Digital Mapping Techniques '03
— Workshop Proceedings
U.S. Geological Survey Open-File Report 03–471
The National Geologic Map Database Project: Overview and Progress
1U.S. Geological Survey, 926-A National Center,Reston, VA 20192
Telephone (703) 648-6907; fax (703) 648-6977;e-mail: firstname.lastname@example.org
2Ohio Geological Survey, 4383 Fountain Square Dr.,Columbus, OH 43224
Telephone (614) 265-6988; fax: (614) 268-3669; e-mail email@example.com
The National Geologic Map Database (NGMDB) project continues to fulfill its mandate. Some of its accomplishments are specific and tangible, and others are more general in nature — for example, the NGMDB contributes to advancements in digital mapping techniques and database design by agencies in the United States and internationally. However, without extensive collaboration from enthusiastic and highly skilled members of the state geological surveys and the Geological Survey of Canada, these accomplishments would not have been possible. Highlights of the past year include:
This project provides an unusual if not unique opportunity to foster better relations and technical collaboration among all geological surveys in the nation. Given the nature of the issue — the creation and management of geoscience map information in digital format during a period of rapid technological evolution — collaboration is critically important. Perhaps more significant, these are changing times for all geological surveys — funding and staff seem to become more scarce each year — and through collaboration we can share our intellectual and computing resources and not “reinvent the wheel” within each agency.
Before describing the NGMDB components and progress, we wish to highlight the various mechanisms by which we define and accomplish our goals. Because advice, guidance, and technical collaboration are an integral part of this project, we discuss the project plan at numerous venues throughout the year. These include geoscience and related professional society meetings, the Digital Mapping Techniques workshop, and site visits to state geological surveys. Advice gathered at these venues serves to refine and, in some cases, to redirect the project’s goals. Comments from users, generally via our Web feedback form, also provide us with valuable perspectives, and have prompted us to make numerous modifications, especially to our Web interface design.
The National Geologic Mapping Act of 1992 and its reauthorizations in 1997 and 1999 (PL106–148) require a National Geologic Map Database to be built by the USGS in cooperation with the AASG. This database is intended to serve as a “national archive” of standardized geoscience information for addressing societal issues and improving our base of scientific knowledge. The Mapping Act anticipates a broad spectrum of users including private citizens, professional geologists, engineers, land-use planners, and government officials. The Act requires the NGMDB to include these geoscience themes: geology, geophysics, geochemistry, paleontology, and geochronology.
In mid-1995, the general stipulations in the Geologic Mapping Act were addressed in the proposed NGMDB design and implementation plan developed by the USGS and AASG. Summaries of this plan are listed in Appendix B. Because of the mandate’s broad scope, we proposed a phased, incremental design for the NGMDB. A phased approach has two benefits: 1) it enables us to identify the nature and quality of existing information and quickly serve it to the public; and 2) it gives us time to build consensus and expertise among the database designers in the state geological surveys and the USGS. Furthermore, it enables us to more effectively consider and respond to evolving technology and user needs. These phases, and our progress, are shown in figure 1.
|Figure 1. Diagram showing the three NGMDB Phases, and progress toward our goals (for example, documenting in the Geoscience Map Catalog all maps and related products for the United States and its territories and possessions).|
In the first and most fundamental phase of the project, we are building a set of easy-to-use reference databases; for example, a comprehensive, searchable map catalog of all geoscience maps in the United States, whether in paper or digital format. The second phase of the project focuses on the development of standards and guidelines needed to improve the utility of digital maps. The third phase proposes to, in the long term, develop an online database of vector-based geologic map information at various scales and resolution.
In late 1995, work began on Phase One. The formation in mid-1996 of several AASG/USGS Standards Working Groups initiated work on Phase Two. The project opened its Web site to the public in January, 1997, as a prototype intended to solicit comments on the Map Catalog. At the Digital Mapping Techniques ‘98 through ‘03 workshops, a series of presentations and discussion sessions provided updates on the NGMDB and, specifically, on the activities of the Standards Working Groups. These progress reports are listed in Appendix B. This report summarizes accomplishments since the project’s inception, and therefore repeats material from previous reports, but it focuses on activities since mid-2002. Additional and more current information may be found at the NGMDB project-information Web site, at http://ncgmp.usgs.gov/ngmdbproject/. The searchable databases are available at http://ngmdb.usgs.gov/.
