Remote Sensing and Image Processing: 5

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Remote Sensing and Image Processing 5

• Dr. Mathias (Mat) Disney
• UCL Geography
• Office 301, 3rd Floor, Chandler House
• Tel [phone] (x24290)
• Email mdisney_at_geog.ucl.ac.uk
• www.geog.ucl.ac.uk/mdisney

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EMR arriving at Earth

• We now know how EMR spectrum is distributed
• Radiant energy arriving at Earths surface
• NOT blackbody, but close
• This lecture..
• Interactions of EMR with atmosphere
• scattering
• Atmospheric windows and choosing the right
  place for bands
• Interactions at the surface
• Scattering and angular effects

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Departure from blackbody assumption

• Interaction with gases in the atmosphere
• attenuation of solar radiation

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Interactions with the atmosphere

• Notice that target reflectance is a function of
• Atmospheric irradiance
• reflectance outside target scattered into path
• diffuse atmospheric irradiance
• multiple-scattered surface-atmosphere interactions

From http//www.geog.ucl.ac.uk/mdisney/phd.bak/f
inal_version/final_pdf/chapter2a.pdf
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Interactions with the atmosphere scattering

• Caused by presence of particles (soot, salt,
  etc.) and/or large gas molecules present in the
  atmosphere
• Interact with EMR and cause to be redirected from
  original path.
• Scattering amount depends on
• ? of radiation
• abundance of particles or gases
• distance the radiation travels through the
  atmosphere (path length)

After http//www.ccrs.nrcan.gc.ca/ccrs/learn/tuto
rials/fundam/chapter1/chapter1_4_e.html
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Atmospheric scattering 1 Rayleigh

• Particle size ltlt ? of radiation
• e.g. very fine soot and dust or N2, O2 molecules
• Rayleigh scattering dominates shorter ? and in
  upper atmos.
• i.e. Longer ? scattered less (visible red ?
  scattered less than blue ?)
• Hence during day, visible blue ? tend to dominate
  (shorter path length)
• Longer path length at sunrise/sunset so
  proportionally more visible blue ? scattered out
  of path so sky tends to look more red
• Even more so if dust in upper atmosphere
• http//www.spc.noaa.gov/publications/corfidi/sunse
  t/
• http//www.nws.noaa.gov/om/educ/activit/bluesky.ht
  m

After http//www.ccrs.nrcan.gc.ca/ccrs/learn/tuto
rials/fundam/chapter1/chapter1_4_e.html
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Atmospheric scattering 2 Mie

• Particle size ? ? of radiation
• e.g. dust, pollen, smoke and water vapour
• Affects longer ? than Rayleigh, BUT weak
  dependence on ?
• Mostly in the lower portions of the atmosphere
• larger particles are more abundant
• dominates when cloud conditions are overcast
• i.e. large amount of water vapour (mist, cloud,
  fog) results in almost totally diffuse
  illumination

After http//www.ccrs.nrcan.gc.ca/ccrs/learn/tuto
rials/fundam/chapter1/chapter1_4_e.html
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Atmospheric scattering 3 Non-selective

• Particle size gtgt ? of radiation
• e.g. Water droplets and larger dust particles,
• All ? affected about equally (hence name!)
• Hence results in fog, mist, clouds etc. appearing
  white
• white equal scattering of red, green and blue ?
  s

After http//www.ccrs.nrcan.gc.ca/ccrs/learn/tuto
rials/fundam/chapter1/chapter1_4_e.html
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Atmospheric absorption

• Other major interaction with signal
• Gaseous molecules in atmosphere can absorb
  photons at various ?
• depends on vibrational modes of molecules
• Very dependent on ?
• Main components are
• CO2, water vapour and ozone (O3)
• Also CH4 ....
• O3 absorbs shorter ? i.e. protects us from UV
  radiation

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Atmospheric windows

• As a result of strong ? dependence of absorption
• Some ? totally unsuitable for remote sensing as
  most radiation absorbed

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Atmospheric windows

• If you want to look at surface
• Look in atmospheric windows where transmissions
  high
• BUT if you want to look at atmosphere ....pick
  gaps
• Very important when selecting instrument channels
• Note atmosphere nearly transparent in ?wave i.e.
  can see through clouds!
• BIG advantage of ?wave remote sensing

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Atmospheric windows

• Vivisble NIR part of the spectrum
• windows, roughly 400-750, 800-1000, 1150-1300,
  1500-1600, 2100-2250nm

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Recap

• Signal we measure contains atmospheric
  contamination (or information depending on your
  point of view!)
• Rayleigh (fine dust and gases), Mie (bigger
  particles) and non-selective scattering (water
  vapour and the rest)
• Perform atmospheric correction to get at surface
  signal
• Part of pre-processing steps (see later)
• So what happens at the surface?

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Reflectance

• When EMR hits target (surface)
• Range of surface reflectance behaviour
• perfect specular (mirror-like) - incidence angle
  exitance angle
• perfectly diffuse (Lambertian) - same reflectance
  in all directions independent of illumination
  angle)

From http//www.ccrs.nrcan.gc.ca/ccrs/learn/tutori
als/fundam/chapter1/chapter1_5_e.html
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Surface energy budget

• Total amount of radiant flux per wavelength
  incident on surface, ?(?) W?m-1 is summation of
• reflected r?, transmitted t?, and absorbed, a?
• i.e. ?(?) r? t? a?
• So need to know about surface reflectance,
  transmittance and absorptance
• Measured RS signal is combination of all 3
  components

After Jensen, J. (2000) Remote sensing of the
environment an Earth Resources Perspective.
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Angular distribution of reflectance

• Real surfaces usually display some degree of
  reflectance ANISOTROPY
• Lambertian surface is isotropic by definition
• Most surfaces have some level of anisotropy
• Described by Bidirectional Reflectance
  Distribution Function (BRDF)

From http//www.geog.ucl.ac.uk/mdisney/phd.bak/f
inal_version/final_pdf/chapter2a.pdf
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Directional Information
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Features of BRDF

• Bowl shape
• increased scattering due to increased path length
  through canopy

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Features of BRDF

• Bowl shape
• increased scattering due to increased path length
  through canopy

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Features of BRDF

• Hot Spot
• mainly shadowing minimum
• so reflectance higher

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Directional reflectance BRDF

• Good explanation of BRDF
• http//geography.bu.edu/brdf/brdfexpl.html

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Summary

• Top-of-atmosphere (TOA) signal is NOT target
  signal
• function of target reflectance
• plus atmospheric component (scattering,
  absorption)
• need to choose appropriate regions (atmospheric
  windows)
• Surface reflectance is anisotropic
• i.e. looks different in different directions
• described by BRDF
• angular signal contains information on size,
  shape and distribution of objects on surface