Download rt_lecture1 - UCL Department of Geography

UCL
DEPARTMENT
OF GEOGRAPHY
UCL DEPARTMENT
OF GEOGRAPHY
GEOGG141/ GEOG3051
Principles & Practice of Remote Sensing (PPRS)
Radiative Transfer Theory at optical wavelengths
applied to vegetation canopies: part 1
Dr. Mathias (Mat) Disney
UCL Geography
Office: 113, Pearson Building
Tel: 7679 0592
Email: [email protected]
http://www2.geog.ucl.ac.uk/~mdisney/teaching/GEOGG141/GEOGG141.html
http://www2.geog.ucl.ac.uk/~mdisney/teaching/3051/GEOG3051.html
Notes adapted from Prof. P. Lewis [email protected]
UCL DEPARTMENT OF GEOGRAPHY
Aim of this section
• Introduce RT approach as basis to understanding
optical and microwave vegetation response
• enable use of models
• enable access to literature
UCL DEPARTMENT OF GEOGRAPHY
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
UCL DEPARTMENT OF GEOGRAPHY
Reading
Full notes for these lectures
http://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
Books
Jensen, 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.
Papers
Feret, 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, 113-121
Walthall, 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.
UCL DEPARTMENT OF GEOGRAPHY
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
UCL DEPARTMENT OF GEOGRAPHY
UCL DEPARTMENT OF GEOGRAPHY
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
UCL DEPARTMENT OF GEOGRAPHY
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…..?
UCL DEPARTMENT OF GEOGRAPHY
Building blocks
for a canopy model
• Require descriptions of:
– canopy architecture
– leaf scattering
– soil scattering
UCL DEPARTMENT OF GEOGRAPHY
Canopy Architecture
• 1-D: Functions of depth from the top of the canopy (z).
UCL DEPARTMENT OF GEOGRAPHY
Canopy Architecture
•
1-D: Functions of depth from the top of the canopy
(z).
1. Vertical leaf area density ul (z ) (m2/m3)
2. the leaf normal orientation distribution function
(dimensionless).
3. leaf size distribution (m)
UCL DEPARTMENT OF GEOGRAPHY
Canopy Architecture
• Leaf area / number density
–
2 leaf per m3
(one-sided)
m
ul (z )
z=H
LAI
L=
ò u (z )dz
l
z =0
UCL DEPARTMENT OF GEOGRAPHY
Canopy Architecture
• Leaf Angle Distribution
z
Inclination to vertical
ò p g (W )d W
2 +
l
l
l
º1
ql
Wl
Leaf normal vector
y
fl
azimuth
x
UCL DEPARTMENT OF GEOGRAPHY
Leaf Angle Distribution
• Archetype Distributions:
 planophile

gl (Jl )= 3 cos 2 Jl
 erectophile

 spherical

æ 3ö 2
g l (Jl ) = ç ÷ sin Jl
è 2ø
 plagiophile

 extremophile 
gl (Jl )= 1
æ 15 ö 2
g l (Jl ) = ç ÷ sin 2Jl
è8ø
æ 15 ö 2
g l (Jl ) = ç ÷ cos 2Jl
è7ø
UCL DEPARTMENT OF GEOGRAPHY
Leaf Angle Distribution
• Archetype Distributions:
3.0
2.5
g_l(theta_l)
2.0
1.5
1.0
0.5
0.0
0
10
20
30
40
50
60
70
leaf zenith angle / degrees
spherical
plagiophile
planophile
extremophile
erectophile
80
90
UCL DEPARTMENT OF GEOGRAPHY
Leaf Dimension
• 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
UCL DEPARTMENT OF GEOGRAPHY
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
UCL DEPARTMENT OF GEOGRAPHY
Scattering properties of leaves
• Leaf surface properties and internal structure
optical
Specular
from surface
Dicotyledon leaf structure
Smooth (waxy) surface
- strong peak
hairs, spines
- more diffused
UCL DEPARTMENT OF GEOGRAPHY
Scattering properties of leaves
• Leaf surface properties and internal structure
optical
Diffused
from scattering at
internal air-cell
wall interfaces
Depends on total area
of cell wall interfaces
Dicotyledon leaf structure
Depends on refractive index:
varies: 1.5@400 nm
1.3@2500nm
UCL DEPARTMENT OF GEOGRAPHY
Scattering properties of leaves
• Leaf surface properties and internal structure
optical
More complex structure (or thickness):
- more scattering
- lower transmittance
- more diffuse
Dicotyledon leaf structure
UCL DEPARTMENT OF GEOGRAPHY
Scattering properties of leaves
• Leaf biochemstry
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 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 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.
Scattering properties of leaves
UCL DEPARTMENT OF GEOGRAPHY
• 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.
UCL DEPARTMENT OF GEOGRAPHY
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
UCL DEPARTMENT OF GEOGRAPHY
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
wavelengths.
reflectance
and
transmittance
at
these
UCL DEPARTMENT OF GEOGRAPHY
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.
UCL DEPARTMENT OF GEOGRAPHY
Scattering properties of leaves
• Optical Models
– flowering plants: PROSPECT – extension of plate
model to N layers
http://teledetection.ipgp.jussieu.fr/opticleaf/models.htm
UCL DEPARTMENT OF GEOGRAPHY
Scattering properties of leaves
• leaf dimensions
– optical
• increase leaf area for constant number of leaves
increase LAI
• increase leaf thickness - decrease transmittance
(increase reflectance)
UCL DEPARTMENT OF GEOGRAPHY
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)
UCL DEPARTMENT OF GEOGRAPHY
soil moisture content
• Optical
– effect essentially proportional across all wavelengths
• enhanced in water absorption bands
UCL DEPARTMENT OF GEOGRAPHY
soil texture/type
• 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.
UCL DEPARTMENT OF GEOGRAPHY
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)
rsoil = po (q 2 + q 2v ) + p1q 2q 2 cos f + p3
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
Soil roughness effects
UCL DEPARTMENT OF GEOGRAPHY
• Rough roughness:
– optical surface scattering
• Note backscatter reflectance peak (‘hotspot’)
• minimal shadowing
• backscatter peak width increases with increasing roughness
Soil roughness effects
UCL DEPARTMENT OF GEOGRAPHY
• 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)
UCL DEPARTMENT OF GEOGRAPHY
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