To submit general comments about project scope and direction, please address the authors directly. For technical comments on the databases or Web page design, please use our Web feedback form; this form is linked from many of our search pages (see “Your comments are welcome”, at http://ngmdb.usgs.gov/).
Through ongoing discussions with private companies, citizens, government officials, and research geologists, it is clear that first and foremost, we need to provide reference databases so that geoscience maps and descriptive information can be found and used. Many people want to better understand the geologic framework beneath their home, business, or town, and so we are building several databases that support general, “data-discovery” questions posed by citizens and researchers alike (fig. 2). These reference databases are: 1) the Geoscience Map Catalog; 2) GEOLEX, the U.S. geologic names lexicon; 3) Geologic Mapping in Progress, which provides information for ongoing National Cooperative Geologic Mapping Program (NCGMP) mapping projects, prior to inclusion of their products in the Map Catalog; and 4) the prototype version of our Geologic Map Image Library — this new initiative is briefly described below, and in other papers in this volume. Plans for the prototype National Paleontology Database also are discussed below.
|Figure 2. Many people want to know whether the geologic framework and the geoscience characteristics (for example, earthquake hazard, geochemistry) of an area have been studied and published. The reference databases built under NGMDB Phase One provide users with access to that information.|
Figure 3 shows the number of people (actually, the number of unique IP addresses or computers) who have used the NGMDB, per month since it opened to the public in January, 1997. These numbers indicate that the site has become a useful resource. Additional increases in use are expected as the Map Catalog, Geolex, and Image Library become fully populated.
|Figure 3. Web usage for the Geoscience Map Catalog, GEOLEX, and Mapping in Progress Databases. This diagram shows that the number of people (actually, the number of unique IP addresses or computers) using the NGMDB has gradually increased as these resource databases become more widely known; this usage trend is punctuated by sharp increases after essentially all USGS maps were entered into the Catalog and after many state geological surveys began to enter map records. The Catalog accounts for about 75–80% of user visits to the NGMDB site.|
The Geoscience Map
“I want to know if a map exists for an area, and where I can find it. . .”
Many organizations produce paper and digital geoscience maps and related products. Discovering whether a product exists for an area, and if so, where it can be purchased or obtained online, can be a time-consuming process. In the past, people found this information by contacting various agencies and institutions, and by conducting extensive library searches. To increase accessibility and use of these paper and digital products, we built the Geoscience Map Catalog as a comprehensive, searchable database of all maps and related products for the United States and its territories and possessions.
The Geoscience Map Catalog contains bibliographic records for more than 61,000 products from at least 270 publishers (see Appendix C or our most current list of publishers at http://ngmsvr.wr.usgs.gov/ngmdb/pub_series.html). Most of these products are from the USGS and 43 state geological surveys. Other publishers include state agencies, federal agencies, scientific societies, park associations, universities, and private companies. Products range from digital maps to books that don’t contain maps but describe the geology of an area, and can be formal series products, Open-File reports, or unpublished dissertations (fig. 4). Because there are many types of geoscience maps and related products, we categorize them by theme (fig. 5).
|Figure 4. Bibliographic records in the Geoscience Map Catalog are drawn from a diverse group of more than 270 publishers.|
|Figure 5. A portion of the Geoscience Map Catalog search page, showing the types of products included.|
The Geoscience Map Catalog provides links to more than 1,300 published, downloadable products of the USGS and the state geological surveys. These links are established only to stable Web pages that provide the official copy-of-record for the publication — in the USGS, links are established only to the Publications Server and the NSDI Clearinghouse node.
