Transcript Vegetation monitoring for climate studies

Remote Sensing and Image
Processing: 5
Dr. Mathias (Mat) Disney
UCL Geography
Office: 301, 3rd Floor, Chandler House
Tel: 7670 4290 (x24290)
Email: [email protected]
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 Earth’s 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
R4
R1
R2
target
R3
target
target
•Notice that target reflectance is a function of
•Atmospheric irradiance
•reflectance outside target scattered into path
•diffuse atmospheric irradiance
•multiple-scattered surface-atmosphere interactions
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From: http://www.geog.ucl.ac.uk/~mdisney/phd.bak/final_version/final_pdf/chapter2a.pdf
target
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)
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After: http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/fundam/chapter1/chapter1_4_e.html
Atmospheric scattering 1: Rayleigh
•Particle size <<  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/sunset/
•http://www.nws.noaa.gov/om/educ/activit/bluesky.htm
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After: http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/fundam/chapter1/chapter1_4_e.html
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
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After: http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/fundam/chapter1/chapter1_4_e.html
Atmospheric scattering 3: Non-selective
•Particle size >>  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
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After: http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/fundam/chapter1/chapter1_4_e.html
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”
Landsat TM
bands in
atmospheric
windows
•As a result of strong  dependence of absorption
•Some  totally unsuitable for remote sensing as most
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radiation absorbed
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 nonselective 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)
Natural surfaces
somewhere in
between
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From http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/fundam/chapter1/chapter1_5_e.html
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
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After: Jensen, J. (2000) Remote sensing of the environment: an Earth Resources Perspective.
Angular distribution of reflectance
•Real surfaces usually
display some degree of
reflectance ANISOTROPY
•Lambertian surface is
isotropic by definition
(a)
(b)
(c)
(d)
•Most surfaces have some
level of anisotropy
•Described by Bidirectional
Reflectance Distribution
Function (BRDF)
Figure 2.1 Four examples of surface reflectance: (a) Lambertian reflectance (b)
non-Lambertian (directional) reflectance (c) specular (mirror-like) reflectance (d)
retro-reflection peak (hotspot).
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From: http://www.geog.ucl.ac.uk/~mdisney/phd.bak/final_version/final_pdf/chapter2a.pdf
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
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