This study introduces an analytical approach to derive a relationship between values of Normalized Difference Vegetation Index (NDVI) obtained from datasets of two sensors with different band passes in the context of inter-sensor cross-calibration of such vegetation index (VI) products. Derivation of the relationship has been performed based on a concept of soil isolines. Starting from the soil isoline equations, an inter-sensor NDVI relationship was derived and represented by a system of equation with a single common parameter. In the derived form of NDVI relationship, all the coefficients were written by the soil reflectances (independent of canopy layer.) The functional form of the relationship becomes rational function of the first-order polynomials, when the soil isoline equations are approximated by the form of first-order polynomials. Those results indicate a functional form suitable to model a relationship of NDVI from two sensors
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