U.S. Geological Survey Open-File Report 2009-1298

The Geologic Mapping Act of 1992 mandates the U.S. Geological Survey (USGS) and Association of American State Geologists (AASG) to design and build a National Geologic Map Database (NGMDB), as an archive of mapbased, standardized geoscience information. The science and technical standards for this archive have been under development since that time, and mostly consist of revisions and modifications to standards that have evolved in the geosciences since the 1800s. Numerous reports of progress have been included in previous Proceedings of the Digital Mapping Techniques workshops and elsewhere (see, for example, Soller and Stamm, this volume). The NGMDB project delivers geoscience information in several ways including via a Geoscience Catalog which provides, for each of ~80,000 publications, one or more web links to the publishing agency and in many cases to downloadable data and images (http://ngmdb.usgs.gov/ngmdb/ ngm_catalog.ora.html). In a recent initiative made feasible after the development of certain science and technical standards, the project began to design a Web-mapping portal (the NGMDB “Data Portal,” http://maps.ngmdb.us/dataviewer/, see Soller, 2008) where the full richness and variability of map information content is managed by the publishing agencies or other repositories, with a subset of the information made available in a standardized and coherent fashion via this Portal, for browsing and querying and, in the future and on a limited basis, for downloading. In this paper I address, but do not presume to solve, a particularly challenging task —that is, how to effectively portray many geologic maps together, in a coherent fashion, via a Web-mapping system. Geologic maps vary significantly in content owing to factors including the purpose (e.g., mineral exploration, economic development, groundwater modeling), map scale, geologic terrane, and the geologic concepts in use when the map was made. Even if we examine the most recent intermediateto detailed-scale maps for a region, the differences among them commonly are sufficient to limit the effectiveness of standard rock classifications for bringing the maps together into a synoptic view. Fault doesn’t lie with the classifications but with their application to a situation for which they were not designed—that is, interactive, dynamic Web display of multiple and disparate geologic maps. Because the geologic classification described in this paper was developed specifically for application in a Webmapping system, and because those systems are relatively new and unevolved, it may be somewhat unconventional. Suggestions for improvement to this classification and how it is applied on the Web are welcome; I do not suppose the classification to be ideal, but it is clear that more effective methods for cartographic display of geologic information in Web-mapping systems are needed. From my limited personal experience, I find the process of developing a classification to be difficult, fraught with ambiguities and second-guessing. It gives me newfound respect for those who have done it successfully. This paper describes how the classification was developed, in order to place it in context and to lend some small measure of insight into the process.