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Digital Mapping Techniques '00 -- Workshop Proceedings
U.S. Geological Survey Open-File Report 00-325

Limitations in the Use of Map Geometry as the Foundation for Digital Geologic Database Design

By Michele E. McRae

U.S. Geological Survey
National Center, MS 926A
12201 Sunrise Valley Drive
Reston, VA 20192
Telephone: (703) 648-6349
Fax: (703) 648-6953
e-mail: mmcrae@usgs.gov

INTRODUCTION

For most of the last century, analog maps have been the geologists' primary instrument to communicate their understanding of the geologic environment. These products have proven their utility in a wide variety of societal and scientific applications such as natural hazards mitigation, water and resource management, and land-use planning. With the advent of geographic information systems (GIS) technology and its improved ability to integrate diverse geospatial data, the digital database is now challenging the role of the traditional geologic map.

Digital datasets facilitate many map-oriented activities such as updating and reprinting existing maps, rescaling data, recombining map units based on common attributes, and overlaying geologic data with other geographic information. Of course, these technological advances have altered neither our understanding of geologic information nor its role in decision-making. Since geologic maps have proven their ability to effectively communicate knowledge of geologic environment, database design practices have focused on translating the geologic map model into the digital arena so that individual paper map elements (i.e., lines, polygons, and symbols) become the geometric building blocks of their corresponding digital database. However, a digital database is not a map. Although applicable to the same problems, the publication media and methods of presenting, exploring, visualizing, and analyzing digital data are significantly different. These differences directly impact how the user perceives and applies the information. Consequently, a digital database whose geometry adheres strictly to the conventions of a paper map is less effective at communicating information than its analog counterpart. This paper attempts to characterize these differences and to suggest alternative models for database design.

THE GEOLOGIC MAP

In order to improve digital database design, we first need to understand how data is modeled on a geologic map and how one perceives that model. Bernknopf and others (1993), define a geologic map as "a graphical information display that uses a combination of colors, lines, and symbols to depict the composition and structure of geologic materials and their distribution across and beneath the landscape. The graphical display contains both descriptive information about geologic units and structures and an interpretive model of how they were formed. This combination of descriptive and interpretive geologic map information provides a conceptual framework that relates all the geologic elements of an area together so that the position, characteristics, and origin of each element are understood in relation to all other elements." The scientific content that one expects to find includes physical and chemical properties of rock units, three-dimensional geometry, relative age relationships, and relationships between geologic structures and processes. The primary graphic components of geologic maps are a planimetric view of the distribution of rock units at the Earth's surface (the map itself) and a legend. Additional graphic elements include a variety of cross-sections, fence diagrams, stratigraphic sections, correlation diagrams, etc.

This combination of individual, 2-dimensional graphic elements forms a single, cohesive product. In order to correctly apply geologic map information, one must understand that the geographic relationships of geologic units are not fixed, but rather change with depth below or height above the Earth's surface. The user must understand how to reconstruct the 3-dimensional framework from these components. Since this interpretation is largely visual, it is the author's responsibility to maximize this understanding by controlling the selection of graphic elements, their layout, and symbolization.

THE DIGITAL DATABASE

Contents of geologic databases vary widely, but generally include a graphic representation of the distribution of geologic features and tabular information describing properties of those features. The graphic elements within a GIS are georeferenced, so that an exact coordinate for any feature (or part of a feature) can be obtained. The locations of objects with respect to each other are understood in terms of these coordinates. For this reason, the positional accuracy of features is of prime importance in the development of any GIS database.

Current database development practices focus mainly on the map and legend components of the paper map. Typically, an existing paper map is scanned or digitized, separated into thematic layers, and attributed according to the map legend information. Due to the importance of positional accuracy, a great deal of effort is expended in 'quality control', i.e., ensuring that the source map's lines and polygons are accurately reproduced and attributed consistently. Although the cross-sections and other diagrams are often included as graphics files, they receive less attention. Consequently, the finished product accurately reproduces the map geometry and descriptive content, but with less emphasis on the interpretive information and geologic relationships.

THE PAPER MAP MODEL AND THE DIGITAL DATABASE

Many components of geologic maps are represented in digital databases. However, the product as a whole lacks the visual cohesiveness of the parent product. Although there is a visual component to GIS, the tools for exploring, querying, and analyzing digital data are not as visually oriented. GIS interprets geographic distribution and relationships through coordinate information and geometry. Consequently, database models must encode geologic relationships within this context.

This section outlines two conceptual issues that need to be addressed in order to improve geologic knowledge representation in digital databases: thematic separation of data layers and the geometric representation of geologic objects. (Note: For the purposes of this discussion, the terms 'feature' and 'object' have distinct meanings. An object generally refers to an entity that is identifiable by particular physical characteristics, relationships, and behaviors, while the term 'feature' generally refers to the geometric element used to represent that object.) Each issue is discussed separately, although in practice, they are interrelated and difficult to isolate. The context of this discussion is conceptual rather than practical; however, two recent publications (McRae, 1999; and Cannon, McRae, and Nicholson, 1999) provide some examples of how existing GIS tools and data structures can be implemented to address the issues presented here.

Thematic Separation of Related Geologic Features
Digital databases are frequently published as a series of files that contain different geologic 'themes'. Thematic separation is usually dictated by feature type (i.e., point, line or polygon) rather than by the geologic relationships between objects. For example, since faults are usually modeled as lines and geologic units as polygons, they are often placed in separate data layers. On a geologic map, of course, faults that act as geologic contacts would be represented by a single line segment and symbolized accordingly. Conceptually, this is an instance of a single feature having two functions (i.e., that of fault and contact). By placing faults and contacts in separate coverages, each function is effectively represented by a unique feature. This obscures the geologic interpretation. Further, database size is negatively impacted by unnecessarily maintaining the same feature in two separate data layers.