Figure 6 shows how the Geoscience Map Catalog can be used to find particular products — upon searching it and identifying the needed product(s), the user is linked to the downloadable data and metadata, to a depository library, or to the appropriate organization for information about how to purchase the product. We address the diverse needs of our user audience through four search options. The easy-to-use Place Name Search is based on the USGS Geographic Names Information System (GNIS); it is designed mostly to address the needs of non-geologists who want to use a simple interface to find information about their home, town, or worksite (fig. 7). In contrast, other choices such as the Comprehensive Search offer more search criteria.
|Figure 6. Diagram showing how a user navigates the Geoscience Map Catalog. Interested in knowing something about the geology of an area (such as the land beneath their house), the user queries the Catalog, which returns a hit list of possibly useful maps and related products. The user selects one of these and, from the Product Description Page, obtains further information and can then choose to buy the product, view and download it, inspect the metadata, or find it at a depository library.|
|Figure 7. The first page of the Geoscience Map Catalog’s Place Name Search.|
The U.S. Geologic Names Lexicon (“GEOLEX”)
“I want to know more about the geologic units shown on this map. . .”
This is the nation’s lexicon of geologic nomenclature. GEOLEX contains information for more than 16,000 geologic units in the U.S. (Stamm and others, 2000). It is an excellent resource for finding significant publications that defined and described geologic units mapped in the U.S. These publications can be critically important in field studies, enabling students and mappers to compare these published descriptions with what they see in the field.
GEOLEX includes the content of the four geologic names databases on USGS Digital Data Series DDS-6 (Mac Lachlan and others, 1996). Before incorporating into GEOLEX, those databases were consolidated, revised, and error-corrected. Our work now focuses on:
Many state geological surveys have been registering new geologic names with the USGS for decades, and are encouraged to continue this practice. In order to promote standardized geologic nomenclature within the U.S., the GNC is being reconstituted. Formerly a committee that focused on nomenclature issues within the USGS, the new GNC will include members from each state geological survey (fig. 8). When a conflict arises, GNC members from the USGS and those states affected will resolve it, and any changes will be recorded in GEOLEX. Through this mechanism, we anticipate that GEOLEX will serve the entire U.S. geoscience community.
|Figure 8. The purpose and membership of the reconstituted Geologic Names Committee. Background image is an index card from the files of the USGS Geologic Names Committee, ca. 1903, showing decisions recorded regarding the use of the Pierre Shale in the USGS Geologic Atlas of the United States folios.|
Geologic Mapping in Progress Database
“I see from the Map Catalog that a map hasn’t been published for this area — is anyone mapping there now?”
Our Geologic Mapping in Progress Database provides users with information about current mapping activities (mostly at 1:24,000- and 1:100,000-scale, but at 1:63,360- and 1:250,000-scale in Alaska) that is funded by the National Cooperative Geologic Mapping Program. We are re-engineering and repopulating this database, and will be linking it directly to the state geological survey fact sheets and Web sites.
Geologic Map Image Library
“I want to see a picture of this geologic map, online. . .”
Through discussions with users, and from comments received via our Web feedback form, it became clear that many people are interested in viewing and/or obtaining maps “online.” Interpretation of the phrase “providing maps online” varies widely — to some people, it implies access to fully attributed, vector-based map databases, whereas to other people, it implies access to map images. Regarding the vector-based map database, we address this large task in Phase Three, below. With the Image Library, we have begun to provide map images to users, as described in two papers in this volume. We hope this new initiative will further strengthen the cooperative relationship between the AASG and USGS.
“I want to know if there is any fossil data from this area. . .”
The NGMDB project has designed and is planning to develop a National Paleontology Database (see Wardlaw and others, 2001). Our general plan is to build prototypes of this database in areas where geologic mapping is underway, so that we can work with mapping projects to design a database useful to science as well as to the public. Plans for a prototype have been delayed somewhat, while we assess ways that the project might interact with the National Science Foundation’s CHRONOS project (described in a paper by Wardlaw in this volume).
Phase Two focuses on development of standards and guidelines needed to assist the USGS and state geological surveys in efficiently producing digital geologic maps, in a more standardized and common format. Our profession encourages innovation and individual pursuit of science, and so the question may be posed — why do we need these standards? Clearly, standards should not impede science but instead should help us efficiently communicate our science to the public. The need for communication was perhaps best articulated by former USGS Director John Wesley Powell, while planning for the new Geologic Atlas of the United States:
“. . .the maps are designed not so much for the specialist as for the people, who justly look to the official geologist for a classification, nomenclature, and system of convention so simple and expressive as to render his work immediately [understandable]. . .” (Powell, 1888).
