GeMS (Geologic Map Schema) is the United States Geological Survey's standardized data model for encoding geologic maps in a GIS geodatabase. It defines a consistent set of feature classes, tables, fields, and controlled vocabularies so that geologic map data from different authors and agencies is structured, interoperable, and ready for publication.
Why it matters
Geologic maps are dense with information — units, contacts, faults, structure measurements, descriptions, and confidence — that is easily lost or made inconsistent when each mapper invents their own schema. GeMS imposes a common structure, which makes maps easier to integrate, query, validate, and archive. It is the required publication standard for geologic maps produced under the USGS National Cooperative Geologic Mapping Program.
A concrete example
A GeMS dataset centers on key feature classes such as MapUnitPolys (map unit polygons), ContactsAndFaults (linework), and OrientationPoints (strike/dip and other measurements), linked to non-spatial tables including DescriptionOfMapUnits and a Glossary of defined terms. Each linework feature carries attributes like Type, IsConcealed, LocationConfidenceMeters, and ExistenceConfidence. GeMS is distributed as an Esri file geodatabase template, with open transcriptions available for use in other tools.
Common pitfall
GeMS is a data schema, not a symbology or cartographic standard. It governs how attributes and relationships are stored, not how the map is drawn; you still apply FGDC symbology separately for presentation. A common error is populating polygon/line attributes inconsistently with the DescriptionOfMapUnits and Glossary tables, which breaks the referential links that validation tools check.