On a geologic map, the author controls the physical layout of individual components in order to facilitate the visual interpretation of the geologic relationships. Current database design practices require the user to reassemble individual components in some meaningful way. Recent policies adopted by the USGS have attempted to overcome this problem by recommending that a print quality graphic file of the geologic data be included with each dataset. This provides the database user with the opportunity to view the data as the author intended. Although this is a valuable visual reference, the issue of how to encode the author's interpretation within the database structure still needs to be addressed.

Cartographic Features Versus Geologic Objects
According to the geologic map data model (Johnson and others, 1999) adopted by the North American Data Model Steering Committee (http://geology.usgs.gov/dm), geologic objects in a database can be either singular or compound. Singular objects are said to be those that have been observed at a single location or are represented by a single cartographic feature. Compound objects are said to result from the interpretation or classification of multiple observations at multiple locations, such as a fault consisting of individual fault traces observed at multiple outcrops. The data model treats singular and compound objects differently. The geometry of singular geologic objects is stored directly in the Spatial Object Archive, while the geometric representation of compound objects must be formed by the aggregation of multiple features within the Spatial Object Archive. Although implementation details are left to the database designer, the examples cited in the data model generally use cartographic representation as the basis for modeling an object as singular or compound. This convention has been widely adopted in the production of digital databases.

A negative consequence of this practice is that what the geologist considers "singular" can become "compound" due to either the limitations of its analog geometry or the digitization process. For example, consider the case of a fault that has been offset by another. The geologist views the crosscutting fault as a singular object and the offset fault as a compound object consisting of two line segments. However, some GIS software packages place nodes at all line intersections. Consequently, both faults will be divided into multiple line segments, effectively creating two compound objects. Without some mechanism to 'reassemble' the crosscutting fault's segments back into a single feature, the geologic interpretation is obscured. Similar problems occur with polygonal data. Paper map constraints force geologic units to appear mutually exclusive, so that their cartographic representation reflects only that portion of the unit not covered by another. On a geologic map, a volcanic unit that underlies a sedimentary unit may appear as multiple, disjointed polygons where the sedimentary unit has eroded to expose it. Common symbolization, annotation, and accompanying cross-sections help inform the map user that the unit is contiguous at depth. Current database production practices typically digitize and attribute each polygon individually. Again, this fragmentation obscures the geologic interpretation that the individual exposures are really part of a single, underlying unit.

In some cases, an object's cartographic representation may serve as the foundation for its digital geometry, if combined with the appropriate data structure. The behavior of the crosscutting fault, for example, can be modeled by using network geometry to aggregate the individual line segments into a single feature. However, many cartographic representations fail to reflect the real geographic extent of the objects being modeled. This is particularly true for geologic units. For example, the aggregation of the volcanic unit's individual polygons would still misrepresent the geologist's knowledge of its distribution.

On a geologic map, any knowledge of the distribution or understanding of how one geologic unit relates to another will be based on an individual's ability to interpret the 3-dimensional distribution from the 2-dimensional representation. A GIS can interpret the distribution of an object only through coordinate information and geometric properties. Hence, the 3-dimensional framework must be encoded in a way interpretable by GIS software. A key to accomplishing that is to ensure that the geometry of an object fully reflects the geologist's knowledge of its distribution. In many cases, that will involve a geometry not constrained by an object's cartographic representation.

CONCLUSIONS

A recent article states, "With the adoption of GIS, many analog records have been computer encoded without considering the limitations of the underlying analog-oriented conceptual models. The result may be an accurate encoding of analog records, but it rarely will be a comprehensive model of reality given the inherent limitations of analog records... The new geospatial data management paradigm is about creating meaningful models that effectively capture the geographic knowledge that defines an organization's version of reality. It's much less about maps or how to convert all those old analog records in the back room." (Levinsohn, 2000). GIS also has its limitations, particularly in its ability to model true 3-dimensional relationships. However, technological advances continually provide new tools for the modeling, visualization, analysis, and publication of spatial data. As GIS tools continue to evolve, so will our ability to model the behavior and relationships of the geologic environment. Despite these advances, a paper map model continues to dominate the design and production of geologic databases. Although geologic maps have been effective tools for communicating geologic data, they are an ineffective model for digital data. The unique properties and constraints of GIS must be considered in developing databases that adequately model our knowledge of the geologic world.

REFERENCES

Bernknopf, R.L., Brookshire, D.S., Soller, D.R., McKee, M.J., Sutter, J.F., Matti, J.C., and Campbell, R.H., 1993, Societal Value of Geologic Maps: U.S. Geological Survey Circular 1111, p. 13-14.

Cannon, W.F., McRae, M.E., Nicholson, S.W., 1999, Geology and Mineral Deposits of the Keweenaw Peninsula, Michigan: U.S. Geological Survey Open-File Report 99-149, https://pubs.usgs.gov/openfile/of99-149/.

Johnson, B.R., Brodaric, B., Raines, G.L., Hastings, J.T., and Wahl, R., 1999, Digital Geologic Map Data Model: AASG/USGS Geologic Map Data Model Working Group Report, p. 13, http://geology.usgs.gov/dm/model/Model43a.pdf.

Levinsohn, A.G., 2000, Use Spatial Data to Model Geographic Knowledge: GEOWorld. v.13, no.4, p. 28.

McRae, M.E., 1999, Using Regions and Route Systems to Model Compound Objects in Arc/Info, in D.R. Soller, ed., Digital Mapping Techniques '99 -- Workshop Proceedings: U.S. Geological Survey Open-File Report 99-386, p.195-196, https://pubs.usgs.gov/openfile/of99-386/mcrae.html.

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