At that time, and throughout the early 20th century, Powell and others guided the USGS and the Nation’s geoscientists toward a set of robust, practical standards for classifying geologic units and materials and representing them on maps. Those standards endured and evolved, and continue as basic guidelines for geologic mapping. Although today we commonly record in the field and laboratory far more complex information than during Powell’s era, the necessity to provide it to the public in a standardized format remains unchanged. Newly evolving data formats and display techniques made feasible by computerization challenge us to revisit Powell’s vision, and to develop standards and guidelines appropriate to today’s technology and science.
In mid-1996, the NGMDB project and the AASG convened a meeting to identify the types of standards and guidelines that would improve the quality and utility of digital maps produced by the nation’s geological surveys. From that meeting, Standards Working Groups were formed to address: 1) standard symbolization on geologic maps; 2) standard procedures for creating digital maps; 3) guidelines for publishing digital geologic maps; 4) documentation of methods and information via formal metadata; and 5) standard data structures and science terminology for geologic databases. The working group results will help provide a set of national standards to support public use of standard, seamless geologic map information for the entire country. In essence, Powell’s pragmatic vision for the Geologic Atlas of the U.S. has been applied a century later to the National Geologic Map Database.
The tasks assigned to these Standards Working Groups are interrelated, as shown in figure 9 — when in the field, a geologist makes observations and (often, provisionally) draws geologic features on a base map; at that time, the accuracy with which these features are located on the map can be estimated. Further, the information may be recorded digitally in the field; if so, it can be structured similar to, or compatible with, the map database’s structure (the “data model” in this figure). Returning to the office, the geologist commonly organizes and interprets field observations and prepares for map production — descriptions may be standardized according to an agency or project-level terminology or “science language,” the map data may be structured according to the standard data model implemented by the agency, and procedures may be documented with metadata both in the office and when gathering data in the field. The descriptive information then is combined with the feature location information in a GIS, and digital cartography is applied to create a map that is published according to agency policies. Finally, the map is released to the public and accessed through various mechanisms including the NGMDB.
|Figure 9. Diagram showing how the standards and guidelines under development by the NGMDB and related groups relate to the process of making a map.|
As described below, since 1996 these Working Groups and their successor organizations have made significant progress toward developing some of the necessary standards and guidelines. General information about the Working Groups and details of their activities are available at http://ncgmp.usgs.gov/ngmdbproject/standards/. Working Group members are listed in Appendix A.
Internationally, the NGMDB participates in venues that help to develop and refine the U.S. standards. These venues also bring our work to the international community, thereby promoting greater standardization with other countries. Examples include:
Geologic Map Symbolization
A draft standard for geologic map line and point symbology and map patterns and colors, published in a USGS Open-File Report in 1995, was reviewed in 1996 by the AASG, USGS, and Federal Geographic Data Committee (FGDC). It was revised by the NGMDB project team and members of the USGS Western Region Publications Group, and in late 1997 was circulated for internal review. The revised draft then was prepared as a proposed federal standard, for consideration by the FGDC. The draft was, in late 1999 through early 2000, considered and approved for public review by the FGDC and its Geologic Data Subcommittee. The document was released for public comment within the period May 19 through September 15, 2000 (see http://ncgmp.usgs.gov/fgdc_gds/mapsymb/ for the document and for information about the review process). This draft standard is described in some detail in Soller and Lindquist (2000). Based on public review comments, in 2002 a new section was added to the draft standard to address uncertainty in locational accuracy of map features. This section was presented for comment (Soller and others, 2002) and revised accordingly. With assistance from a Standing Committee to oversee resolution of review comments and long-term maintenance of the standard, the document is being prepared for submittal to FGDC, for final discussion and adoption as a Federal standard.
