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Chapter I Concepts and Foundations of Remote Sensing

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Title: Chapter I Concepts and Foundations of Remote Sensing


1
Chapter IConcepts and Foundations of Remote
Sensing
Geography 4260Remote Sensing
GEOG 4260
2
Introduction to Remote Sensing
GEOG 4260
  • 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.

3
Introduction to Remote Sensing
GEOG 4260
  • 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.

4
Introduction to Remote Sensing
GEOG 4260
  • Why is remote sensing defined as both a science
    and an art?

5
Introduction to Remote Sensing
GEOG 4260
  • 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?

6
Introduction to Remote Sensing
GEOG 4260
  • The definition of remote sensing refers to both
    data and information. Whats the difference?

7
Introduction to Remote Sensing
GEOG 4260
  • 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.

8
Introduction to Remote Sensing
GEOG 4260
  • 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?

9
Introduction to Remote Sensing
GEOG 4260
  • Figure 1.1 illustrates the generalized remote
    sensing process

10
GEOG 4260
  • Electromagnetic Energy

11
Electromagnetic Energy
GEOG 4260
  • 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.

12
Electromagnetic Energy
GEOG 4260
  • 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?

13
Electromagnetic Energy
GEOG 4260
  • Different forms of electromagnetic energy have
    different wavelengths, i.e. distances between
    wave crests (or other identical points on the
    wave).

14
Electromagnetic Energy
GEOG 4260
  • Wavelength is related to wave frequency because
    all electromagnetic radiation travels at the same
    speed in any given medium.

15
Electromagnetic Energy
GEOG 4260
  • Because the speed of light (c) is constant, the
    relationship between wavelength (?) and frequency
    (v) is given by c v?. What does this imply?

16
Electromagnetic Energy
GEOG 4260
  • c v? implies that specifying either wavelength
    or frequency is sufficient to describe a
    particular form of electromagnetic radiation.

17
Electromagnetic Energy
GEOG 4260
  • In remote sensing, it is most common to describe
    particular forms of electromagnetic radiation by
    their wavelength rather than frequency.

18
Electromagnetic Energy
GEOG 4260
  • Additionally, wavelength is normally specified in
    micrometers (um). One micrometer is one millionth
    of a meter.

19
Electromagnetic Energy
GEOG 4260
  • Sometimes, wavelength is specified in
    nano-meters, i.e. billionths of a meter,
    particularly for wavelengths of visible and
    shorter radiation.

20
Electromagnetic Energy
GEOG 4260
  • Incidentally, the term micron is an obsolete term
    formally used synonymously with micrometer.

21
GEOG 4260
  • The Electromagnetic Spectrum

22
The Electromagnetic Spectrum
GEOG 4260
  • 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.

23
The Electromagnetic Spectrum
GEOG 4260
  • 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.

24
The Electromagnetic Spectrum
GEOG 4260
  • 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.

25
The Electromagnetic Spectrum
GEOG 4260
  • 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.

26
The Electromagnetic Spectrum
GEOG 4260
  • 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.

27
The Electromagnetic Spectrum
GEOG 4260
  • The point is that the divisions of the
    electromagnetic spectrum are artificial even
    though they are meaningful to us.

28
The Electromagnetic Spectrum
GEOG 4260
  • 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.

29
The Electromagnetic Spectrum
GEOG 4260
  • Cosmic rays and gamma rays represent the shortest
    wavelengths of electromagnetic radiation.

30
The Electromagnetic Spectrum
GEOG 4260
  • AM radio waves have the longest wavelengths, but
    the spectrum continues beyond the longest
    wavelengths that are used by radio stations.

31
The Electromagnetic Spectrum
GEOG 4260
  • 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.

32
The Electromagnetic Spectrum
GEOG 4260
  • Visible light is a continuous spectrum, but is
    subdivided into named colors based on the way
    different wavelengths create visual sensations.

