Soil isoline equation in red-NIR reflectance space for cross calibration of NDVI between sensors

An underlying assumption in retrievals of biophysical and geological parameters from remotely sensed satellite data is an existence of inter-band relationship. Such relationships are often visualized as isolines in cross plots of reflectance spectra. One type of isolines observed under constant soil properties is the soil isoline, which would provide useful in- formation in retrievals of soil parameters. The purpose of this study is to derive such a relationship observed in a red-NIR reflectance subspace under a fixed condition of soil surface (constant reflectance spectrum of the soil surface). The de- rived expression is then used to relate two NDVI values observed by different pairs of wavelengths in the context of inter-sensor cross calibrations of NDVI. The derived expressions are demonstrated numerically with radiative transfer models of leaf and canopy. The results indicate validity of the derived expression and their potential to be used for determining an appropriate functional form in modeling a relationship between two NDVIs from different sensors.