The Data Capture Working Group has coordinated seven annual “Digital Mapping Techniques” (DMT) workshops for state, federal, and Canadian geologists, cartographers, managers, and industry partners. These informal meetings serve as a forum for discussion and information-sharing, and have been quite successful. They have significantly helped the geoscience community converge on more standardized approaches for digital mapping and GIS analysis, and thus agencies have adopted new, more efficient techniques for digital map preparation, analysis, and production. In support of DMT workshops, an email listserver is maintained to facilitate the exchange of specific technical information.
The most recent DMT workshop, held in Millersville, Pennsylvania, and hosted by the Pennsylvania Geological Survey, was attended by 90 representatives of 36 state, federal, and Canadian agencies and private companies. Workshop proceedings are published (see Appendix B and http://ncgmp.usgs.gov/ngmdbproject/standards/datacapt/). Published copies of the proceedings may be obtained from David Soller or Thomas Berg.
Map Publication Requirements
Through the USGS Geologic Division Information Council, the NGMDB led development of the USGS policy “Publication Requirements for Digital Map Products” (enacted May 24, 1999; see link under Map Publication Guidelines, at http://ncgmp.usgs.gov/ngmdbproject/standards/). A less USGS-specific version of this document was developed by the Data Information Exchange Working Group and presented for technical review at a special session of the Digital Mapping Techniques ‘99 workshop (Soller and others, 1999a). The revised document (entitled “Proposed Guidelines for Inclusion of Digital Map Products in the National Geologic Map Database”) was reviewed by the AASG Digital Geologic Mapping Committee. In 2002, it was unanimously approved via an AASG resolution, and has been incorporated as a guideline for digital map product deliverables to the STATEMAP component of the National Cooperative Geologic Mapping Program (see link under Map Publication Guidelines, at http://ncgmp.usgs.gov/ngmdbproject/standards/).
Among the geological surveys there are many approaches to determining authorship credit and citation format for geologic maps, digital geologic maps, and associated databases. It is prudent for agencies to adopt policies that preserve the relationship of the geologist-authors to their product, the map image, and to identify the appropriate authorship (if any) and/or credit for persons responsible for creating the database files. A summary of this issue and a proposed guideline was discussed at the Digital Mapping Techniques workshop in 2001 (Berquist and Soller, 2001).
The Metadata Working Group developed its final report in 1998. The report provides guidance on the creation and management of well-structured formal metadata for digital maps (see http://ncgmp.usgs.gov/ngmdbproject/standards/metadata/metaWG.html). The report contains links to metadata-creation tools and general discussions of metadata concepts (see, for example, the metadata-creation tools, “Metadata in Plain Language,” and other helpful information at http://geology.usgs.gov/tools/metadata/.
Geologic Map Data Model
In early 1999, the Data Model Working Group had concluded its work with release of a draft version of a data model (Johnson and others, 1998). The Group then was succeeded by the North American Data Model Steering Committee (NADMSC, http://geology.usgs.gov/dm/). State and USGS collaborators on the NGMDB continue to participate in this activity, helping to develop, refine, and test the North American Geologic Map Data Model (“NADM”) and the standard science language that must accompany it. This work recently has produced a significant accomplishment, the NADM Conceptual Data Model. This model is available for perusal and comment, at http://geology.usgs.gov/dm/steering/teams/design/NADM-C1.0/NADMC1_0.pdf. Information about other Committee activities is provided in two papers in this volume: 1) the development of a XML-based interchange format; and 2) the development of standard science language to describe the lithology of earth materials.
To provide templates for building GIS data, ESRI is designing ArcGIS data models for many industries and applications (see http://esri.com/software/arcgisdatamodels/index.html). Through discussions that involved the NGMDB, ESRI plans to structure the ArcGIS data model at least in part on concepts in the NADM Data Model.
