Abstract
Analytical canopy reflectance (CR) models have reached the level of adequacy that makes it possible to estimate vegetation parameters by the inversion of such models. The increasing efficiency of algorithms, and the increasing power of computers incite to develop procedures for the estimation of vegetation phytometrical parameters on large areas using satellite data and the inversion of theoretical CR models. A computer code has been developed for the inversion of the Markov chain canopy reflectance model by Kuusk (1995) on a NOAA AVHRR or Landsat Thematic Mapper (TM) image. The straightforward procedure applies a pixel-by-pixel inversion. Estimation of two CR model parameters LAI and leaf chlorophyll content on a 512x512 pixel image takes some hours on a Sun-4 workstation. Another, more efficient procedure uses the clusterisation of the satellite image using the GIS software package TopoL by Help Service Mapping ^TM (1995). The inversion procedure is to run for every cluster only once. Consequently, the amount of computations depends on the number of clusters and not on the image size. As a rule, 100-200 clusters give sufficient discrimination of canopies. A sample image of the test site near Tartu Observatory, Estonia reproduces well the LAI of field crops. The MCRM does not work on forests, therefore the LAI of forests is underestimated systematically. The procedure uses the spectral information of the six optical channels of TM unlike the NDVI which can use the reflectance values in two spectral channels only. The necessary atmospheric correction has been made with the atmosphere radiation transfer code 6S by Vermote et al. (1994). As the MCRM is a directional model, the off-nadir directional data of future satellite instruments (POLDER) can be used as well.
References
Kuusk, A. (1995) A Markov chain model of canopy reflectance. Agric. Forest Meteorol., 76: 221-236.
TopoL for Windows, version 3.0. Help Service Mapping s.r.o., Cz.R., 1995.
Vermote, E., Tanre, D., Deuze, J.L., Herman, M., and Morcrette, J.J. Second Simulation of the Satellite Signal in the Solar Spectrum (6S). User Guide Version 0, 1994.