Multi-angular Remote Sensing

Application 1:

Global Mapping of Foliage Clumping Index Using Multi-Angular Satellite Data

Global mapping of the vegetation clumping index is attempted for the first time using multi-angular POLDER 1 data based on a methodology that has been demonstrated to be applicable to Canada's landmass. The clumping index quantified the level of foliage grouping within distinct canopy structures, such tree crowns, shrubs, and row crops, relative to a random distribution. Vegetation foliage clumping significantly alters its radiation environment and therefore affects vegetation growth as well as water and carbon cycles. The clumping index is useful in ecological and meteorological models because it provides new structural information in addition to the effective LAI retrieved from mono-angle remote sensing and allows accurate separation of sunlit and shaded leaves in the canopy. The relationship between an angular index (normalized difference between hotspot and darkspot) and the clumping index is explored using a geometrical optical model named "4-Scale". A simplified version of the mechanistic hotspot model used in 4-Scale is developed to derive the hotspot reflectance from multi-angle measurements for mapping purposes. An accurate clumping map for areas with significant tree (shrub) covers has been achieved, although further research is required to reduce topographic effects.

Figure 1. Modeled NDHD vs. clumping index (Omega) for a wide range of parameters highlighting the distributions of individual parameters within the cluster. The SZA was chosen as 35 for all simulations

Figure 2. Modeled NDHD vs. clumping index (Omega) for a range of parameters covering the full range for both variables to highlight the effect of crown shape and solar illumination angle as well as the results for Grassland.

Figure 3. POLDER (NIR and Red) directional reflectance from June 1997 along the principal plane for a single resolution element in A) Broadleaved, evergreen; B) Needle-leaved, evergreen; C) Herbaceous cover, open; and D) Shrub Cover, deciduous; land cover types (GLC2000). A negative scattering angle was assigned to indicate the backscattering direction

Figure 4. Global vegetation clumping index map derived from POLDER 1 data using the normalized difference between interpolated hotspot and darkspot NIR reflectance and applied to vegetated land cover. Vegetation clumping increases with decreasing values of the index

Application 2:

Retrieving Forest Background Reflectance in a Boreal Region from Multi-angle Imaging SpectroRadiometer (MISR) Data

Studies of the bidirectional behavior of forest canopy have shown that the total reflectance of a forest canopy is the combination of illuminated and shaded components of the tree crown as well as the background. In this study, we estimate the background potion from the bidirectional reflection observed by Multi-angle Imaging SpectroRadiometer (MISR) instrument which scans the earth in nine different view angles in an oblique plane relative to the sun. The nadir and 60o forward directions of the MISR images were used to derive the reflectivity of the forest background based on the probabilities of viewing the illuminated tree crown and background on those view angles. The probabilities were estimated using the Four-Scale model. In the study, background reflectivity mosaic images in red and NIR wavelengths covering the BOREAS region during winter and spring seasons were obtained. The mosaic images of winter show high background reflectivity in both wavelengths, and in most of the areas the reflectivity was more than 0.3. In mosaic images of spring the spatial variations in the background reflectivity were considerable. The seasonal changes in the background reflectivity were also studied with multi temporal MISR data, and a similarity in the temporal pattern was found between the retrieved forest background reflectivity and grass land reflectance. These spatial and temporal patterns of the background component retrieved from MISR would be critically important in retrieving the biophysical parameters of vegetation and in ecosystem modeling.

Figure 1: The variation of the total reflectance of a forest canopy in a red band with view zenith angle on the perpendicular plane for two contrasting background types

Figure 2: Background reflectivity of forest area (a) RED band in winter (b) RED band in spring (c) NIR band in winter (b) NIR band in spring

Figure 3: Background reflectivity and its temporal changes of (a) coniferous forest in the red band, (b) coniferous forest in the NIR band, (c) deciduous forest in the red band; (d) deciduous forest in the NIR band

Figure 4: Comparison of retrieved background reflectivity in coniferous and deciduous forests with open grassland reflectance in (a) red and (b) NIR wavelengths

Figure 5: RGB color composites of forest areas in spring. (a) NIR, Red and Green bands of MISR nadir reflectance data, and (b) NIR, Red and Green background reflectivities retrieved using MISR data.



Chen, J. M., C. H. Menges, and S. G. Leblanc, 2005. Global derivation of the vegetation clumping index from multi-angular satellite data. Remote Sensing of Environment, 97: 447-457

© Revised: July, 2006