Finding the Sparkle of GeMS: Using the USGS Geologic Map Schema

If you love maps, a geologic map is one of the most beautiful examples of cartography to ever exist. They are full of vivid colors, special symbols, and different line styles. To the geologist, they tell a tale of deep time and an evolving Earth. Within the map are stories about the birth of continents and the powers of water and wind, all contained on one page. Learning to read a geologic map is like learning another language, and many hours can easily be spent poring over the map to unravel the meaning of it all.

The majority of geologic maps were originally produced before the widespread use of GIS. In 2009, in an effort to bridge the gap between traditional geologic mapping and GIS-based methods, the Geologic Map Schema (GeMS) was introduced. The schema provided a standardized way to organize geologic map data and content into a modern database format. Importantly, it paved the way for meeting the requirements of the Geologic Mapping Act of 1992, which mandated “the creation of a national archive of standardized geologic map content” (USGS, 2020). One advantage of this national archive is the ability to more readily work with continuous, correlated geologic data and attributes that span across quad sheets, states, and other man-made boundaries.

The typical GeMS dataset is a geodatabase that contains all map features and their unique attributes, plus non-spatial tables that contain critical information about the geologic units, map references, and map index. The USGS provides a GIS-ready GeMS Toolbox that supports data development, validation, error checking, and metadata creation that helps to standardize each map geodatabase. Converting a single non-GeMS map into a compliant one can take hours to days, depending on the state of the source data. Some maps we’ve worked on have no digital data aside from their digital scan, while others are almost fully GeMS Level 3 compliant.

CivicMapper has been assisting State Geological Surveys across the US to transform their geologic maps into the compliant GeMS format. One lesson learned in our GeMS conversion work is the variety and uniqueness of geologic maps, and the need for expert geologic knowledge to guide the conversions. This is because there is no “typical” geologic map. While all maps conform to a basic geologic mapping standard, their symbology, level of detail, features, and scan quality vary significantly. Geologic maps are as unique as fingerprints and each one imparts a sense of the geologist’s interests and objectives for that mapping project.?

At CivicMapper, our CEO and licensed Professional Geologist, Emily Mercurio, PhD, PG is closely involved in our GeMS conversions, and reviews all data at each stage of their development. This step is critical for creating a quality deliverable. While our work is underway, we also remain in frequent contact with the client to ask questions and leverage their expertise on the State's geology. As a result, we typically include a log of answered questions with the final deliverable so that the client can retain a record of our agreed-upon approach and logic used in the conversion process. This Q&A log is a particularly helpful touchpoint for future conversions.?

A link to our interactive demo version of a basic converted GeMS Level 1 dataset can be found here. This app lets you explore the symbology created from scanned geologic maps in Pennsylvania, for both bedrock and surficial geologic maps. If you are interested in learning more or working with CivicMapper for your GeMS conversion project, please get in touch!?

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