Vegetation impedes the geological analysis of band ratio images, because it is both widely distributed in the surficial environment and can be spectrally similar to ferric oxides and clays when sampled by broad-band imaging devices. We address this problem by a technique we call #x2018;directed principal component analysis#x2019; (DPCA) that involves calculating principal components on two input band ratio images. One ratio is a geological discriminant, conTused by the presence of vegetation; the second ratio is chosen for its suitability as a vegetation index. Once computed, the second DPC has the properties of a geological discriminant, but is less influenced by vegetation. The effects of vegetation, which are strongly correlated between the two input ratios, contribute chiefly to DPC#1. This simple method, applied selectively to airborne thematic mapper data, substantially reduces the effects of vegetation.
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