Notes adapted from Prof. P. Lewis [email protected]
description
Transcript of Notes adapted from Prof. P. Lewis [email protected]
UCL DEPARTMENT OF GEOGRAPHYUCL DEPARTMENT OF GEOGRAPHY
GEOGG141/ GEOG3051Principles & Practice of Remote Sensing (PPRS)Radiative Transfer Theory at optical wavelengths applied to vegetation canopies: part 1
Notes adapted from Prof. P. Lewis [email protected]
Dr. Mathias (Mat) DisneyUCL GeographyOffice: 113, Pearson BuildingTel: 7679 0592Email: [email protected]://www2.geog.ucl.ac.uk/~mdisney/teaching/GEOGG141/GEOGG141.htmlhttp://www2.geog.ucl.ac.uk/~mdisney/teaching/3051/GEOG3051.html
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Aim of this section
• Introduce RT approach as basis to understanding optical and microwave vegetation response
• enable use of models• enable access to literature
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Scope of this section
• Introduction to background theory– RT theory– Wave propagation and polarisation– Useful tools for developing RT
• Building blocks of a canopy scattering model– canopy architecture– scattering properties of leaves– soil properties
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Reading
Full notes for these lectureshttp://www2.geog.ucl.ac.uk/~mdisney/teaching/GEOGG141/rt_theory/rt_notes1.pdf http://www2.geog.ucl.ac.uk/~mdisney/teaching/GEOGG141/rt_theory/rt_notes2.pdf
BooksJensen, J. (2007) Remote Sensing: an Earth Resources Perspective, 2nd ed., Chapter 11 (355-408), 1st ed chapter 10.Liang, S. (2004) Quantitative Remote Sensing of Land Surfaces, Wiley, Chapter 3 (76-142).Monteith, J. L. and Unsworth, M. H. (1990) Principles of Environmental Physics, 2nd ed., ch 5 & 6.
PapersFeret, J-B. et al. (2008) PROSPECT-4 and 5: Advances in the leaf optical properties model separating photosynthetic pigments, RSE, 112, 3030-3043.Jacquemoud. S. and Baret, F. (1990) PROSPECT: A model of leaf optical properties spectra, RSE, 34, 75-91.Nilson, T. and Kuusk, A. (1989) A canopy reflectance model for the homogeneous plant canopy and its inversion, RSE, 27, 157-167.Price, J. (1990), On the information content of soil reflectance spectra RSE, 33, 113-121Walthall, C. L. et al. (1985) Simple equation to approximate the bidirectional reflectance from vegetative canopies and bare soil surfaces, Applied Optics, 24(3), 383-387.
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Why build models?
• Assist data interpretation• calculate RS signal as fn. of biophysical variables
• Study sensitivity• to biophysical variables or system parameters
• Interpolation or Extrapolation• fill the gaps / extend observations
• Inversion• estimate biophysical parameters from RS
• aid experimental design• plan experiments
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Radiative Transfer Theory
• Applicability– heuristic treatment
• consider energy balance across elemental volume– assume:
• no correlation between fields– addition of power not fields
• no diffraction/interference in RT– can be in scattering
– develop common (simple) case here
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Radiative Transfer Theory
• Case considered:– horizontally infinite but vertically finite plane
parallel medium (air) embedded with infinitessimal oriented scattering objects at low density
– canopy lies over soil surface (lower boundary)– assume horizontal homogeneity
• applicable to many cases of vegetation• But…..?
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Building blocks for a canopy model
• Require descriptions of:– canopy architecture– leaf scattering– soil scattering
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Canopy Architecture• 1-D: Functions of depth from the top of the canopy (z).
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Canopy Architecture
• 1-D: Functions of depth from the top of the canopy (z).
1. Vertical leaf area density (m2/m3)2. the leaf normal orientation distribution function
(dimensionless).3. leaf size distribution (m)
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Canopy Architecture
• Leaf area / number density– (one-sided) m2 leaf per m3
LAI
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Canopy Architecture
• Leaf Angle Distribution
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• Archetype Distributions:· planophile
· erectophile
· spherical
· plagiophile
· extremophile
Leaf Angle Distribution
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• Archetype Distributions:
Leaf Angle Distribution
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• RT theory: infinitesimal scatterers– without modifications (dealt with later)
• In optical, leaf size affects canopy scattering in retroreflection direction– ‘roughness’ term: ratio of leaf linear dimension to canopy
height
also, leaf thickness effects on reflectance /transmittance
Leaf Dimension
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Canopy element and soil spectral properties
• Scattering properties of leaves– scattering affected by:
• Leaf surface properties and internal structure; • leaf biochemistry; • leaf size (essentially thickness, for a given LAI).
