Transcript Physical principles

Physical principles
Exo-atmospheric
radiance
Atmospheric
scattering and
absorption
Adjacency
effects
surface
reflectance
substrate
reflectance
% ground
cover
Canopy
structure
Leaf Area Index
LAI
Photosynthetic
activity
Biomass/Primary
Production
Leaves
Factors affecting reflectance/transmittance of
plant elements
 pigment type/concentration
 e.g. Chlorophyll a,b - absorb visible radiation
 surface features:
 hairs, spines, veins, cuticular wax - scattering
(e.g. specular)
 cell morphology and content
 NIR - high refl. due to scattering at
refractive index discontinuities (nair = 0; ncell=1.47)
 MIR - water absorption features
1.45 m  1.95 m  2.5 m
Canopy
 reflectance varies with viewing and illumin.
angles (and wavelength)
 refl. in NIR high, lower in visible
Canopy
 features of canopy directional reflectance:
 backscattering peak (hotspot) - mostly shadow
hiding
 general upward bowl shape - longer path length
at higher angles in volume-scattering media
 general downward bowl shape for opaque
solids (some soils, forest)
Canopy
 features of canopy directional reflectance:
 symmetry about principal plane (for
homogeneous canopies)
 shape in nadir region influence by (% cover)
and soil reflectance
 forward scattering peak if specular refl.
factors affecting canopy
reflectance
 reflectance / transmittance of soil/vegetation
elements
 spatial distribution of plants
 regular, clumped, row, plant density
factors affecting canopy
reflectance
 %cover of canopy, LAI
 increase in LAI
 decrease in visible refl.
 increase in NIR refl.
 threshold
 LAI 2 (vis)
 LAI 6-8 (NIR)
factors affecting canopy
reflectance
 individual plant structure
 arrangement and shape of leaves on plant
 leaf angle distribution
 canopy multiculture/layers
– heterogeneity of canopy within pixel
modelling approaches
 statistical / empirical
 statistical correlations between canopy
variables & refl. ‘signature’ or transformations
(Vis)
 strengths:
 simple, fast
 not reliant on understanding
 weaknesses:
 effects may be coupled
 correlation may be site/veg. specific
modelling approaches
 physical approach
(Goel, 1988; Strahler, 1991)
 model physics of interaction of radiation with
vegetation
 defined as fn of vegetation parameters
 forward modelling - know parameters, simulate
refl.
 inverse modelling - sample reflectance (RS),
derive estimates of canopy variables
modelling approaches
 physical approach
 strengths:
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relate directly to biophysical parameters
very flexible
understand mechanisms
range of models appropriate
 ‘simple’ analytical RT
 GO shadowing
 hybrid approaches
 complex 3D numerical models
modelling approaches
 physical approach
 weaknesses:
 need to understand scattering mechanisms
 different for different structures e.g volume
scattering/shadowing (RT equation/GO theory)
 inversion of non-linear models can be complex,
unstable, costly
 neural nets?/LUTs?