Over the past few decades, significant advances in computer technology have begun to permit complex spatial information (especially vector-based) to be stored, managed, and analyzed for use by a growing number of geoscientists. At the beginning of the NGMDB project, we judged that computer-based mapping was not a sufficiently mature discipline to permit us to develop an online, vector-based map database. In particular, technology for display and query of complex spatial information on the Web was in its infancy, and hence was not seriously considered by the NGMDB project as a viable means to deliver information to the general public. However, there now exists sufficient digital geologic map data; sufficient convergence on standard data formats, data models, GIS and digital cartographic practices and field data capture techniques; and sufficient technological advances in Internet delivery of spatial information to warrant a research effort for a prototype, online vector-based map database.
Before beginning to design this database, project personnel held numerous discussions with geoscientists and the general public to gauge interest in an online database and to define its scope. Based on these discussions, it was clear that this database should be:
This database will integrate with other databases developed under the NGMDB project. For example, a user accessing the online, vector-based map database might identify a map unit of interest, and then want to purchase or download the original published map product, or inquire about fossils found within that unit, or learn about the history of the geologic unit. Also, a user might access the Map Catalog and identify a map of interest, and then be linked to the online map database in order to browse and query it.
The NGMDB project has begun a series of prototypes, to advance our understanding of the technical and management challenges to developing the operational system; an introduction is given in Soller and others (2000). In 1999, we outlined some basic requirements for the prototype and tested them using map data for the greater Yellowstone area of Wyoming and Montana (Wahl and others, 2000). The second prototype (Soller and others, 2001) was conducted in cooperation with the Kentucky Geological Survey. In this prototype, we demonstrated in a commercial database system (GE-Smallworld) how the geologic database could be analyzed over the Web in concert with local datasets. The data model for the second prototype is described in Soller and others (2002) and was a significant contributor to the design of the new NADM Conceptual Data Model noted above.
Before proceeding further with plans for the publicly-accessible map database, we need to define a set of standardized terminology for the properties of earth materials (the science language). This science language must be sufficiently robust to accommodate terminology generated through today’s field mapping, and terminology found in map unit descriptions on older and on smaller-scale maps, where descriptions tend to be highly generalized. Also, we need to collect enough standardized geologic map data to justify the cost of developing the database. Therefore, in our third prototype we will create map data with a standardized data model and science language, using available mapping in disparate field areas (central Arizona, northern Virginia, Kentucky, southern California, and the Greater Yellowstone Area; see fig. 10). To achieve this, we are writing data-entry software tools supported by science language derived from the NADMSC.
|Figure 10. The goals of the current prototype are to: 1) create map data that has a standardized data model and science language, beginning with some national-scale maps and available mapping in disparate field areas shown above, and 2) create data-entry tools that are flexible and readily modified, enabling geologists to enter detailed, more standardized descriptive information.|
What is a data model, and how does it apply to geologic maps?
A data model provides organization to the descriptive and spatial information that constitute a geologic map. The relations between a data model, science language, and the geologic map require some explanation. A data model may be highly conceptual, or it may describe the data structure for managing information within a specific hardware/software platform. In either case, it is a central construct because it addresses the database design for geologic maps in GIS format. In figure 11, the data model is simplified to four locations, or “bins”, where information can be stored, with each bin containing many database tables and fields:
|Figure 11. Simplified representation of the data model and its application to a typical, 2-D geologic map. The presence of a geologic unit on the map, referred to in the data model as an “occurrence” of that map unit, is described by: 1) its bounding contacts and faults, whose coordinates are stored as the unit’s “geometry”; and 2) its physical properties, which are stored as the unit’s “descriptors.”|
Will the U.S. have a single standard data model and science language?
The NGMDB online map database is envisioned as a distributed system that will provide seamless access to, and display of, map data served by many agencies. If all agencies used the same science language and exactly the same data model, and if it were implemented on the same hardware and software platform, a functional system would be relatively easy to build. That, however, is not a realistic scenario. Each agency has a unique history, set of objectives, and budget that will dictate the nature of their map database. (It should be noted that not all geological surveys in the U.S. can now afford to build such a system.) A more realistic approach is to assume a heterogenous computing environment, and to build software that can translate data structure and science language from one agency’s system to another (fig. 12). This translation mechanism ensures “interoperability” between systems, and is the most realistic approach for the NGMDB.
