a height of $4$m. For each measured mixed pixel, the plot-specific pure endmember spectra and ground cover fraction distributions were determined. Specifically, to mitigate the impact of nonlinear mixing from endmember variability, plot-specific endmembers were acquired by measuring a number of pure spectra in each plot, as illustrated in Fig. \[fig:setup\]. One set of soil, weed and tree endmember spectra is depicted in Fig. \[fig:insitu\_endmembers\].

![Experimental set-up to determine plot-specific soil and tree endmember signatures for each plot (from [@Somers2009]). The areas T1, T2 and T3 (S1, S2, and S3, resp.) identify the sub-plots selected for the measurements of pure tree (soil, resp.) spectra. These measurements are averaged to provide the plot-specific tree (soil, resp.) endmember signature.[]{data-label="fig:setup"}](fig_setup_new_small.\figextension){width="0.75\columnwidth"}

![Two- and three-endmember in-situ measurements. Example of the measured endmember spectra: soil (black), weed (red) and tree (green).[]{data-label="fig:insitu_endmembers"}](endmembers_insitu_one_instance.pdf){width="0.75\columnwidth"}

Information on the ground cover composition of each of the measured mixed pixels was extracted from digital photographs (SONY DSC-P8/3.2 megapixel cyber shot camera, positioned in nadir). A more detailed description on the experimental setup, depicted in Fig. \[fig:setup\], can be found in [@Somers2009].

Experimental results
====================

The relevance of the mixing models under test, namely LMM, FM, NM, GBM and PPNM, and associated unmixing 