Investigation of the variability of the directional reflectance of Australian land cover types∗

First results from a study of the spectral directional reflectance across the Australian continent are presented. The Roujean model parameters from the POLDER/ADEOS Level 3 Surface Directional product are compared with a map of Australian vegetation structural types. The model parameters show variations between vegetation classes that follow the geographic distribution of the classes. While for any one parameter the within‐class differences are at least as large as the between‐class differences, a statistical analysis shows that when all three model parameters for all four bands (twelve variables) are considered then nearly all pairs of classes are highly separable. In summer, the model parameters show spatial fluctuations that are absent from maps of reflectance at a fixed view‐illumination geometry that were derived from the POLDER product by forward evaluation of the Roujean model. It is suggested that the spurious parameter fluctuations might arise from a similarity of shape of the two kernels of the model that is accentuated at small solar zenith angles.