|Figure 12. A single, monolithic system design shared by all agencies is unlikely. Rather, interoperability among the many agency databases linked together by the NGMDB database is the most logical design philosophy. In this diagram, we envision that map data from one agency (the Kentucky Geological Survey, http://www.uky.edu/KGS/) will be translated into reference standards (the data model and science language standards adopted by the NGMDB) and translated out to the criteria required by another agency (the Idaho Geological Survey’s Geologic Map Data Model, http://www.idahogeology.org/Lab/datamodel.htm). This approach also could permit the NGMDB to coordinate the translation and display of multiple agency databases. In this diagram, the reference standards are represented by a schematic of the draft NGMDB data model (discussed in another paper in this volume) and an example of science language from Folk (1954, fig. 1a) showing a rock classification based on mud-sand-gravel content.|
To facilitate interoperability among systems, the NGMDB will define and maintain a set of reference standards (for data model, science language, time scale) based in part on those produced by the NADMSC. Interoperability software that enables disparate systems to appear to the user as a single system is being evaluated by groups including the NADMSC, NGMDB, and the National Science Foundation-funded GEON project. We anticipate collaborative research, especially with GEON, on XML-based “wrapper/mediator” technology to address these needs for the NGMDB. Through this technology, agencies should be able to correlate their unique data structure and scientific terminology to the reference standard, and the translator (presumably XML-based) will enable us to display the information to the user in a single view.
Extending the data model to include three-dimensional(3-D) map information
The data model was designed for the typical geologic map, which provides a two-dimensional representation of the geologic framework. On most geologic maps, this framework is expressed generally, in cross-sections and map unit descriptions. The data model can accommodate more detailed and location-specific 3-D information, although it has not yet been applied in this fashion.
Three-dimensional geologic map information can be represented by various methods. The most traditional approach is vector-based stack-unit mapping, where a vertical stack of surface and subsurface geologic units are combined into a two-dimensional (2-D) map unit (fig. 13a). The stack-unit characterizes the vertical variations of physical properties in each 3-D map unit. These maps are readily managed in the data model, like a traditional geologic map (fig. 11).
Figure 13. Approaches for representing three-dimensional map information, and for managing it in the data model.
A. Vector-based stack-unit maps depict the vertical succession of geologic units to a specified depth (here, the base of the block diagram). This mapping approach characterizes the vertical variations of physical properties in each 3-D map unit. In this example, an alluvial deposit (unit “a”) overlies glacial till (unit “t”), and the stack-unit labeled “a/t” indicates that relationship, whereas the unit “t” indicates that glacial till extends down to the specified depth. In a manner similar to that shown in figure 11, the stack-unit’s occurrence (the map unit’s outcrop), geometry (the map unit’s boundaries), and descriptors (the physical properties of the geologic units included in the stack-unit) are managed as they are for a typical 2-D geologic map.
B. Raster-based stacked surfaces depict the surface of each buried geologic unit, and can accommodate data on lateral variations of physical properties. In this example from Soller and others (1999), the upper surface of each buried geologic unit was represented in raster format as an ArcInfo Grid file. The middle grid is the uppermost surface of an economically important aquifer, the Mahomet Sand, which fills a pre- and inter-glacial valley carved into the bedrock surface. Each geologic unit in raster format can be managed in the data model, in a manner not dissimilar from that shown for the stack-unit map. The Mahomet Sand is continuous in this area, and represents one occurrence of this unit in the data model. Each raster, or pixel, on the Mahomet Sand surface has a set of map coordinates that are recorded in a GIS (in the data model bin that is labeled “Pixel coordinates”, which is the raster corollary of the “Geometry” bin for vector map data). Each pixel can have a unique set of descriptive information, such as surface elevation, unit thickness, lithology, transmissivity, etc.).
Map unit descriptions, whether on traditional 2-D geologic maps or vector-based stack-unit maps, apply to the entire unit. As a consequence, if a map unit’s texture is described as “generally sandy, although fining to the east,” the unit cannot be readily subdivided into areas that are sandy and those that are finer. This can be a limitation to users, especially when using the map for detailed studies. In contrast to vector-based stack-unit maps, voxel maps show every part of a geologic unit as a unique point known as a volume-pixel or voxel. Each voxel can have a unique set of attributes, therefore lateral and vertical variations in texture within the geologic unit can be described in great detail. Such information is difficult to collect at depth, and so in studies where this type of representation is needed, voxel attributes tend to be computed from a few point measurements within the geologic unit.
