Summary of the GeMS Document


The following text has been taken directly from the GeMS document to highlight summary information. Use the link icon directly following section headers to open up that section in the document. In the elements section below, clicking text will take you to the feature class or table's section in the document. Other helpful resources that summarize the document can be found in the green box.


GeMS (Geologic Map Schema) is a standardized database schema for the digital publication of geologic maps. It stipulates that an Esri database format be used to adhere to USGS policy. GeMS is designed for a single-map database, however it is intended to provide a stepping stone toward the development of multiple-map databases.

Publication Package

For a digital geologic map publication named “mapXYZ”, the publication package should include the elements listed in this checklist. As-needed elements are mandatory only if they are part of the content of the map report. Optional elements may or may not be present, at the discretion of the author or publisher.

To make database content available without the requirement of having an ArcGIS license, now or in the future, an open shapefile version of the database is required. This version uses the well-documented shapefile and .dbf formats, which have length limitations of fields (must have ≤255 characters) and field names (must be ≤10 characters). The checklist describes this version and how to create it using the tool.


As with the overall digital data package, some geologic map database elements are required, some are as-needed, and some are optional. Additional optional elements are permitted.

This database design relies on relations (joins, or relationship classes) between the various feature classes and nonspatial tables.

Field Hygiene


Most feature classes in the GeMS schema contain the fields Type, Label, and Symbol; however the MapUnitPolys polygon feature class contains the field MapUnit in lieu of the Type field.

What goes where?

Map-unit polygons are bounded by contacts, faults, shorelines, snowfield or glacier boundaries, scratch boundaries, and/or the map boundary. Contacts do not normally separate polygons of the same map unit, although faults commonly do. In addition, a map-unit polygon may be partly bisected by a fault whose trace terminates within that polygon.

The distribution of units on a particular map is recorded in the polygon feature class MapUnitPolys. Contacts that separate map units, faults that bound map units, and dangling faults are recorded in the line feature class ContactsAndFaults.

Some maps show contacts and faults that are concealed beneath covering map units (for example, beneath thin surficial deposits or open water). These concealed contacts and faults are recorded in the line feature class ContactsAndFaults, and their concealed status should be encoded as IsConcealed = “Y”. Such concealed contacts and faults may or may not be involved in topology with MapUnit polygons; in addition, concealed contacts and faults may dangle.

Many geologic maps contain other feature classes that are not involved fully in map topology (for example, fossil localities, fold axes, or bedding-orientation measurements). Feature classes for such features are described in the “As-Needed Elements” section.

Directional Lines

Many types of geologic lines have directionality, equivalent to handedness. Examples are thrust faults, which (by convention) have sawteeth on the side of the upper (overlying) plate, and normal faults, which (also by convention) have half-circle or ball-and-bar decorations on the downthrown (overlying) side of the fault. The direction that the decorations point indicates the geometry of that feature.

To preserve this directionality in the database, we subscribe to the right-hand rule for such line features: that is, lines should be created or edited such that the decoration points to the right while one travels from the start of the line segment to its end. In the case of up-down (U–D) notation on faults, the direction that has the right side of the fault in the down (D) direction should be considered as the right-hand direction.


It is required that geologic-map databases include symbols and symbolization instructions to achieve a preferred visualization of the map. In GeMS, symbolization consists of (1) values of Symbol for all features (use null values where a database feature is not symbolized on the map), (2) an ArcGIS .style file that contains the symbols (for areas, lines, and points) used on the preferred visualization, and (3) an Esri map-composition (.mxd) file. Alternatively, authors may choose to use Esri’s cartographic representations to symbolize one or more of the map’s layers, in which case a .style file may not be needed.

Generally speaking (and to the extent that it is possible), line and point symbolization should follow the FGDC Digital Cartographic Standard for Geologic Map Symbolization (FGDC, 2006). Most of the symbols in the FGDC cartographic standard have been implemented as an ArcGIS .style file (“”). Note that both the .style file and all its associated font files need to be installed for the .style file to function correctly. Note also that, to use this .style file, it is necessary to left-pad the original FGDC symbol identifiers with zeroes so that each part of the identifier has a two- or three character width (for example, symbol 1.1.3 becomes 01.01.03, and symbol 1.1.25 becomes 01.01.25).

Although not a requirement, CMYK color fills for map-unit polygons may be selected from the FGDC cartographic standard’s CMYK Color Chart (FGDC, 2006). An ArcGIS .style file (“”) of this chart is available.

Also not a requirement, geologic age symbol characters are best displayed using the FGDCGeoAge font (U.S. Geological Survey, 2006). The OpenType version (“FGDCGeoAge.otf”) of this font is recommended.

If the FGDC cartographic standard does not define a suitable symbol for a particular feature on a map, the standard may be supplemented with either customized symbols or with FGDC symbols that have been repurposed for the map. Such repurposed symbols need to be identified in the RepurposedSymbols table, which is required if FGDC symbols have been repurposed.


Required, as-needed, and optional contents of a GeMS-structured database. As-needed elements must be present in the database if they are part of the content of a map report. Optional elements may or may not be present, at the discretion of the author and publisher.

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