Pattersons curse (PC, Echium plantagineum L.) is a major invasive weed in Australia. In this work, spectral characteristics of PC plants were used to quantify PC abundance in paddocks. Reflectance spectra between 350 and 2500 nm of three paddocks were measured and PC abundance was estimated. In one of the three paddocks, reflectance was also measured from two opposite viewing directions under an angle of 45.^o. Based on the spectral characteristics of PC flowers, a normalised difference blue index (NDBI) is proposed to quantify PC abundance. Effects of dead material on abundance estimates were simulated using spectral mixtures created with flower, green and dead material spectra. The potential of PC detection with air- or space-borne systems was evaluated by convolving spectra to spectral responses of MODIS, HyMap and CHRIS.
The abundance of PC flowers when viewed from nadir was low, even in heavily infested fields. It was found that spectral unmixing and the NDBI index resulted in similar fractions of explained variation (R² of 0.65 and 0.66) in PC flower abundance. The abundance estimates from the opposite viewing directions differed strongly. Also, the fraction of dead material in the paddock strongly affected the NDBI index. Spectra based on CHRIS and HyMap specifications resulted in similar relationships between NDBI and PC flower abundance. It is concluded that detection of PC flowers with hyperspectral air- and spaceborne remote sensing systems are feasible, but effects of dead material and viewing angle should be considered carefully.