An intensively used agricultural test site in Switzerland is covered by the DAIS 7915 imaging spectrometer in summer 1997. Three different methods of collecting endmembers for spectral unmixing are selected and compared against each other. The methods include a soil-vegetation-atmosphere-transfer approach (SVAT) based on a leaf optical properties model (PROSPECT) and a canopy model (SAIL), image based endmember selection and in-situ reflectance measurements using a ground spectroradiometer. The presented methods are discussed and verified with an extensive ground truth. A rejection procedure for classification of unmixing results is proposed on the acceptance of constraint spectral unmixing results using the uncertainty, expressed by the RMS, of the endmember selection.