Geoscience Reference
In-Depth Information
The results of the accomplished classification are discussed in detail in the
articles by Vasilyev A et al. (1997a, 1997b, 1997c). Figure 3.17 illustrates the
typical spectra of the SBC for the natural surfaces. The SBC of three surface
types (snow, sand, and pure lake water) are presented in Fig. 3.17. In addition,
we should mention that the albedo coincides with the SBC for the orthotropic
surface. However, we will analyze to what extent this approximation is correct
for the considered surfaces. The analysis of the accomplished classification has
shown that there is no dependence upon the viewing direction for the snow
surface, i. e. the snow surface is close to the orthotropic. The uncertainty of the
approximation is determined by the standard deviation for the class “snow”
and is equal to 8% in average over the spectrum (Vasilyev A et al. 1997a, 1997b,
1997c). The description of the main classes of the SBC and their spectral values
are presented in Tables A.4-A.7 of Appendix A.
The water surface is the most anisotropic among all natural surfaces as
Fig. 3.18 demonstrates. The sharp maximum is a solar glare in the viewing
direction to the Sun (
0 ) formed by the solar beams mirror-reflected
from the waving water surface (curves 2 and 3 in Fig. 3.18). There is a weak
minimum in the opposite direction (curve 4) that has been explained in the
topic by Mulamaa (1964). Regretfully there was no possibility to measure
SBCdependenceoftheviewingangleandazimuthforthesandsurface.The
ϕ =
Fig. 3.18. Dependence between the spectral brightness coefficients of the water surface and
the viewing direction. Average values of the SBC of the relevant classes and one standard
deviation interval are shown: 1 - observation to nadir; 2 - the mirror reflection direction;
3 - the viewing angle corresponded to the mirror reflection at azimuth 45 ; 4 - the same as
3 for azimuth 180
 
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