Remote Sensing

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Remote Sensing

• Topic 2 Photographic Principles
• Chapter 2 Lillesand and Keifer

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• Advantages of Aerial Photography Over Ground
  Observation
• Improved vantage point
• Capability to stop action
• Permanent recording
• Greater spectral sensitivity
• Increased spatial resolution
• Photogrammetry

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Basic B/W Photography

• Film and photographic paper consists of a light
  sensitive emulsion on a base
• paper, plastic, transparency, or glass
• Emulsion consists of silver halide crystals
  suspended in gelatin

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Basic B/W Photography

• Emulsion undergoes a photochemical reaction in
  the presence of light to form a latent image
• When developed, silver halide crystals are
  converted to silver grains, which are black
• Number of silver grains is dependent on amount of
  light or exposure
• results in light and dark tones on photo
• enables us to identify features
• essential for extracting info from aerial
  photography

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A Simple Camera

• Consists of a lens of defined focal length
• Diaphragm controls size of opening and amount of
  light entering
• Shutter controls length of time light enters

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Exposure

• Exposure at any point on a photograph is
  dependent on
• Scene brightness/spectral reflectivity features
• Diameter of the camera lens diaphragm
• Time of exposure shutter speed
• Focal length of camera lens
• E sd2t / 4f2
• where E exposure, s scene brightness
• d diameter t time f focal length
• Note for a given scene, exposure can be
  controlled by regulating the diameter of the
  opening, d, or the speed of the shutter, t

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F-stop and Exposure

• The diameter of the lens opening (d) is adjusted
  by setting the aperture or f-stop
• F f-stop f / d
• So E sd2t / 4f2 can be rewritten as E st /
  4F2
• This leaves just shutter speed (t) and f-stop (F)
• Shorter exposure times prevent blurring but
  require high speed films
• Longer exposure times allow more light, and
  slower films, but may result in blurring

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Extraneous Effects on Exposure

• Geometric and atmospheric factors also affect the
  amount of exposure and consequently the tone or
  intensity of colour

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Exposure Falloff

• Exposure falloff is the result of variations in
• Effective light collecting area of the lens
• Distance from lens to focal plane
• Size of film area exposed
• Exposure decreases with increasing distance from
  the centre of the focal plane

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Sun-Object-Sensor Geometry

• The position of a feature(s) with respect to
  camera view angle, solar elevation, and
  horizontal angle between camera and sun affects
  exposure

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Sun-Object-Sensor Geometry

• These effects include
• Differential irradiance (shading)
• Differential scattering
• Specular reflection

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Film Density Measurements

• Provided geometric and atmospheric effects have
  been accounted for
• measures of tonal variation or density across a
  photograph (transparency) may be useful
• Correlations between optical density and certain
  attributes of a feature may be established to
  estimate
• Type
• Condition
• Amount
• Value
• and/or extent of a feature.
• E.g.. biomass, crop yield, soil moisture, soil
  type, species type, etc.

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Film Density Measurements

• Density is a measure of the darkness, opacity, or
  transmittance of a photographic image
• Print or Film
• Density is directly related to exposure
• Determined using a densitometer
• measures transmittance (through a transparency)
• or reflectance (off a photograph)

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Characteristic Curves

• The silver halide grains of a panchromatic photo
  are equivalent to many small detectors
• Theoretically, exposure should vary linearly with
  reflectance, however, this is not the case
• A characteristic curve maybe developed for each
  type and batch of film to identify the
  relationship between exposure and density

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Characteristic Curves

• Exposure varies at the toe and shoulder and is
  linear only though the straight line portion of
  the curve.
• Slope of straight line portion is gamma (?)

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Characteristic Curves

• Gamma (?), is directly proportional to the
  contrast of the film and inversely proportional
  to radiometric resolution
• Film 1 has high contrast and smaller (better)
  radiometric resolution, film 2 has low contrast
  and larger (poorer) radiometric resolution

High Contrast High Slope (?) Better Radiometric
Resolution
Low Contrast Low Slope (?) Poorer Radiometric
Resolution
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Characteristic Curves

• However, higher ? means lower exposure latitude
• Range over which exposure will result in a good
  photo

Low Exposure Latitude
High Exposure Latitude
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Characteristic Curves

• Higher speed films have characteristic curves
  shifted to the left
• Less light (exposure) is required
• Film 1 is fast film, film 2 is slow film
• However, faster films have larger silver grains
  so poorer spatial resolution

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B/W Aerial Films

• B/W aerial films include both panchromatic and
  infrared films
• Panchromatic is most commonly used film for
  aerial photography

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B/W Panchromatic Films

• Panchromatic films are sensitive to EMR from the
  UV (0.3 ?m) through to the red (0.7 ?m) portions
  of the electromagnetic spectrum
• Since UV and blue EMR is effectively scattered in
  the atmosphere, filters are used to limit these
  wavelengths

