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

The Making of the 1:250,000-Scale Digital Geologic Map of Pennsylvania

By Christine E. Miles and Thomas G. Whitfield

Pennsylvania Geological Survey
P. O. Box 8453
Harrisburg, PA 17105-8453
Telephone: (717) 783-7269
Fax: (717) 783-7267

In 1980, the Pennsylvania Geological Survey (PaGS) published a new edition of Map 1, Geologic Map of Pennsylvania. This full-color map, which was compiled and printed at 1:250,000 scale, shows the areal distribution and extent of the bedrock geologic units in Pennsylvania. Since its publication, Map 1 has remained the most current, available source of bedrock geology for the entire state and has had widespread use for regional studies. The increasing use of GIS technology in the past five years has led to numerous requests from federal and state government agencies and the private sector for a digital version of the map. In response to these requests, the PaGS has converted the geologic features depicted on Map 1 to a digital format. Two data sets were prepared using ARC/INFO software, one for geologic units and faults, and the other for dikes. The data sets contain 194 geologic units, more than 12,000 polygons, and more than 30,000 arcs.

Production of the data sets involved preparation of scanned images of the 1:250,000-scale geologic units and faults from the 1980 map. Because of inherent problems with the 1:250,000-scale base map, scans were also made of the source materials that were used to compile the state geologic map in order to more accurately locate the geologic contacts in relation to streams, roads, and other base features. The images were georeferenced and projected to transverse mercator, the projection of the 1980 map. Vector files were edited on screen on a computer workstation with the images in the background for reference.

In compiling the data set for the geologic units, the PaGS extensively modified an earlier data set of geologic formational contacts prepared by the U.S. Geological Survey, Water Resources Division. The PaGS edited or redigitized all arcs, relocated and corrected polygon label points, digitized all fault lines, and added several items to the polygon and arc attribute tables. Polygon attributes include map symbol, name of geologic unit, age, and three lithology designations. Arc attributes include type of geologic contact line, fault line, and boundary line. The data set for dikes was prepared entirely by the PaGS and contains only line attributes: map symbol, name, age, lithology, and the type of dike line. The accuracy of the attributes and positioning of the arcs and polygons were checked by preparing and proofreading numerous check plots, verifying the data on screen, and preparing a script in Arc Macro Language to ensure correctness of polygon attributes.

It is anticipated that the data sets will be of use for a variety of regional or statewide GIS applications, including water-resource and environmental studies, land use planning, conservation and ecosystem management, mineral exploration, and industrial development.

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