Directional Reflectance Distribution Research Articles

New approaches in multi-angular proximal sensing of vegetation: Accounting for spatial heterogeneity and diffuse radiation in directional reflectance distribution models

The development of tower-mounted automated multi-angular hyperspectral systems has brought new opportunities and challenges for the characterization of the Bidirectional Reflectance Distribution Function (BRDF) on a continuous basis. This study describes the deployment of one of these systems in a Mediterranean savanna ecosystem (AMSPEC-MED), and proposes new approaches for modeling of directional effects. In this study, a Hemispherical-Directional Reflectance Distribution Function (HDRDF) was introduced in order to quantify the effect of diffuse radiation on the estimation of BRDF. The HDRDFs of the two covers of the ecosystem - trees and grasses - were un-mixed using a 3-Dimensional (3-D) model of the observed scene. Up-scaling HDRDF estimates to MODIS BRDF product (r2∈[0.74, 0.86]) and down-scaling to hand held spectral measurements (r2=0.88) achieved a reasonable accuracy (RMSE∈[1.81, 3.14]). Despite the uncertainties in the estimation of diffuse irradiance and the 3-D representation of the scene, HDRDF un-mixing demonstrates the potential of automated multi-angular proximal sensing to study vegetation properties in heterogeneous ecosystems and the correction of directional effects of different sources.

A hemispherical–directional reflectance model as a tool for understanding image distinctions between cultivated and uncultivated bare surfaces

This paper discusses a model to predict the normalized hemispherical–directional reflectance function for soil or rocky surfaces of a given roughness under conditions of outdoor illumination. These surfaces are simulated by geometrical shapes similar to beads merging into each other, characterized by three parameters. In addition, the shape of the surface is characterized by the directivity factor D R, expressing the differences between the maximum and the minimum deviations of its height, calculated along all possible directions. The surface is illuminated by a hemispherical light source created by a number of point sources of given light intensities. The light energy is scattered from the surface, in accordance the quasi-Lambertian function. The distribution of the surface reflectance, as viewed from all the possible directions, can be described for all the possible illumination conditions expressed by the solar zenith and the horizontal angles for a given hemisphere light distribution of a definite optical thickness. This represents the hemispherical–directional reflectance distribution function, HDRDF, of the surface. The HDRDF function is normalized to the nadir viewpoint and visualized for a given illumination condition. The model assumes that the HDRDF of a surface contains information about the directivity of the surface shape, as described by the directivity factor of the surface hemispherical–directional reflectance function D HDRDF. This factor, expressing the asymmetry of the HDRDF with respect to the solar principal plane (SPP), is strongly correlated with the D R. The use of both factors, the D R and D HDRDF, enables us to understand the distinctions between soil surface images with height irregularities of directional character that create a furrow microrelief, and irregularities spread non-directly, randomly, depending on whether the soil has been cultivated or not. The model was tested on directional reflectance data measured in the visible, the near and the middle infrared spectra for cultivated surface with furrows, as well as for three uncultivated desert loess and rocky surfaces situated in Israel.

Biophysical controls on the directional spectral reflectance properties of bracken (Pteridium aquilinum) canopies: Results of a field experiment

Bracken presents a serious environmental problem in many regions of the world and remote sensing offers the potential of monitoring the invasion and management of this plant. This study evaluates this potential by reporting the results of an investigation of the relationships between the biophysical properties of a bracken canopy and its spectral reflectance and how such relationships are effected by view angle. An experiment was carried out through the bracken growing season at sites in West Sussex, UK. In the principal plane of the Sun, red reflectance becomes lower and relatively less variable with view angle as the canopy develops, while near-infrared reflectance increases and becomes relatively more variable with view angle. As Leaf Area Index (LAI) increases, the variability of the Normalized Difference Vegetation Index (NDVI) with view angle decreases, while the simple ratio vegetation index (SR) behaves in almost the opposite manner. Polar plots, illustrating full directional reflectance distributions, reveal that as the canopy develops the hot spot in red reflectance at the retro-solar angle diminishes, to be replaced by enhanced red reflectance at high zenith angles in the forward-scatter direction. Near-infrared reflectance behaves very differently, as a hot spot at the retro-solar angle becomes more pronounced as the canopy grows. The angular distribution of NDVI and SR varies markedly over the growing season, however both indices are well correlated with canopy biophysical properties for most view angles sampled, apart from the extreme off-nadir. Red edge position was largely insensitive to view angle, but had a good correlation with LAI, as did percenage reflectance at the red edge position.

Directional reflectance of vegetation measured by a calibrated digital camera

Obtaining directional reflectance information for vegetation canopies is often an expensive and time-consuming process. We present here a simple approach based on the use of an inexpensive digital camera equipped with a wide-angle lens. By the imaging of a large homogeneous area, a single image captures multiple views of a vegetation canopy. This gives a directional reflectance distribution fully sampled for view direction and free of variations in Sun elevation and azimuth. We determined the radiometric response of the camera sensor CCD's at the focal point and then extended this calibration to the full CCD array by using averaged images of clear blue sky. We evaluated the utility of the system by obtaining directional reflectance distributions of two vegetation targets, grass (Lolium spp) and pine forest (Pinus radiata), for red visible light. The precision of the derived biangular pattern of reflectance was +/-7%.

