PRECISE CONVERSION OF PAPER GEOLOGIC MAPS TO VALUE-ADDED DIGITAL PRODUCTS: THE MASSACHUSETTS METHOD FOR SURFICIAL GEOLOGY

The Office of the Massachusetts State Geologist, MASSGIS, and the U.S. Geological Survey are cooperating to compile digitally the surficial geology of Massachusetts from 189, 7.5’ geologic quadrangle maps. This process includes: 1) precise digital capture of geologic features from 93 published maps, 2) digitization of 96 compiled field maps, 3) interpretation of all maps with a consistent regional stratigraphic framework, 4) revision of maps to include subsurface data, sedimentary facies and textures, and layered 3-D map compilation. The main issue in digital conversion of published maps is the separation of map line work (geologic contacts) from the CMYK-process-screened map polygon fills. For this project, scanned and rectified images of the surficial geologic maps are processed in Photoshop using the program’s editing tools. The resulting line work is exported to ArcScan and adjusted using the original raster image as a background before vectorizing the polygons. Reviewers can edit and approve the vector linework and polygon attribution by direct overlay comparison with the original raster map. This methodology provides a set of precise, noise-free contact lines by “on the fly” editing, eliminating the large number of errors and artifacts that “automated” vectorizing processes inevitably generate. Similar processing techniques are used for compiled field maps. Linework is digitized and edited on screen in Adobe Illustrator on basemap images georeferenced in Map Publisher. Original map units of Late Wisconsinan glaciogenic deposits are revised and placed in a regional deglaciation chronology that shows moraines, morphosequences, extent of glacial fluvial, lake, and marine basins, and correlation of ice-margin retreat positions. For all maps, geologic map units are compiled in separate layers to aid drawing and editing. Subsurface data and overprint patterns are applied to show the extent of sedimentary facies within map units. The map-unit layers are then stacked by age, leading to a 3-D technique for showing subsurface information. This compilation digitally locates the surficial stratigraphic units across the State and can be used for future 3-D analyses such as the location of high-production aquifers and sand and gravel aggregate resources.