The preprocessor (PREPRO) computer program offers the exploration geologist a variety of options for encoding regional exploration data into ternary form for use in characteristic analysis. PREPRO's options include variable-input formats, cursor input, ordering among a set of input variables, and selectable ternary transformations. Moreover, the program provides for the display of results which, in turn, makes possible review, reselection, and retransformation of variables. Most important, the performance of the listed steps in an interactive computing environment results in rapid and efficient preprocessing of the data.