Geologic Map Data

Geologic map data is complex, and we have several options for publishing such information. The focus for this project should be on maps that have some special interest for geothermal energy development. Here are some options and a discussion of a possible roadmap for developing services, starting from the most basic.

File-based schemes:

1. Create a metadata record for a published paper map. This is already being done, and is handled very well through the USGS National Geologic Map Database map catalog. You've probably already been doing this for your state data.
2. Scan a geologic map and make the scanned image available online. Lots of this is being done already, and the USGS NGMDB Map catalog is creating links to those that it knows about. In many cases these are dumped in directories on web sites, and standard web searches (Google, Yahoo, Bing...) find them quite nicely if the title of the map is pretty informative, even better if the web site includes some text describing the map.
3. Scan and georeference the map, make it available online. This might produce a world file that accompanies the scanned image, or the georeferencing information may be put in a header that is part of the image file (e.g. geoTiff). Similar to 2, but when the map is downloaded, it can be put directly in a geographic information system client that understands the georeferencing scheme used (these are pretty standard).
4. Digitize the map using GIS software to produce a database file that has vector geometry for the points, lines and polygons on the map, and associates them with attribute data allowing query and user defined portrayal of the data. This might be made available online as ESRI shape files, an ESRI personal geodatabase, ESRI Arc/Info Export files (.e00)., MapInfo files, OGC GML (xml) files, or any of a large number of other possible formats. There is currently no standardization for how such data files are constructed (the data schema), or the vocabularies used, so the data consumer must be provided sufficient metadata describing what they're getting that they can make sense out of it. Such data publication is a common practice for most currently generated geologic map data, and the USGS NGMDB Map catalog is creating links to such datasets that it knows about.

Service-based schemes

Map service

1. Set up a map service using the scanned, georeferenced map image. Open Geospatial Consortium (OGC) Web Map Service (WMS) servers will generally do this without problem. Even better, de-collar the map so its just the map area with the geologic information and expose that as a raster WMS. Easy, but clicking on the map in the client software only return an RGB value for the pixel (unless you start customizing the service...). Presenting legend information to the user can be tricky. Map service GIS client software allows a user to browse the map displayed in a geographic framework with other GIS data, sending requests to the server for an image of the map that is within the current user view frame each time the user zooms or pans. Various caching schemes (client and server side...) are used to improve performance.
2. Set up a map service based on a vector GIS dataset. This requires producing a portrayal of the data based on the information in the data, allowing some harmonization of the portrayal scheme between adjacent quads, or duplication of the presentation on an existing published map from which the data were digitized. User information clicks on the map will result in a presentation of whatever attribute data is encoded in the GIS dataset used to drive the map service. Sophisticated services can be configured to allow users to customize the portrayal of the data based on the attribute data (using e.g. OGC SLD files).

Feature service

User can request specific features from the service. Definition of feature types can be specific to a particular service (not very interoperable), or based on standardized information schemes. Examples of feature types might be faults, mapped outcrop areas, Geologic units, structure observations (e.g. strike and dip data measurement points). Interoperability requires conventions for what the features represent, what attributes are associated with each feature, and the vocabulary and syntax used to specify the values fort those attributes. Examples of feature services include OGC Web Feature Service (WFS) and ESRI ArcGIS server feature services.

1. Simple Feature service. Simple features have single-valued attributes that are specified by numeric or string values. In the simplest case, such services can be set up using vector GIS data in a flat-file format (e.g. ESRI shape file), for which each record is a feature that has the attributes that are in the flat-file data source. A simple feature content model and xml schema has been developed by the CGI Interoperability Working Group, and this schema will be used for geologic map data simple features in AAGS geothermal data.
   • Geologic Unit Feature v. 1.0
   • Contact feature v 1.0
   • Fault feature v 1.0 (See also the Quaternary Fault template)
2. Complex feature service. Content model (data schema) for features allow attributes with multiple values (e.g. a geologic unit is composed of several kinds of rocks), or attributes that are represented by a more granular data structure (e.g. a geologic unit has a rock type, which has its own grain size and mineralogy attributes). GeoSciML uses a complex content model, thus requires complex feature service implementation.

Features for a geologic map service

Mapped features

• Polygons represent mapped outcrop areas assigned to a geologic unit
• Lines can represent mapped structures (contacts, faults, hinge surface traces) that associated with a geologic structure description or dikes that may be considered structures, but are also associated with a geologic unit.
• Point mapped features are used to represent features that are too small to represent as a polygon or line feature (at the scale of mapping) and have a geologic structure or geologic unit description, or may represent some other areally restricted geologic feature (volcanic vent).

Geologic feature

Geologic features are geoscience domain entities that may have a complex extent that may not actually be known. Examples include named formations (a body of rock, originally interpreted to have been contiguous, but may now be complexly dismembered), or particular faults (the Lewis Thrust) or contacts (the Redwall-Supai contact). Mapped features represent particular occurrences of some geologic feature.

Observations

Points are commonly used to represent observation locations where data were collected--orientation of some structural feature, sample collection, photographs.