Excellent review here:http://www.photobiology.info/Jacq_Ustin.html
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Scattering properties of leaves
• Leaf surface properties and internal structure
opticalSpecular
from surface
Smooth (waxy) surface- strong peak
hairs, spines- more diffused
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Scattering properties of leaves
• Leaf surface properties and internal structure
opticalDiffused
from scattering at internal air-cell wall interfaces
Depends on total areaof cell wall interfaces
Depends on refractive index:varies: 1.5@400 nm
1.3@2500nm
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Scattering properties of leaves
• Leaf surface properties and internal structure
optical
More complex structure (or thickness):- more scattering- lower transmittance- more diffuse
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Scattering properties of leaves
• Leaf biochemstry
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Scattering properties of leaves• Leaf biochemstry
Feret, Jacquemoud et al. (2008) PROSPECT-4 and 5: Advances in the leaf optical properties model separating photosynthetic pigments, RSE, 112, 3030-3043.
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Scattering properties of leaves• Leaf biochemstry
Feret, Jacquemoud et al. (2008) PROSPECT-4 and 5: Advances in the leaf optical properties model separating photosynthetic pigments, RSE, 112, 3030-3043.
UCL DEPARTMENT OF GEOGRAPHY
Scattering properties of leaves• Leaf biochemstry
Feret, Jacquemoud et al. (2008) PROSPECT-4 and 5: Advances in the leaf optical properties model separating photosynthetic pigments, RSE, 112, 3030-3043.
UCL DEPARTMENT OF GEOGRAPHYScattering properties of leaves
• Leaf water
Feret, Jacquemoud et al. (2008) PROSPECT-4 and 5: Advances in the leaf optical properties model separating photosynthetic pigments, RSE, 112, 3030-3043.
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Scattering properties of leaves• Leaf biochemstry
– pigments: chlorophyll a and b, a-carotene, and xanthophyll • absorb in blue (& red for chlorophyll)
– absorbed radiation converted into:• heat energy, flourescence or carbohydrates through
photosynthesis
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Scattering properties of leaves• Leaf biochemstry
– Leaf water is major consituent of leaf fresh weight,• around 66% averaged over a large number of leaf types
– other constituents ‘dry matter’• cellulose, lignin, protein, starch and minerals
– Absorptance constituents increases with concentration• reducing leaf reflectance and transmittance at these
wavelengths.
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Scattering properties of leaves
• Optical Models– flowering plants: PROSPECT – a generalised
plate model
Figure from: http://teledetection.ipgp.jussieu.fr/opticleaf/models.htm & see for more detail on various approaches to leaf optical properties modelling
Jacquemoud. S. and Baret, F. (1990) PROSPECT: A model of leaf optical properties spectra, RSE, 34, 75-91.
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Scattering properties of leaves
• Optical Models– flowering plants: PROSPECT – extension of plate
model to N layers
http://teledetection.ipgp.jussieu.fr/opticleaf/models.htm
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Scattering properties of leaves• leaf dimensions
– optical• increase leaf area for constant number of leaves -
increase LAI• increase leaf thickness - decrease transmittance
(increase reflectance)
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Scattering properties of soils
• Optical and microwave affected by:– soil moisture content
– Wetter soils are darker (optical); have lower dielectric (microwave)
– soil type/texture– soil surface roughness
– shadowing (optical)– coherent scattering (microwave)
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soil moisture content• Optical
– effect essentially proportional across all wavelengths• enhanced in water absorption bands
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• Optical– relatively little variation in spectral
properties– Price (1990):
• PCA on large soil database - 99.6% of variation in 4 PCs
– Stoner & Baumgardner (1982) defined 5 main soil types:• organic dominated• minimally altered• iron affected• organic dominated• iron dominated
Price, J. (1990), On the information content of soil reflectance spectra RSE, 33, 113-121.
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Soil roughness effects
• Affects directional properties of reflectance (optical particularly)
• Simple models:– as only a boundary condition, can sometimes use simple
models• e.g. Lambertian• e.g. trigonometric (Walthall et al., 1985; Nilson and Kuusk 1990)
where θv,i are the view and illumination (sun) zenith angles; ϕ is relative azimuth angle (ϕi - ϕv).
UCL DEPARTMENT OF GEOGRAPHY Soil roughness effects
• Rough roughness:– optical surface scattering
• clods, rough ploughing– use Geometric Optics model (Cierniewski)– projections/shadowing from protrusions
UCL DEPARTMENT OF GEOGRAPHYSoil roughness effects
• Rough roughness:– optical surface scattering
• Note backscatter reflectance peak (‘hotspot’)• minimal shadowing• backscatter peak width increases with increasing roughness
UCL DEPARTMENT OF GEOGRAPHYSoil roughness effects
• Rough roughness:– volumetric scattering
• consider scattering from ‘body’ of soil– particulate medium– use RT theory (Hapke - optical)– modified for surface effects (at different scales of roughness)
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Summary
• Introduction– Examined rationale for modelling– discussion of RT theory– Scattering from leaves
• Canopy model building blocks– canopy architecture: area/number, angle, size– leaf scattering: spectral & structural– soil scattering: roughness, type, water