Assessment of Multiple Forward Mode Inversion Solutions from a Forest Canopy Reflectance Model

Abstract

Multiple Forward Mode (MFM) is a technique for deriving inversion results for canopy reflectance models that are not invertible directly. MFM consists of a set of well defined methods for indirect inversion using look-up tables (LUT) produced from a series of controlled forward-mode canopy reflectance model runs. Biophysical-structural inversion results are determined by choosing inversion solutions from the LUT where satellite image data and modeled reflectance values coincide. This work refines the MFM technique with regard to (i) model parameterization for the initial forward mode geometric-optical canopy reflectance model executions and (ii) methods for limiting or describing multiple solutions. The MFM procedure was tested in a sub-alpine montane forest in the front range of the Canadian Rocky Mountains. Inversion solutions for pine canopy structure (horizontal and vertical crown radius) were obtained using the MFM procedure with SPOT 5 image input. Absolute average structural error (RMSE) was within 0.4m for horizontal crown radius and 0.8m for vertical crown radius for inversion methods based on a spectral distance function with 80% and 60% of the estimates falling within a 0.5m difference of measured values for horizontal and vertical dimensions respectively.