Compiling digital geo-referenced geologic maps; a method for the non-specialist

Producing publication-quality digital, georeferenced geologic maps can now be done using high-end versions of GIS software. Not all of the geoscience community, however, has sufficient training in the use this software, and many geoscience departments do not have the licenses necessary to access high-end editing capabilities. To address this problem, we present a simplified procedure that utilizes the basic features of ArcGIS for map compilation, and the graphics capabilities of Adobe Illustrator for editing. This method allows those with limited cartographic experience and/or resources to produce publication quality maps. We used this approach during the compilation of a 1:10,000 scale digital map of a 30 km-long by 3 km-wide segment of the Hudson Valley fold-thrust belt in eastern New York State. This compilation incorporates data from 4 smaller maps (2 in ArcGIS, 1 in Illustrator, and 1 hand-drawn) that were prepared at different scales. The first step is to create a single composite topographic base map in Illustrator by edge matching the different quadrangles. Edge matching involves alignment of map features (e.g., contour lines) across the different quadrangles. We used the new series of digital USGS topo maps, US Topo. These maps come as vector-based PDF documents with editable layers and provide significant advantages over commonly used image formats, including Digital Raster Graphics. In the second step, the Illustrator-based map and the scanned version of the paper map along with the ArcGIS based maps were georeferenced to match the datum of the composite base map. Lithological units were digitized as polygons in ArcGIS. Strike-and-dip data (in right-hand-rule format) and measurement locations were compiled in an Excel database and were then used to prepare an X-Y plot in ArcGIS. The map was then imported into Illustrator for manual smoothing of polygons and editing of dip magnitudes and label positions. Unit contacts, faults and structural elements (e.g., fold axes) were digitized in Illustrator, which provides more flexibility in editing of line features. Similarly, Illustrator was used to layout text and the legend elements. Once the map is complete, it can be exported into ArcGIS to be usable for future applications.