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The ASTER sensor, which has been aboard the TERRA satellite since late 1999, has 15 spectral bands that cover 14 different wavelength regions in the 0.52- to 11.65-μm range. Because there are only three primary colors that humans can directly observe simultaneously, displays of triplets of spectral parameters as red, green, and blue are commonly used to enhance the appearance of specific minerals of interest, where they are exposed on the ground. To aid this process, we have created 14 brightness codes (one for each different ASTER spectral band) and 46 spectral ratio codes (36 nonreciprocal spectral ratios of nine spectral bands between 0.52–2.43 μm and 10 nonreciprocal spectral ratios of five spectral bands between 8.125–11.65 μm), which divide the mineral library spectra into deciles. Each decile of each spectral band or ratio is labeled from 9 for the highest decile, down to 0 for the lowest decile. A triplet combination with codes of 9, 0, 0 can be displayed as red, blue, green (RGB), respectively, which makes the mineral of interest red in the resulting image, with few (usually well less than 10% of the minerals in the library) false positives. Examples of how spectral ratio codes may be applied are demonstrated using ASTER data of the west-central Powder River basin in Wyoming for the enhancement of hematite (in red beds), gypsum, quartz (in sandstone), and calcite (in limestone). Supervised classification derived from training sets identified on spectral ratio images selected on the basis of ratio codes of ASTER data produced a lithologic map of the study area in the west-central Powder River basin that had an accuracy of 63.3 percent, compared with field data. We conclude that the supervised classification provides a more accurate lithologic map than we could have produced by using a traditional geologic map from which to pick training sets.

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