Digital Mapping Techniques '99 -- Workshop Proceedings
U.S. Geological Survey Open-File Report 99-386

Digital Geologic Map of the Harrodsburg 30 x 60-Minute Quadrangle, Central Kentucky

By Warren H. Anderson and Thomas N. Sparks

Kentucky Geological Survey
228 Mining and Minerals Resources Building
University of Kentucky
Lexington, KY 40506-0107
Telephone: (606) 257-5500
Fax: (606) 257-1147

The digital geologic map of the Harrodsburg 30 x 60 minute quadrangle was compiled from 32 U.S. Geological Survey 7.5-minute geologic quadrangle maps (GQ's). The GQ's are products of a cooperative mapping project between the U.S. Geological Survey and the Kentucky Geological Survey from 1960 to 1978. This map is a compilation of existing maps, and no additional geologic field work took place. When there were problems in stratigraphic correlation between quadrangles, the best current data available were used to resolve these differences.

The geology of the mapped area consists of flat-lying sedimentary rocks of Silurian, Devonian, and Ordovician age. These rocks extend across the north-trending Cincinnati Arch, crossing Ohio through Kentucky to Tennessee. The Jessamine Dome is a regional domal structure along the Cincinnati Arch in central Kentucky, and is reflected in the outcrop pattern of the Upper Ordovician units in the center of the map. Three major fault systems are also present in the Harrodsburg 30 x 60-minute quadrangle: the Lexington, Kentucky River, and Irvine-Paint Creek Fault Systems.

The individual 7.5-minute quadrangle maps (scale 1:24,000) were vectorized and attributed in Arc/Info from raster images of the stable-base Mylar composites, using a semi-automated data-capture technique. Compiling 32 individual 7.5-minute maps into a 30 x 60-minute map (scale 1;100,000) required resolving significant problems, such as (1) correlating geologic formations across quadrangle boundaries, (2) displaying nonuniform structure-contour horizons, and (3) resolving discrepancies in Quaternary alluvium boundaries and inferred contacts. Resolving the differences between quadrangles was necessary for topological analysis in a geographic information system (GIS). In addition, lithologic members mapped on the individual 7.5-minute quadrangle maps that were deemed too small to be mapped at a scale of 1:100,000 were mapped and consolidated with their adjoining members at the formation level.

A scaled schematic digital cross section was created in Arc/Info, using ARCPLOT's SURFACEXSECTION command on the grid lattices created from the available digital elevation models (DEM's) and the structure horizon grid created from the compiled structure contours. After the resultant two line sections were created, the structure-contour horizon was intersected with the DEM surface horizon, and the subsurface horizons were copied and pasted the appropriate distance below the surface during the editing process.

The final product consists primarily of an ArcView layout of the compiled formation polygon coverage, compiled fault and structure-contour arc and annotation coverages, economic point and arc coverages (quarries and mineral veins), city and town annotations, and associated coverages of digital line graphs (DLG's) of hydrology, roads, railroads, powerlines, pipelines, and county boundaries. Included on the final layout are a stratigraphic column, base maps, text, and the digital cross section. The final layout was created in ArcView on a 36 x 65 inch page layout and plotted on a Hewlett Packard DesignJet 755CM printer.

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