The space-borne POLDER instrument has provided the first opportunity to sample the BRDF of every point on Earth for viewing angles up to 60°-70°, and for the full azimuth range, at a spatial resolution of about 6km (see examples for POLDER 1 and POLDER 2 below).
A database compiles the most significant BRDFs acquired by ADEOS-1/POLDER-1 during 8 months, from November, 1996 to June, 1997, on a maximum number of sites describing the natural variability of continental ecosystems, at several seasons whenever possible. The POLDER-1 bidirectional reflectances have been reprocessed with the advanced Level 2 algorithms developed for the processing line of ADEOS-2/POLDER-2, and including all atmospheric corrections. The BRDF database has been implemented on the basis of the 22 vegetation classes of the GLC2000 land cover map. The POLDER-1 BRDF database I2.00, a tool of visualization (Visu_BRDF), a User Manual can be downloaded on the POSTEL web site.
The same work has been done with the ADEOS-2/POLDER-2 measurements (from april to october 2003) and the corresponding BRDF data base I1.00 can also be downloaded on the POSTEL web site.
This set of two database is an exceptional collection of bi-directional reflectances measured from space, providing exclusive information about the anisotropy of the continental ecosystems. Then, it is an incomparable tool for many environmental studies.
The figures below present BRDFs of 5 various ecosystems acquired at 5 different periods: the bi-directional reflectances (%) at 670nm (at left) and 865nm (in the center) are represented in the angular space. The major features of the BRDF are a peak of reflectance when the sun and the viewing directions coincide (the hot spot effect) and a minimum of reflectance in the forward scattering direction. These patterns are clearly displayed on the data collected in the principal plane (at right). The width and magnitude of the hot spot phenomenon vary according the ecosystems. It can be very useful to retrieve structural parameters of the vegetation or to quantify the spatial distribution of the major elements of the landscape (Lacaze et al., 2002).