Abstract
In its most general sense, remote sensing is the science of deducing information about a surface or medium by measuring radiation that has been emitted or scattered by that surface or medium. Specifically, remote sensing usually implies measurements of the earth. With the advent of sensors aboard aircraft and satellites, the potential to acquire information about the earth’s surface and resources increased on an unprecedented scale. The realization of that potential involves building the necessary links between surface characteristics and emanating radiation. This chapter contains a discussion of the relationship between the structure of vegetation and the measured brightness of that vegetation as viewed from above. In particular, how does the angle of view and the angle of illumination affect the brightness of a vegetative canopy?
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Abbreviations
- Aj :
-
albedo of the jth layer in the random model
- As :
-
soil reflectance
- Ci :
-
fraction of leaves that are sunlit at ith grid point
- Dj :
-
downward diffuse irradiance above the jth layer in the random model
- Fd(r⃗i):
-
downward diffuse irradiance at the ith grid point
- Ft :
-
total incident irradiance on the horizontal at the top of the canopy
- Fu(r⃗i):
-
upward diffuse irradiance at the ith grid point
- G(Ω̱):
-
canopy extinction coefficient: the fraction of area projected toward (Ω̱)
- Hmt, Hmb :
-
fraction of mth ellipsoid chopped in the top or bottom half
- I(r⃗i,Ω̱):
-
scene radiance from the ith grid point. Scene radiance is either from soil or foliage
- I(r⃗i,Ω̱, θL):
-
total radiance of the foliage in angle class (α, β) at the ith grid point
- I(Ω̱):
-
radiance leaving the canopy in direction (θV, ϕv)
- Ib(r⃗i,Ω̱, θL):
-
Radiance from foliage at ith point due to direct beam
- Id(r⃗i,Ω̱, θL):
-
radiance from foliage in angle class α at ith point due to diffuse irradiance
- L:
-
leaf area index
- ΔL:
-
layer LAI for random model
- M:
-
path length through the canopy
- ΔM:
-
grid point spacing
- Nb :
-
number of replicated boxes involved in a particular ray’s path
- Ns :
-
number of subcanopies in one box
- Ng :
-
number of grid points that lie within any subcanopy
- T:
-
diffuse noninterceptance for a random model layer
- T̛:
-
beam noninterceptance for a random model layer
- Tu(r⃗i):
-
diffuse noninterceptance, upper hemisphere, for the ith grid point
- Tl(r⃗i):
-
diffuse noninterceptance, lower hemisphere, for the ith grid point
- Uj :
-
upward diffuse irradiance above the jth layer in the random model
- fb :
-
fraction of incident total irradiance that is direct beam
- gL(θL):
-
fraction of leaf area at leaf angle class (θL)
- p(r⃗i,Ω̱):
-
probability of a ray from direction (Ω̱) reaching point (r⃗) without being intercepted
- r⃗1, r⃗2 :
-
entry and exit points for a box
- r⃗3, r⃗4 :
-
entry and exit points for a ellipsoid
- r̄L :
-
weighted leaf reflectance for a layer of leaves
- r:
-
random model layer reflectance
- rx, ry, rz :
-
radii in the x, y, and z directions of an ellipsoid
- rL :
-
leaf reflectance
- Sj, S̛j :
-
source terms up and down for the random model above the jth layer
- t̄L :
-
weighted leaf transmittance for a layer of leaves
- t:
-
random model layer transmittance
- tL :
-
leaf transmittance
- uL :
-
foliage area density (m-1): leaf area per canopy volume
- ϕO :
-
solar azimuth angle
- ϕL :
-
leaf azimuth angle
- ϕ:
-
view azimuth angle
- ω̛L :
-
leaf transmittance or reflectance, depending on view and sun angles
- ρ(θ — ϕO :
-
soil bidirectional reflectance function for view angle θ and azimuth angle ϕ
- ρ (Ω̱, θO):
-
bidirectional reflectance distribution function
- θO :
-
solar zenith angle
- θL :
-
leaf tilt angle
- θ:
-
view zenith angle
- Ω̱L · Ω̱:
-
cosine of the angle between the leaf normal and the viewer
- Ω̱L · Ω̱O :
-
cosine of the angle between the leaf normal and the sun
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Welles, J.M., Norman, J.M. (1991). Photon Transport in Discontinuous Canopies: A Weighted Random Approach. In: Myneni, R.B., Ross, J. (eds) Photon-Vegetation Interactions. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-75389-3_12
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DOI: https://doi.org/10.1007/978-3-642-75389-3_12
Publisher Name: Springer, Berlin, Heidelberg
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