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B/W Infrared Films

• Infrared films are sensitive to UV, visible, and
  infrared portions of the electromagnetic spectrum
  up to about 0.9 ?m
• Films sensitive to wavelengths beyond 0.9 ?m
  exist but are unstable

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Panchromatic Photo
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B/W Infrared Photo
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Colour Aerial Films

• The advantage of using colour films is the
  increased interpretability of the resulting
  photographs
• We can discriminate between many more different
  colours than shades of grey
• More spectral information due to increased
  spectral resolution

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Colour Theory

• Blue, green, and red are referred to as additive
  primaries
• Variations in the relative proportion of all
  three reflected back to our eyes results in all
  possible colours
• The combination of all three results in white
• Absence of all three is perceived as black

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Colour Theory

• Yellow, magenta, and cyan are referred to as
  subtractive primaries
• Subtractive primaries filter blue, green, and red
  light from white to produce all possible colours
• The combination of all three results in black

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How Colour Film Works

• Colour photography is based on the principle of
  subtractive primary colours
• Yellow, magenta, and cyan dyes are used to filter
  differing proportions of blue, green, and red
  light from incident white light
• The dyes are introduced into separate layers of
  colour film each containing silver halide
  crystals modified by organic molecules known as
  spectral sensitizers
• The subtractive primaries are used to control the
  relative proportions of additive primaries
  reflected back to our eye

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How Colour Film Works

• When developed the blue, green, and red sensitive
  layers absorb differing proportions of yellow,
  magenta, and cyan dyes inversely proportional to
  the amount of blue, green, and red light
  reflected from the features photographed

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Spectral Sensitivity
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How Colour IR Film Works

• The assignment of yellow, magenta and cyan dyes
  to an emulsion layer of specific spectral
  sensitivity is arbitrary
• Colour infrared films are manufactured such that
  each dye layer is assigned to a different portion
  of the EM spectrum
• Also called false colour film

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Spectral Sensitivity
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How Colour IR Film Works

• Blue wavelengths are filtered out of incoming
  light using a filter placed in front of the
  camera lens
• Near infrared ? cyan dye - controls red
  reflectance to our eye
• Red ? magenta dye controls green reflectance to
  our eye
• Green ? yellow dye - controls blue reflectance to
  our eye

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Filters

• Made of glass or gelatin
• Enable us to select wavelengths of reflected
  energy allowed to reach film
• Example
• yellow filter absorbs blue wavelengths
• allows green and red to pass through
• commonly used to reduce haze

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Filters

• Other filters only allow longer wavelengths to
  pass
• called shortwave blocking filters or high-pass
  filters
• often used when taking b/w infrared photography
• Filters reduce total light allowed to reach film
• exposure may have to be adjusted to allow for
  more light

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Aerial Cameras

• Hand held 35 mm cameras
• Single and multiple lens frame cameras
• Digital cameras
• Digital multispectral scanners
• Analog and digital video recorders

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Single Lens Frame Camera

• Most common aerial camera
• Photos are (9.5 in) on a side film 150 ft spools
• Use vacuum frame and image motion compensation
• Focal lengths vary from 300 mm (12 in) to 90 mm
  (3.5 in) 152 mm (6 in) is most common.
• Focal length also affects angle of coverage.
• Narrow 300 mm (12 in) lt 60º
• Normal 210 mm (8.25 in) 60º - 75º
• Wide 152 mm (6 in) 75º - 100º
• Super Wide 90 mm (3.5 in) gt 100º

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Single Lens Frame Camera
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Multi-Lens Frame Camera

• More than one photo at a time
• Different film/filter combinations.
• Referred to as multiband or multispectral
  photographs
• Film/filter combinations selected to obtain
  narrow portions of the electromagnetic spectrum

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Strip Camera

• Records photographic images by moving film past a
  slit (aperture) perpendicular to the flight line
• Slit remains open while the aircraft is in motion
• Image motion eliminated by moving the film past
  slit
• Commonly used for photographing linear features
• e.g. rivers, highways, utility lines, etc.

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Panoramic Camera

• Lens rotates/oscillates perpendicular to flt.
  Line
• Slit (aperture) is parallel to flt. Line
• Able to cover large ground areas
• Lacks geometric fidelity
• severe scale distortion

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Digital Camera

• Utilize an array of solid state silicon chips
  called charge-coupled devices (CCDs)
• Sensitive from UV to mid-infrared portion of EMS
• CCDs can be calibrated to record in very narrow
  band widths
• Size of CCD array determines image resolution
• Resolution is approaching photographic systems
• Storage space no longer a limitation
• Increased spectral and radiometric resolution and
  ability to digitally process/analyze imagery is a
  major advantage

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Video Camera

• Single or multiple cameras can be used
• Sensitive to visible, near, and mid-infrared
• Single frames can be captured and analyzed
• Comparatively poor spatial resolution
• Cheap do it yourself approach to remote sensing