographic film. A greater number of photons
  incident on a particular part of the film
  produces a stronger chemical reaction.
• The initial image is an invisible latent image,
  but later chemical processing of the film
  produces a visible image that contains data about
  the features that were the subject of the
  photograph.

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• Electronic sensors use solid state electronic
  devices that produce an electrical signal when
  exposed to photons. The strength of the
  electronic signal is recorded to create a
  permanent record of the data produced.
• With photography, a separate recording device is
  not needed because the film acts as both the
  sensor and the recording device.

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• Because photographs can be scanned to create
  digital images, data produced by photographic
  sensor systems can be digitally analyzed in the
  same way that the digital data produced by
  electronic system is analyzed.
• Likewise, because a visual image can be created
  from digital data, visual interpretation
  techniques can be used on both photographs and
  digital imagery.

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• Both visual and digital interpretation techniques
  have their own advantages and disadvantages. The
  human mind is far superior to a computer in
  interpreting spatial patterns and the spatial
  relationships between features, both of which are
  useful in image interpretation.
• However, computers are superior in detecting
  subtle intensity variations and in comparing
  spectral reflectance patterns.

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• Digital images, whether acquired electronically
  or scanned from photographs, consist of one or
  more raster arrays of digital numbers.

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• Each cell in the array (pixel) contains a single
  integer number corresponding to the total energy
  reflected and scattered into that portion of the
  image.
• This total intensity is referred to as radiance
  or brightness.

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• In the case of a black-and-white image, the
  numbers determine the intensity of a shade of
  gray used to display that portion of the image.

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• In the case of a color image, there are normally
  three rasters so that each pixel is associated
  with three integer numbers, each corresponding to
  the radiance over three different wavelength
  bands.

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• With a normal color image, these three bands
  correspond with blue, green and red radiance.
• With false color infrared images, the wavelength
  bands are normally green, red and infrared, but
  other combinations are not uncommon.

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• The digital numbers are ultimately derived from
  the strength of an electrical current generated
  by a single detector element.
• Current strength is converted to a digital number
  through an analog-to-digital converter.

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• The range of digital numbers depends on the
  characteristics of the particular recording
  device, but they typically range from 0 to 255, 0
  to 511 or 0 to 1023.
• These numbers are stored as binary integers
  (bits) and these particular ranges represent the
  ranges available with 8-, 9- and 10-bit numbers.
  Binary computers are adept at handling images
  stored as binary integers.

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• Reference Data

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• Reference data are data derived from other
  sources that are used in image interpretation.
  Without reference data in some form,
  interpretation of remote sensing data would be
  impossible.
• However, life experiences and academic training
  are an important sources of reference data that
  are stored in the mind. Therefore, the success of
  many remote sensing investigations depends
  heavily on the training and experience of the
  people involved.

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• Other sources of reference data include such
  things as
• Hardcopy maps,
• Tabular reports,
• Descriptive text documents, and
• Other forms of remote sensing images.

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• Other sources of reference data include such
  things as
• Field measurements,
• Hardcopy maps,
• Tabular reports,
• Descriptive text documents, and
• Other forms of remote sensing images.
• Reference data are also known as ground truth.

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• The Global Positioning Satellite System

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• The Global Positioning Satellite (GPS) System has
  recently revolutionized position determination.
• GPS allows users to easily determine their
  latitude and longitude virtually anywhere on
  Earth, day or night, regardless of weather
  conditions.

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• GPS uses a constellation of 24 satellites,
  Earth-based control stations, and user receivers.

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• Determining your location with a GPS receiver
  requires that a minimum of four satellites be
  above the horizon.
• The GPS receiver determines your distance from
  each of the four satellites and is able to
  calculate your latitude and longitude from those
  distances.

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• One satellite provides your location on the
  surface of an imaginary sphere

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• Measurements from two satellites provide your
  location along an imaginary circle

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• Measurements from three satellites provide your
  location at one of two points

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• Trick question
• Why do GPS receivers need to have four satellites
  above the horizon to provide an accurate location?

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• Each GPS satellite has an extremely accurate (and
  extremely expensive) atomic clock onboard.
  Therefore, the time that each satellite transmits
  signals is very accurately known.
• However, your GPS receiver has an inexpensive and
  inaccurate clock. Therefore, it must have some
  way to reset its time frequently to match the
  time onboard the satellites.

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• Over short time intervals, the receivers clock
  maintains time nearly as well as an atomic clock.
  
• Therefore, it can compare the differences in
  times of arrival of the satellite signals even
  though it doesnt know exactly when those signals
  were transmitted.

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• The data from three of the four satellites is
  sufficient to define your location with respect
  to those satellites, i.e. your relative location.
• The data from the fourth satellite can therefore
  be used to reset your clock to agree with all
  four atomic clocks aboard the satellites. It then
  becomes possible to determine your location in an
  absolute coordinate system on the surface of the
  Earth.

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• Using Data Obtained with a GPS Receiver

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• A good GPS receiver can
• Display your location in latitude and longitude
  coordinates,
• Report location in other coordinate systems,
• Display a map using Geographic Information
  Systems (GIS) technology,
• Provide GIS functions such as routing or speed
  calculations.

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• GPS is currently in wide use for
• Surveying and mapping,
• Automobile, ship, aircraft and missile
  navigation,
• Vehicle tracking, e.g. trucking, school buses,
  emergency response, and other vehicles, and
• Recreational uses such as hiking, fishing,
  hunting, and geocaching.

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• Geographic Information Systems

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• Read about geographic information systems in
  Section 1.11 of your text. Several questions on
  the first midterm exam will be based on this
  information.

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• STOP!
• The following notes are under development and may
  not contain accurate information.

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• Next Chapter 2
• Photographic Remote Sensing Systems