CREATION OF THE BEDROCK GEOLOGIC MAP OF VERMONT – AN EVOLUTION FROM ANALOG TO DIGITAL MAPPING TECHNIQUES

In the 20 plus years since the beginning of the project to create a new bedrock geologic map of Vermont, cartographic methods have evolved from analog to digital. Initially, 1:12,000- and 1:24,000-scale maps were compiled on paper, drafted with pen and ink onto mylar, hand-colored, and published using lithographic and photographic film separation techniques. In Vermont in the early 1990s, the introduction of GIS to geologic mapping included compilation of preliminary 1:24,000 geologic maps in PC Arc/Info and Unix-based host Arc/Info. The mylar separates were scanned, auto-vectorized, and edited in Arc/Info. Preliminary maps and GIS data were cooperatively published by the USGS and the Vermont Geological Survey in an effort to quickly release information to the public and to support state-wide compilation. To date, about 40 percent of Vermont has been published at 1:24,000-scale or larger and about 25 percent of the 7.5-minute quadrangles have been published as GIS databases.

As techniques evolved from analog to digital, preliminary maps first created in GIS format were later used to publish maps in formal map series. In these cases, the GIS data were converted to Adobe Illustrator where final cartography and digital film separation techniques were employed. Scale changes for compilation at 1:100,000 were achieved by printing paper copies from scanned or photocopied source maps at multiple scales, requiring re-drafting in some places, and by plotting maps from 1:24,000-scale vector GIS data. Linework was edge-matched and hand-drafted with pen and ink on mylar greenlines of the 30 x 60-minute topographic base maps. Then the greenlines were scanned and vectorized, and compiled in Illustrator as two oversize sheets. The description and correlation of map units, cross-sections, tectonic map, index map, and references were hand-drafted and compiled in a word processor and Adobe Illustrator. Conversion of the Illustrator files to a seamless 1:100, 000 GIS database began once the map was submitted for formal peer review in August 2008, and is currently ongoing in ArcGIS. The final product will include a printed paper map in 3 sheets, Adobe PDF files of each sheet, and a GIS database that meets the requirements of the NGMDB model and Vermont’s GIS data standards.