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T he use of microwave radiometry for the retrieval of land use distribution (subpixel proportions) and forest stem volume for 25 km and 50 km pixels is investigated, with new algorithms introduced for the spaceborne multichannel SSM/I instrument. The test sites are located in the boreal forest zone in Finland. SSM/I data from snow-free periods (from May to September) are used in the study, together with a digital high-resolution land use map as reference data. A stepwise algorithm is developed, in which the surface emissivity is estimated for each of the pixels in the first stage of the algorithm. By using a training data set (50% of all data), empirical coefficients for the subpixel land use and forest stem volume are determined. The results for subpixel land use classification (retrieval of fractions of individual land use categories contained within study pixels) are promising. When there are two or three categories to retrieve, the rms errors range from 3% to 9%, and the respective relative errors from 3% to 32%. When there are six categories to retrieve, the performance is less satisfactory, with relative errors exceeding 100% for some categories. For the retrieval of forest stem volume, a similar approach is used. The validation data set shows rms errors (standard errors) of approximately 20 m 3 /ha when the mean estimated forest stem volume is around 70 m 3 /ha. When compared to reported Landsat TM-based biomass estimates for a study area in Finland, the r 2 (coefficient of determination) values are comparable. The methods are also validated for a wider use by limiting the training data set to either northern or southern Finland, since the surface and forest types in these areas are systematically different (subgroups of boreal forests). The results show that the methods are applicable to remote areas with less training data. This is especially true for the stem volume estimation and limited (two or three classes) subpixel land use classification. These can be considered as promising results for a coarse resolution, high-frequency microwave instrument such as the SSM/I. |
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