Implementing the Geologic Mapping Database Standard

This blog post was written by UW-Madison undergraduate Benji Johnson, who worked at WGNHS as a Social Media and Geology Assistant from 2021-2022. Includes May 2024 update from Pete Schoephoester and Mel Reusche.

1869 geological map of Wisconsin by T.A. Lapham, featuring color-coded geological formations, counties, cities, rivers, and Lake Superior.
Figure 1. First detailed, statewide geologic map of Wisconsin—by I.A. Lapham in 1869.

Caroline Rose, a Geographic Information System (GIS) specialist and cartographer with the Wisconsin Geological and Natural History Survey (WGNHS), is leading an effort to convert printed maps showing Wisconsin’s geology into digital versions that align with the GeMS national database standard. GeMS—the Geologic Map Schema—is the standard schema for geologic maps in the USGS National Cooperative Geologic Mapping Program and was formally published in September 2020. With the GeMS standard, geological surveys across the country hope to create consistency and accessibility among their maps.

Before the advent of computers and internet connectivity, geologic maps were produced by hand. This was a laborious process that relied on the accumulation of observational data without the aid of GPS or satellite imagery. Figure 1 depicts a product of this method—the first detailed, statewide geologic map of Wisconsin.

Technological advancements in computing, the internet, satellites, GPS, and more led to the development of GIS—computers and databases for mapping and spatial analysis. GIS incorporates many layers of geographic data in an integrated system. For example, with GIS you can combine satellite imagery, elevation data, political boundaries, and field observations from geologists together to help create new maps.

How the GeMS standard works

In many cases, a mapping project will need to use both modern forms of mapping (like GIS) as well as existing non-digitized observational data (like hand-drawn maps). The GeMS project aims to translate both analog and digital geologic map data into a consistent format.

In a typical database for a geologic map, a combination of points, lines, and polygons are used to define mapped geologic features. Points show outcrops, lines show boundaries (contacts) between geologic formations, and polygons show the geographic extent of the rock formations themselves. A number of rules must be followed when making a geologic map. For example, polygons must not overlap, nor can there be gaps between the polygons. Each point, line, and polygon is associated with a row in a table of information. In the past, individual mappers or organizations would use their own naming and organizational conventions. This sometimes resulted in confusion when comparing map databases between different mappers.

Flowchart detailing the many steps and tasks needed to to complete GeMS standardization.
Figure 2. In order to standardize a geologic map, each map undergoes a thorough process. See full documentation here (Caroline Rose).

GeMS standardizes how this information is organized (Fig. 2). Each column in a GeMS database is designated a title and purpose; each data unit is designated a confidence level. Furthermore, four explanatory tables are required, including a legend, glossary, data sources, and the standard table called “GeoMaterials”. GeoMaterials are defined by the USGS so that when data are pulled into a database from different areas, there is literacy among the states and consistency in their data. All told, the GeMS framework should improve the clarity of geologic maps and make the data more accessible to a broader audience.

The challenges and rewards of GeMS

Caroline says that the GeMS work is challenging because there is just so much to do. The work is mainly map by map, and county by county, assessing how the complex information on a map is organized, and how it can be translated into the GeMS standard. This effort began for Caroline in 2018 and has grown to include the hard work of several WGNHS colleagues: GIS specialists Steve Mauel, Matt Rehwald, and Nick Rompa as well as Assistant Director of Technical Operations Pete Schoephoester, and two student interns. The work can be frustrating at times because the guidelines are not exhaustive; there are many moments when the GIS team must use their best discretion or seek input from a geologist with knowledge of the area. Overall, the nationalized effort is a rewarding opportunity. In Caroline’s words, “We hope this [national database] will make life easier for a lot of people in the future.”

Map of Wisconsin overlaid by a colorful patchwork of county-scale geologic maps.
Figure 3. Map of Wisconsin showing where GeMS work is underway (Caroline Rose).

The Geologic Map Schema democratizes geologic maps by providing a common language of consistency. With GeMS, state geological surveys and various agencies across the country can better understand each other and share data. This might pave the way for future interstate collaborative mapping projects. Additionally, all these maps and data are created and published in common GIS formats for public access. It is this accessibility that ultimately motivates geologists and cartographers in the GeMS effort.

Further reading and resources

As of this writing, Caroline and her WGNHS colleagues have converted over 30 maps to the GeMS standard with more underway (Fig. 3). These maps, and future GeMS products, are made available in our Publications Catalog.


View all WGNHS GeMS maps


For full GeMS documentation, visit: “GeMS”—Geologic Map Schema (USGS National Geologic Map Database)