Endmember variability in Spectral Mixture Analysis: A review
The composite nature of remotely sensed spectral information often masks diagnostic spectral features and hampers the detailed identiﬁcation and mapping of targeted constituents of the earth's surface. Spectral Mixture Analysis (SMA) is a well established and eﬀective technique to address this mixture problem. SMA models a mixed spectrum as a linear or nonlinear combination of its constituent spectral components or spectral endmembers weighted by their subpixel fractional cover. By model inversion SMA provides subpixel endmember fractions. The lack of ability to account for temporal and spatial variability between and among endmembers has been acknowledged as a major shortcoming of conventional SMA approaches using a linear mixture model with ﬁxed endmembers. Over the past decades numerous eﬀorts have been made to circumvent this issue. This review paper summarizes the available methods and results of endmember variability reduction in SMA. Five basic principles to mitigate endmember variability are identiﬁed: (i) the use of multiple endmembers for each component in an iterative mixture analysis cycle, (ii) the selection of a subset of stable spectral features, (iii) the spectral weighting of bands, (iv) spectral signal transformations and (v) the use of radiative transfer models in a mixture analysis. We draw attention to the high complementarities between the diﬀerent techniques and suggest that an integrated approach is necessary to eﬀectively address endmember variability issues in SMA.