A method is presented to map spectral reflectance data of altered or unaltered rocks into color space to aid interpretation of color ratio composite (CRC) images for engineering geologic purposes. The method uses in situ spectral reflectance data or multispectral images of known lithologies, normalized ratio values, ternary diagrams, chromaticity diagrams, and color science theory to enable the engineering geologist to display spectral separability of lithologic units and improve the interpretation of CRC images. The normalized ratio values, derived from spectral reflectance data, can be plotted in a ternary diagram. Transforming normalized ratio values into equivalent chromaticity coordinates, the ternary diagram becomes a chromaticity diagram.

From such a presentation, the engineering geologist can determine a priori the approximate color that each lithologic unit will appear in a CRC image. In this study, it was determined that the best CRC images correspond to the ternary/chromaticity diagrams that show the most separation of spectral reflectance data.

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