33
The Electromagnetic Spectrum
GEOG 4260
  • 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.

34
The Electromagnetic Spectrum
GEOG 4260
  • 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.

35
The Electromagnetic Spectrum
GEOG 4260
  • 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.

36
The Electromagnetic Spectrum
GEOG 4260
  • 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.

37
The Electromagnetic Spectrum
GEOG 4260
  • 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.

38
The Electromagnetic Spectrum
GEOG 4260
  • 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).

39
The Electromagnetic Spectrum
GEOG 4260
  • 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).

40
The Electromagnetic Spectrum
GEOG 4260
  • 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.

41
GEOG 4260
  • Sources of Electromagnetic Radiation

42
The Electromagnetic Spectrum
GEOG 4260
  • 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.

43
The Electromagnetic Spectrum
GEOG 4260
  • 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

44
The Electromagnetic Spectrum
GEOG 4260
  • 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).

45
The Electromagnetic Spectrum
GEOG 4260
  • 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.

46
The Electromagnetic Spectrum
GEOG 4260
  • 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.

47
The Electromagnetic Spectrum
GEOG 4260
  • In addition to influencing radiation rates,
    temperature also influences the wavelengths of
    the emitted electromagnetic radiation.

48
The Electromagnetic Spectrum
GEOG 4260
  • The sun, with a radiant temperature of about
    6000K, emits shorter wavelength than the Earth
    whose radiant temperature is closer to 300K.

49
The Electromagnetic Spectrum
GEOG 4260
  • What is the peak wavelength of solar radiation?

50
The Electromagnetic Spectrum
GEOG 4260
  • What type of radiation does the sun emit at its
    peak wavelength of radiation? (Note that the
    wavelength scales use different units).

51
The Electromagnetic Spectrum
GEOG 4260
  • What other wavelengths make up the bulk of the
    suns radiant output?

52
The Electromagnetic Spectrum
GEOG 4260
  • Because of its lower temperature, Earth radiation
    peaks at about 9.7 um (for simplification, this
    is approximately equal to 10-4 meters).

53
The Electromagnetic Spectrum
GEOG 4260
  • What name do we apply to electromagnetic
    radiation at 9.7 um?

54
The Electromagnetic Spectrum
GEOG 4260
  • Specifically, this part of the infrared spectrum
    is referred to as thermal infrared. We sense
    thermal infrared energy as heat.

55
The Electromagnetic Spectrum
GEOG 4260
  • 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.

56
The Electromagnetic Spectrum
GEOG 4260
  • 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.

57
The Electromagnetic Spectrum
GEOG 4260
  • 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.

58
The Electromagnetic Spectrum
GEOG 4260
  • Energy Interactions in the Atmosphere

59
The Electromagnetic Spectrum
GEOG 4260
  • 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.

60
The Electromagnetic Spectrum
GEOG 4260
  • 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.

61
The Electromagnetic Spectrum
GEOG 4260
  • Scattering

62
The Electromagnetic Spectrum
GEOG 4260
  • Scattering is the unpredictable redirection of
    electromagnetic radiation by gases molecules and
    other particles in the atmosphere.

63
The Electromagnetic Spectrum
GEOG 4260
  • 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).

64
The Electromagnetic Spectrum
GEOG 4260
  • Rayleigh Scattering

65
The Electromagnetic Spectrum
GEOG 4260
  • The effect of Rayleigh scattering is much more
    pronounced at shorter wavelength than at longer
    wavelengths.
  • What are the shortest wavelengths of visible
    light?

66
The Electromagnetic Spectrum
GEOG 4260
  • What wavelengths are scattered even more easily
    than blue light (0.4 0.5 um)?
  • What visible wavelengths are least effected by
    Rayleigh scattering?

67
The Electromagnetic Spectrum
GEOG 4260
  • 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?

68
The Electromagnetic Spectrum
GEOG 4260
  • In addition to creating blue skies, Rayleigh
    scattering is responsible for the bluish cast to
    high altitude photographs.

69
The Electromagnetic Spectrum
GEOG 4260
  • 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.

70
The Electromagnetic Spectrum
GEOG 4260
  • Mie Scattering

71
The Electromagnetic Spectrum
GEOG 4260
  • 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.

72
The Electromagnetic Spectrum
GEOG 4260
  • 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.

73
The Electromagnetic Spectrum
GEOG 4260
  • Nonselective Scattering

74
The Electromagnetic Spectrum
GEOG 4260
  • 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.

75
The Electromagnetic Spectrum
GEOG 4260
  • Because scattering is nonselective, fog and
    clouds appear white (a color resulting from the
    presence of equal amounts of red, green and blue
    photons).

76
The Electromagnetic Spectrum
GEOG 4260
  • Nonselective scattering is also responsible for
    much of the haze in the atmosphere when
    atmospheric aerosols are abundant.

