The mapping techniques described here were developed and tested in an IGS-funded study of the mineral resources in Putnam County, Indiana. Putnam County is located about midway between Indianapolis and Terre Haute, and lies on the northeast flank of the Illinois Basin. The computer mapping in Putnam County was based on a database developed from public domain geologic data stored in IGS files and at other public agencies. Data sources included the field notes and work maps of IGS geologists, published maps and reports, measured sections, descriptions of exposures in active and abandoned quarries, core descriptions, water well records, soils maps, petroleum exploration well records, and IGS seismic refraction records. Most of the outcrop and subsurface data sites include at least one direct observation of the spatial location of a stratigraphic boundary.
Some sites record only indirect evidence such as isolated outcrops, float, or slope breaks.
Indirect evidence was included in the database because it helps to establish the presence of a mappable unit at a spatial coordinate. The task of interpreting and computer-mapping sedimentary rock units with public domain data is complicated by variations in spatial and stratigraphic precision. Stratigraphic imprecision occurs where the description of a stratigraphic sequence contains inadequate subdivision and lithologic characterization. These two kinds of imprecision often occur independently. A driller's log, for example, may record a very precise spatial location for a vague and generalized description of a rock sequence.