Empirically testing the use of Directional Area Scattering Factor (DASF) to correct hyperspectral remote sensing data for canopy structural effects (original) (raw)
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Biochemical properties retrieved from remote sensing data are crucial sources of information for many applications. However, leaf and canopy scattering processes must be accounted for to reliably estimate information on canopy biochemistry, carbon-cycle processes and energy exchange. A coupled leaf-canopy model based on spectral invariants theory has been proposed, that uses the so-called Directional Area Scattering Factor (DASF) to correct hyperspectral remote sensing data for canopy structural effects. In this study, the reliability of DASF to decouple canopy structure and biochemistry was empirically tested using simulated reflectance spectra modelled using a Monte Carlo Ray Tracing (MCRT) radiative transfer model. This approach allows all canopy and radiative properties to be specified a priori. Simulations were performed under idealised conditions of directional-hemispherical reflectance, isotropic Lambertian leaf reflectance and transmittance and sufficiently dense (high LAI) ...
An analytical and computationally efficient reflectance model for leaf canopies
Agricultural and Forest Meteorology, 1993
An explicit analytical model for calculating vegetation canopy reflectance is developed in this paper, based on radiative transfer theory through separating the roles of incident direct and diffuse radiation, singly and multiply scattered radiation by foliage and soil. Using principles of scattering from a point source, the contributions of diffuse sky radiation to both first-order and multiple scattering reflectance are accurately specified. In addition, this analytical model incorporates the effects of nonrandom spatial dispersion of foliage, noncircular shape and nonhorizontal orientation of leaves on the canopy hotspot and then on reflectance distributions. Although the reflectance by multiple scattering is simply estimated with the two-stream approximation (Nilson, 1991), the angular distributions of canopy reflectance produced by the model agree well with those measured above soybean and wheat canopies. Some original results from model sensitivity analysis are sketched as follows: (1) leaf shape and orientation have a considerable influence on the canopy hotspot except for planophile canopies or in near nadir viewing directions. The contributions of soil reflectance and multiple scattering increase with the modal inclination angle of leaves; (2) the foliage spatial dispersion pattern changes the magnitude and angular distribution of the canopy reflectance strongly in the RED band; (3) when the ratio of diffuse sky radiation to the total incident radiation is less than 20%, its contribution to canopy reflectance is less than 6% and 8% on average for RED and NIR bands, respectively. This provides a means to approximately calculate the two components signifying the role of diffuse sky radiation and then to conditionally simplify this reflectance model, and therefore improve its practical applicability; (4) the sensitive regions of different structural parameters are usually different and generally change with waveband and leaf orientation. This implies that only by using the reflectance data in its sensitive region, can a specified parameter be accurately estimated by model inversion. Finally, further improvements needed for analytical approaches are briefly discussed.
Modeling the bidirectional reflectance distribution function of mixed finite plant canopies and soil
Journal of Geophysical Research, 1994
An analytical model of the bidirectional reflectance for optically semi-infinite plant canopies has been extended to describe the reflectance of finite depth canopies with contributions from the underlying soil. The model depends on 10 independent parameters describing vegetation and soil optical and structural properties. The model is inverted with a nonlinear minimization routine using directional reflectance data for lawn (leaf area index (LAI) is equal to 9.9), soybeans (LAI, 2.9) and simulated reflectance data (LAI, 1.0) from a numerical bidirectional reflectance distribution function (BRDF) model (Myneni et al., 1988). While the ten-parameter model results in relatively low rms differences for the BRDF, most of the retrieved parameters exhibit poor stability. The most stable parameter was the single-scattering albedo of the vegetation. Canopy albedo could be derived with an accuracy of less than 5% relative error in the visible and less than 1% in the near-infrared. Sensitivity tests were performed to determine which of the 10 parameters were most important and to assess the effects of Gaussian noise on the parameter retrievals. Out of the 10 parameters, three were identified which described most of the BRDF variability. At low LAI values the most influential parameters were the single-scattering albedos (both soil and vegetation) and LAI, while at higher LAI values (> 2.5) these shifted to the two scattering phase function parameters for vegetation and the single-scattering albedo of the vegetation. The three-parameter model, formed by fixing the seven least significant parameters, gave higher rms values but was less sensitive to noise in the BRDF than the full ten-parameter model. A full hemispherical reflectance data set for lawn was then interpolated to yield BRDF values corresponding to advanced very high resolution radiometer (AVHRR) scan geometries collected over a period of nine days. The resulting retrieved parameters and BRDFs are similar to those for the full sampling geometry, suggesting that the limited geometry of AVHRR measurements might be used to reliably retrieve BRDF and canopy albedo with this model.
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Studies that compare modelled reflectances with satellite-measured reflectances for different wavelengths and view angles are still rare. We compared model outputs from three different canopy reflectance models (SLC, FRT and INFORM) with satellite measured reflectances (Chris/PROBA). Comparison of the simulated directional reflectances reveals general agreement but also some differences among the models. In general, the radiative transfer models produce signatures comparable to measured ones for spruce and beech forest of different age and canopy structure.
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A two-layer canopy reflectance model
Journal of Quantitative Spectroscopy and Radiative Transfer, 2001
A computationally e cient canopy re ectance model is developed. A typical two-layer canopy of forest understory communities is addressed in the model where a geometrically thin layer of vegetation of di erent structure and=or optical properties is under the main layer of canopy. The model allows to calculate re ectance spectrum in every given direction for the spectral range 400-2500 nm. The model calculations show that the use of e ective canopy parameters in a homogeneous canopy re ectance model may cause signiÿcant biases in estimated canopy re ectance.
Journal of Geophysical Research, 1992
Leaves of the dominant grass species of the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) site reflect and transmit radiation in a similar manner to other healthy green leaves. Visible reflectance factors (RFs) and transmittance factors (TFs) were lower for older leaves than younger leaves except during senescence, when RF and TF values were higher. Near-infrared (NIR) RF values increased and TF values decreased with leaf age, with the reverse occurring as the leaf underwent senescence. Leaf optical properties were not found to be dependent on leaf water potential in the range from-0.5 to-3.0 MPa. Canopy bidirectional reflectance factor (BRF) values generally increased with increasing view zenith angle (Ov). Maximum values were in the backscatter direction, whereas BRF values in the visible region were lowest at oblique off-nadir Ov in the forward scatter direction and at or near nadir in the NIR region. Solar principal plane BRF values varied most at large solar zenith angles (Os). Visible and mid-infrared canopy BRF values decreased and NIR BRF values increased with leaf area index (LAI). Soil BRF distributions in the solar principal plane varied slightly with Os and Ov and varied considerably for wet and dry surfaces. Spectral vegetation indices (SVIs) varied with Os and Ov; values were lowest in the backscatter direction and highest in the forward scatter direction. The fraction of absorbed photosynthetically active radiation (APAR) increased with increasing Os. APAR had a strong linear relationship to nadir-derived SVI values but not to oblique off-nadir-derived SVI values. The relatively small dependence of off-nadir SVI values on Os should allow daily APAR values to be estimated from measurements made at any time of the day. Factors which affect reflectance from vegetated surfaces and contribute to the non-Lambertian nature of these surfaces include (1) spectral properties of canopy elements and substrate; (2) the canopy architecture (that is, leaf area index (3) illumination and viewing directions [Norman et al., 1985]. Canopy architecture plays a vital role in determining the BRFs from a vegetative canopy. Leaves are oriented at a variety of inclination angles, thereby varying effective illumination and viewing angles. The result is a complex pattern of reflected and transmitted radiation. Canopy architecture can change due to wind, heliotropism, and water stress.