Use of an isoline-based inversion technique to retrieve a leaf area index for inter-sensor calibration of spectral vegetation index

A simple deterministic technique of two-band spectral inversion is introduced to estimate a leaf area index (LAI) by minimizing the influences of canopy background brightness, which is known as a main source of uncertainties in spectral vegetation indices. The technique is based on vegetation isolines that characterize the behavior of canopy reflectance spectra under various background brightness conditions. Analytical expressions of vegetation isolines were used to determine the inversion formula and their coefficients by relating the LAI to the slope and offset of vegetation isolines. The main objective of this simple approach is to produce a quick estimation of biophysical parameter to translate vegetation index products among different sensors such as Terra-MODIS, NOAAA -AVHRR and GLI onboard the ADEOS-II satellite which is planned to be launched late in 2002. This paper explains an inversion technique based on the vegetation isoline equation and demonstrates its performance using the SAIL canopy model combined with the PROSPECT leaf model. The results show that the inversion technique accurately estimates LAI under the simulations regardless the soil brightness. Although the number of parameters used in the simulations was limited for the demonstration purpose, the results indicate the usefulness of this technique by combining the statistical approaches to constrain the possible combinations of vegetation parameters. The results also indicate the potential of this technique toward inter-sensor spectral calibration of standard vegetation indices by accounting the differences in band-pass filters.