A third approach to 3-D mapping, raster-based stacked surfaces, offers a useful compromise between vector-based stack-unit and voxel-based mapping. In this approach, a set of 2-D elevation maps shows, in raster format, the surface of each buried geologic unit. These surfaces are in many cases rasterized from conventional, vector-based maps. Unlike the vector-based stack-unit map, they provide the opportunity to model the surface elevation and thickness of each unit, and to assign unique physical properties to each location on the unit’s surface. Although not as detailed as a voxel representation, this approach requires less information and fewer assumptions about the 3-D variation of properties within the unit, and canmore readily be created using conventional GIS software such as ArcGIS. Lateral variations in a physical property such as texture can be recorded; this is informative for units such as alluvium, which may have distinct subenvironments with different characteristics (for example, coarser material in the main channel, and finer material in overbank areas and tributaries).
Raster-based stacked surfaces (and, by extension, voxel-based maps) can be represented in the data model, as shown in figure 13b. This raster-specific information can significantly improve the value of geologic data when applied to, for example, groundwater modeling. The 3-D geometry of the glacial aquifer shown in figure 13b was provided to a private groundwater consortium in order to develop a regional groundwater flow model. The aquifer is composed of coarse sand and gravel in the main channel but is finer-grained in the tributaries because sediment dammed the margins of the main channel, causing lakes to form in tributaries. When the 3-D information was provided to the consortium, the authors did not have sufficient data to assign to the units any lateral variations in texture. As a result, the groundwater modelers had to assume a homogenous aquifer. Raster surfaces that showed lateral variations in sediment texture would have enabled the modelers to consider the heterogeneity that was known to exist within that aquifer.
National and regional map coverage
The online map database will be more useful if it includes some geologic map coverage for the entire nation. To that end, the NGMDB has supported compilation and GIS development of several regional maps (fig. 14). Most significant is the digital version of the “Geologic Map of North America”. This map is the final product of the Geological Society of America’s (GSA) Decade of North American Geology project. The NGMDB has provided funding and expertise for development of the digital files that will be used to print the map, in order to engage GSA in a plan to develop a database for the map. When compilation and review of the map has been completed, hopefully within the next year, we will propose a database design and begin to populate the digital files made available from cartographic production of the map. This work will be conducted in collaboration with GSA and interested national geological surveys. The other maps shown in figure 14 are published or in press, and we intend to process these for inclusion in the online map database.
|Figure 14. Regional maps whose compilation and/or GIS development is supported by the NGMDB. The uppermost map, the Geologic Map of North America, is discussed in the text. The center map is in press (Soller and Reheis, in press) and must be converted to a database. The database for the lower map is published (Soller and Packard, 1998) and will be adapted to the emerging NGMDB standards.|
The authors thank the members of the NGMDB project staff and collaborators for their enthusiastic and expert support, without which the project would not be possible. In particular, we thank: Nancy Stamm (USGS, Reston, VA; Geolex database, and general support and guidance to the project), Ed Pfeifer, Alex Acosta, Dennis McMacken, Jana Ruhlman, and Michael Gishey (USGS, Flagstaff, AZ; Website and database management), Chuck Mayfield and Nancy Blair (USGS Library; Map Catalog content), Bruce Wardlaw (USGS, Reston, Va.; Paleontology Database), Robert Wardwell (USGS, Vancouver, WA; Image Library) and Kevin Laurent and Jeremy Skog (USGS, Reston, VA; Image Library), Steve Richard (Arizona Geological Survey, Tucson, AZ; data model and science language), Jonathan Matti (USGS, Tucson, AZ; data model and science language), and Jordan Hastings (USGS, Santa Barbara, CA; data model).
We also thank the many committee members who provided technical guidance and standards (Appendix A).
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