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GEOGG141/ GEOG3051 Principles & Practice of Remote Sensing (PPRS) Radiative Transfer Theory at o ptical wavelengths applied to vegetation canopies: part 1. Notes adapted from Prof. P. Lewis plewis@geog.ucl.ac.uk. Dr. Mathias (Mat) Disney UCL Geography Office: 113, Pearson Building

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Notes adapted from prof p lewis plewis@geog ucl ac uk

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 plewis@geog.ucl.ac.uk

Dr. Mathias (Mat) Disney

UCL Geography

Office: 113, Pearson Building

Tel: 7679 0592

Email: mdisney@ucl.geog.ac.uk

http://www2.geog.ucl.ac.uk/~mdisney/teaching/GEOGG141/GEOGG141.html

http://www2.geog.ucl.ac.uk/~mdisney/teaching/3051/GEOG3051.html


Aim of this section
Aim of this section

  • Introduce RT approach as basis to understanding optical and microwave vegetation response

  • enable use of models

  • enable access to literature


Scope of this section
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


Reading
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), 1sted 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.


Why build models
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


  • Radiative transfer theory
    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


    Radiative transfer theory1
    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…..?


    Building blocks for a canopy model
    Building blocks for a canopy model

    • Require descriptions of:

      • canopy architecture

      • leaf scattering

      • soil scattering


    Canopy architecture
    Canopy Architecture

    • 1-D: Functions of depth from the top of the canopy (z).


    Canopy architecture1
    Canopy Architecture

    • 1-D: Functions of depth from the top of the canopy (z).

      1. Vertical leaf area density(m2/m3)

    • the leaf normal orientation distribution function

      (dimensionless).

      3. leaf size distribution (m)


    Notes adapted from prof p lewis plewis geog ucl ac uk

    LAI

    Canopy Architecture

    • Leaf area / number density

      • (one-sided) m2 leaf per m3


    Notes adapted from prof p lewis plewis geog ucl ac uk

    Canopy Architecture

    • Leaf Angle Distribution


    Notes adapted from prof p lewis plewis geog ucl ac uk

    Leaf Angle Distribution

    • Archetype Distributions:

      • · planophile 

      • · erectophile 

      • · spherical 

      • · plagiophile 

      • · extremophile 


    Notes adapted from prof p lewis plewis geog ucl ac uk

    Leaf Angle Distribution

    • Archetype Distributions:


    Notes adapted from prof p lewis plewis geog ucl ac uk

    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


    Notes adapted from prof p lewis plewis geog ucl ac uk

    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


    Notes adapted from prof p lewis plewis geog ucl ac uk

    Specular

    from surface

    Scattering properties of leaves

    • Leaf surface properties and internal structure

    optical

    Smooth (waxy) surface

    - strong peak

    hairs, spines

    - more diffused


    Notes adapted from prof p lewis plewis geog ucl ac uk

    Diffused

    from scattering at

    internal air-cell

    wall interfaces

    Scattering properties of leaves

    • Leaf surface properties and internal structure

    optical

    Depends on refractive index:

    varies: 1.5@400 nm

    1.3@2500nm

    Depends on total area

    of cell wall interfaces


    Notes adapted from prof p lewis plewis geog ucl ac uk

    Scattering properties of leaves

    • Leaf surface properties and internal structure

    optical

    More complex structure (or thickness):

    - more scattering

    - lower transmittance

    - more diffuse



    Notes adapted from prof p lewis plewis geog ucl ac uk

    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.


    Notes adapted from prof p lewis plewis geog ucl ac uk

    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.


    Notes adapted from prof p lewis plewis geog ucl ac uk

    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.


    Notes adapted from prof p lewis plewis geog ucl ac uk

    Scattering 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.


    Notes adapted from prof p lewis plewis geog ucl ac uk

    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


    Notes adapted from prof p lewis plewis geog ucl ac uk

    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.


    Notes adapted from prof p lewis plewis geog ucl ac uk

    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.


    Notes adapted from prof p lewis plewis geog ucl ac uk

    Scattering properties of leaves

    • Optical Models

      • flowering plants: PROSPECT – extension of plate model to N layers

    http://teledetection.ipgp.jussieu.fr/opticleaf/models.htm


    Notes adapted from prof p lewis plewis geog ucl ac uk

    Scattering properties of leaves

    • leaf dimensions

      • optical

        • increase leaf area for constant number of leaves - increase LAI

        • increase leaf thickness - decrease transmittance (increase reflectance)


    Notes adapted from prof p lewis plewis geog ucl ac uk

    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)


    Notes adapted from prof p lewis plewis geog ucl ac uk

    soil moisture content

    • Optical

      • effect essentially proportional across all wavelengths

        • enhanced in water absorption bands


    Notes adapted from prof p lewis plewis geog ucl ac uk

    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.


    Notes adapted from prof p lewis plewis geog ucl ac uk

    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).


    Notes adapted from prof p lewis plewis geog ucl ac uk

    Soil roughness effects

    • Rough roughness:

      • optical surface scattering

        • clods, rough ploughing

          • use Geometric Optics model (Cierniewski)

          • projections/shadowing from protrusions


    Notes adapted from prof p lewis plewis geog ucl ac uk

    Soil roughness effects

    • Rough roughness:

      • optical surface scattering

        • Note backscatter reflectance peak (‘hotspot’)

        • minimal shadowing

        • backscatter peak width increases with increasing roughness


    Notes adapted from prof p lewis plewis geog ucl ac uk

    Soil 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)


    Notes adapted from prof p lewis plewis geog ucl ac uk

    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