A Markov chain model of canopy reflectance

Markov properties of stand geometry are incorporated into an analytical multispectral canopy reflectance model. The correlation of leaf positions in adjacent layers significantly influences the gap probability in a stand and, consequently, the canopy reflectance (CR) and its angular distribution. The sensitivity analysis demonstrated that the effect is greatest on the angular distribution of multiply scattered radiation. Validation of the new CR model that considers the Markov stand geometry demonstrated an improved agreement between model calculations and measured directional reflectance distribution of near infrared (NIR) reflectance for barley and soybean stands. As a consequence, inversion of the new model in the NIR spectral region and in two spectral bands simultaneously allowed significantly better estimation of the leaf area index of a stand than the Nilson-Kuusk model which assumes a Poisson stand geometry.

Extension of off-nadir view angles for directional sensor systems

A knowledge-based system called VEG was expanded to infer nadir or any off-nadir reflectance(s) of a vegetation target given any combination of other directional reflectance(s) of the target. VEG determines the best techniques to use in an array of techniques, applies the techniques to the target data, and provides a rigorous estimate of the accuracy of the inference(s). The knowledge-based system, VEG, facilitates the use of diverse knowledge bases to be incorporated into the inference techniques. In this study, VEG used additional information to make more accurate view-angle extension techniques than the traditional techniques that only use spectral data from the unknown target. VEG used spectral data and a normalized difference technique to infer the percentage of ground cover of the unknown target. This estimate of percentage of ground cover of the unknown target along with information on the sun angle were then used to search a historical data base for targets that match the unknown target in these characteristics. This data captured the general shape of the reflectance distribution of the unknown target. This historical information was used to estimate the coefficients of the techniques for the conditions at hand and to test the accuracy of the techniques. The tests used in this study were difficult ones. For example, techniques were tested that make long angular extensions using one, two, or four input view angles to predict an unknown nadir value. Furthermore, a wide variety of unknown targets were tested. The errors (±proportional rms) obtained were on the order of 0.15. In addition techniques were tested that use seven or nine multiple view angles to predict the entire hemispherical reflectance distribution of an unknown target. The accuracy of these tests was relatively good considering the relatively dynamic and noisy nature of directional reflectance distributions. The accuracy of the techniques in this study depends on the smoothness of the historical reflectance distributions and the amount of historical data available that closely matches the unknown target.

Directional Reflectance Distributions of a Hardwood and Pine Forest Canopy

The directional reflectance distributions for both a hardwood and pine forest canopy at Beltsville, Maryland, were measured in June as a function of sun angle from a helicopter platform using a hand-held radiometer with AVHRR band 1 (0.58-0.68 μm) and band 2 (0.73-1.1 μm). Canopy characteristics were measured on the ground. The reflectance distributions are reported and compared to the scattering behavior of agricultural and natural grassland canopies. In addition, the three-dimensional radiative transfer model of Kimes was used to document the unique radiant transfers that take place in forest canopies due to their special geometric structure. Measurements and model simulations showed that the scattering behavior of relatively dense forest canopies is similar to the scattering behavior of agricultural crops and natural grasslands. Only in more sparse forest canopies with significant spacing between the tree crowns (or clumps of tree crowns) does the scattering behavior deviate from homogeneous agricultural and natural grassland canopies. This clumping of vegetation material has two effects on the radiant transfers within the canopy: A) it increases the probability of gap to the understory and/or soil layers that increases the influence of the scattering properties of these lower layers; and B) it increases the number of low transmitting clumps of vegetation within the scene causing increased backscatter and decreased forward scatter to occur relative to the homogeneous case. Both effects, referred to as phenomenon A and B, respectively, tend to increase backscatter relative to forward scatter.

Modeling the Radiant Transfers of Sparse Vegetation Canopies

The three-dimensional radiative transfer model of Kimes [2] was used to extend our understanding of the physical principles causing the scattering dynamics in sparse vegetation canopies (? 50-percent ground cover). The model was upgraded by including an aniotropic scattering algorithm for soil developed by Walthall et al. [7]. The model was validated using measured directional reflectance data that covered the entire exitance hemisphere. Two canopies were chosen to present in this study-an orchard grass canopy (50-percent ground cover) and a hard wheat canopy (11-percent ground cover). These canopies showed the typical scattering behavior of canopies with low and intermediate vegetation density. A red wavelength (0.58-0.68 pm) band was used throughout the study. A number of phenomena contributed to the directional reflectance distributions observed in the field. These include: 1) the strong anisotropic scattering properties of the soil, 2) the geometric effect of the vegetation probability of gap function on the soil anisotropy and solar irradiance, and 3) the anisotropic scattering of vegetation which is controlled by the phase function (for an infinitely small volume of representative leaves) and geometric Effect 1 (cause by layering of leaves). These phenomena as identified in this paper account for the major scattering behavior of observed data sets of directional reflectance distributions. Such knowledge provides an intelligent basis for defining specifications of earth-observing sensor systems and for inferring important aspects of physical and biological processes of the plant system.