A Landsat spectral classification map of the Chagai Hills, Baluchistan, Pakistan, has been produced by computer processing of Landsat-1 digital data. A supervised classification algorithm was used which employed the Mahalanobis distance as a maximum-likelihood discriminant. A Landsat geologic map has been interpreted from the spectral classification by use of a set of interpretation rules specifically formulated for the Chagai Hills. The Landsat geologic map shows an overall close agreement with a photogeologic map produced by the Hunting Survey Corp. Areas of disagreement have been studied and evaluated as to the reasons for and validity of the discrepancies.

Each pixel of the Landsat data was classified as one of the preselected pixel populations which represent chosen surface types within the Chagai Hills. Pixel populations generated from the control areas of each of the mapped surface types were represented as "characteristic lines" in the Landsat four-dimensional color space. A characteristic line is defined as the line which parallels the pixel population's principal component and passes through the population's center of gravity. The characteristic lines were used to thin out surface types to which the pixel being tested could not be reasonably classified. The Mahalanobis distance classification discriminant was based on the position of a test pixel within the density distributions of the populations. The individual pixel classificat ons were smoothed by reclassifying the central pixel of a 3 × 3 array on the basis of the classification information contained in all nine pixels.

A discriminant similar to one of a tightly constrained parallelepiped was used to classify hydrothermal alteration. The alteration classifications were smoothed by considering only those areas where two or more pixels of a 3 × 3 array were classified as altered. Approximately 100 areas classified as hydrothermally altered were located on the Landsat geologic map. Four of these are known to be altered rock. Two of the known areas of alteration were used as control areas; the correct classification of the other two increases the confidence in the alteration classifications across the Chagai Hills.