77
The Electromagnetic Spectrum
GEOG 4260
  • 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.

78
The Electromagnetic Spectrum
GEOG 4260
  • Many of the preceding slides on scattering assume
    were talking about visible light. This diagram
    illustrates effects for other wavelengths of
    electromagnetic radiation.

79
The Electromagnetic Spectrum
GEOG 4260
  • Absorption

80
The Electromagnetic Spectrum
GEOG 4260
  • 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.

81
The Electromagnetic Spectrum
GEOG 4260
  • 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.

82
The Electromagnetic Spectrum
GEOG 4260
  • 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.

83
The Electromagnetic Spectrum
GEOG 4260
  • 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.

84
The Electromagnetic Spectrum
GEOG 4260
  • 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.

85
The Electromagnetic Spectrum
GEOG 4260
  • 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.

86
The Electromagnetic Spectrum
GEOG 4260
  • 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.

87
The Electromagnetic Spectrum
GEOG 4260
  • 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.

88
The Electromagnetic Spectrum
GEOG 4260
  • 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.

89
The Electromagnetic Spectrum
GEOG 4260
  • 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.

90
The Electromagnetic Spectrum
GEOG 4260
  • 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.

91
The Electromagnetic Spectrum
GEOG 4260
  • 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.

92
The Electromagnetic Spectrum
GEOG 4260
  • 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.

93
The Electromagnetic Spectrum
GEOG 4260
  • Energy Interactions with Earth Surface Features

94
The Electromagnetic Spectrum
GEOG 4260
  • 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.

95
The Electromagnetic Spectrum
GEOG 4260
  • 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.

96
The Electromagnetic Spectrum
GEOG 4260
  • 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).

97
The Electromagnetic Spectrum
GEOG 4260
  • 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?

98
The Electromagnetic Spectrum
GEOG 4260
  • 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.

99
The Electromagnetic Spectrum
GEOG 4260
  • 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.

100
The Electromagnetic Spectrum
GEOG 4260
  • 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?

101
The Electromagnetic Spectrum
GEOG 4260
  • 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.

102
The Electromagnetic Spectrum
GEOG 4260
  • 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.

103
The Electromagnetic Spectrum
GEOG 4260
  • 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.

104
The Electromagnetic Spectrum
GEOG 4260
  • 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.

105
The Electromagnetic Spectrum
GEOG 4260
  • 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.

106
The Electromagnetic Spectrum
GEOG 4260
  • 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.

107
The Electromagnetic Spectrum
GEOG 4260
  • 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.

108
The Electromagnetic Spectrum
GEOG 4260
  • More accurately, a single specular surface
    reflects little energy back to the sensor system.
    Dihedral reflectors and trihedral reflectors
    return very strong signals.

109
The Electromagnetic Spectrum
GEOG 4260
  • Many natural features such as cliffs and many
    engineered structures contain dihedral and
    trihedral reflectors that generate strong radar
    echos.

110
The Electromagnetic Spectrum
GEOG 4260
  • 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.

111
The Electromagnetic Spectrum
GEOG 4260
  • 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.

133
The Electromagnetic Spectrum
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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.

135
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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

139
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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.

140
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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.

142
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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.

143
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  • Atmospheric Influences on
  • Spectral Response Patterns

144
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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.

149
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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.

153
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  • Digital images, whether acquired electronically
    or scanned from photographs, consist of one or
    more raster arrays of digital numbers.

154
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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.

155
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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.

156
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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.

157
The Electromagnetic Spectrum
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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.

160
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  • Reference Data

161
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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.

163
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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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Location
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  • The Global Positioning Satellite System

165
Location
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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.

166
Location
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  • GPS uses a constellation of 24 satellites,
    Earth-based control stations, and user receivers.

167
Location
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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.

168
Location
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  • One satellite provides your location on the
    surface of an imaginary sphere

169
Location
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  • Measurements from two satellites provide your
    location along an imaginary circle

170
Location
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  • Measurements from three satellites provide your
    location at one of two points

171
Location
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  • Trick question
  • Why do GPS receivers need to have four satellites
    above the horizon to provide an accurate location?

172
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.

173
Location
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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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Location
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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.

175
Location
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  • Using Data Obtained with a GPS Receiver

176
Location
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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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Location
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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.

178
Location
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  • Geographic Information Systems

179
Location
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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.

181
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  • Next Chapter 2
  • Photographic Remote Sensing Systems
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