row azimuth of $7.3^{\circ}$ and an average tree height of $3$m. This composition is consistent with the reference orchard, located in Wellington, South Africa ($33.58^{\circ}$S, $18.93^{\circ}$E), used to calibrate the virtual orchard [@Stuckens2009]. Spectral input data for citrus leaves and stems, soils and weeds have been measured using a full-range ($350$-$2500$nm) analytic spectral devices (ASD) Fieldspec JR spectroradiometer with a $25^{\circ}$ foreoptic. The weed spectrum has been chosen as of the Lolium sp. species. A Haplic Arenosol [@FAO1998] typical for commercial citrus orchards in the Western Cape Province in South Africa has been used in the simulations [@Somers2010a]. An example of a high resolution image of $20\textrm{m} \times20\textrm{m}$ of the two-endmember orchard is depicted in Fig \[fig:orchardExample\] (a), while the three-endmember orchard is shown in Fig \[fig:orchardExample\] (b). For a detailed description of the design, modalities and application of the virtual orchard, the reader is invited to consult [@Stuckens2009].

[0.49]{} ![High resolution images of the two orchards with (a) two endmembers, i.e., tree and soil, and (b) three endmembers, i.e., tree, soil and weeds. []{data-label="fig:orchardExample"}](Orchard_noweeds "fig:"){width="\columnwidth"}

[0.49]{} ![High resolution images of the two orchards with (a) two endmembers, i.e., tree and soil, and (b) three endmembers, i.e., tree, soil and 