U.S. Geological Survey Open-File Report 2005-1428

Digital Mapping Techniques '05—Workshop Proceedings

NGMDB Geologic Map Feature Class Model

By Stephen M. Richard,1 Jon A. Craigue,2 and David R. Soller3

1Arizona Geological Survey, 416 W. Congress #100, Tucson, AZ 85701
Telephone: (520) 770-3500; Fax: (520) 770-3305; e-mail: Steve.Richard@azgs.az.gov

2U.S. Geological Survey, Earth Surface Processes Research Institute, Tucson, AZ
e-mail: jcraigue@espri.arizona.edu

3U.S. Geological Survey, 926-A National Center, Reston, VA 20192
e-mail: drsoller@usgs.gov

BACKGROUND

This document describes the spatial data feature classes in the prototype design for the National Geologic Map Database project (NGMDB; http://ngmdb.usgs.gov). The implementation of thematic data table in the NGMDB prototype is described in a separate document (Richard et al., 2004). The design presented here has been submitted as an ESRI geology data model, and is available from the ESRI support web site (see http://support.esri.com/index.cfm?fa=downloads.dataModels.filteredGateway&dmid=30, and find the NGMDB database design documents link).

Geoscience Entities of Interest

As a precursor to defining feature datasets and feature classes in an ESRI geodatabase, this section enumerates the geologic entities of interest and the spatial relationships between them. The term "entity" is used here to denote phenomena of interest observed in the 'real world', as opposed to features, which are the database objects implemented to represent our understanding of those phenomena. A feature in ESRI geodatabase usage is explicitly required to have a geometry property that specifies a location and shape. This discussion uses terminology and basic definitions from the NADM C1 model (NADMSC, 2004).

Geologists are concerned with the three dimensional arrangement of material within the Earth. The entities of interest are bodies of material (geologic units) and surfaces that bound or cut them (geologic surfaces). The 2-D map view that is the framework for a GIS represents the geometry of the intersection of these entities with some map horizon—typically the Earth's surface, but possibly some abstract surface like a mine-level, cross section surface, or some buried surface (e.g. top of Precambrian rock). The basic features that may be implemented in a 2 (or 2.5)-D GIS are points, lines, and polygons (and composite features aggregated from these simple features). Points represent the intersection of a line with the map horizon (e.g. a borehole collar), or an observation location on the map horizon (a station). Lines represent the intersection of a surface with the map horizon (surface trace), the projection of some buried line beneath the map horizon (e.g. the cutoff of a contact at a fault, an inclined borehole, a channel axis), or a line defined within the map horizon (e.g. sand dune crest, geomorphic escarpment). Polygons represent one of several situations, including the intersection of bodies with the map horizon (i.e. the outcrop of geologic units), patches defined on the map horizon, the projection of patches on a surface other than the map horizon into the map plane, or the projection of 3-D bodies that do not intersect the map horizon into the map plane.

The following discussion elaborates on this basic framework to define the entities of interest that need to be mapped into feature classes, feature datasets, and topology rules in an ESRI geodatabase implementation.

GEODATABASE FEATURE CLASSES

Table 1 summarizes ESRI geodatabase feature classes used to specify location in the NGMDB implementation. All spatial data tables include fields to specify a default text label and symbol to use in map displays if no other symbolization is specified. This is to simplify the rapid display of spatial data. OutcropBoundaryTrace and GeologicUnitOutcrop are line and polygon feature classes whose locations represent observable geologic phenomena in or on the Earth. Station is a point feature class that specifies a location at which observations were made, and does not (inherently) represent the location of some phenomena. The term observation is used in the sense of GML Observation and Measurement (Cox et al., 2004). These feature classes are organized according to their semantics, associated properties, and implementation issues for using topology rules in the ESRI geodatabase environment.

 

Table 1. Location specification tables.
Table Description
OutcropBoundaryTrace
Line features that represent the intersection of geologic surface that bounds mapped rock bodies with the depicted map horizon. Subtypes include fault and geneticBoundary. These traces may bound GeologicUnitOutcrop polygons.
DikeVeinMarkerTrace Line features that represent the intersection of a dike, vein, or marker bed with the depicted map horizon; subtypes differentiate these cases.
HingeSurfaceTrace Line features that represent the intersection of the hinge surface of a fold with the depicted map horizon.
EarthFissureTrace Line feature that represents a fissure in the depicted map horizon.
ConcealedBoundaryTrace Line feature that represents the trace of a geologic surface in a map horizon different from the depicted map horizon. Subtypes include concealed faults and genetic boundaries, and structure contours.
GeologicUnitOutcrop Polygons representing the intersection of a geologic unit with the map horizon.
Station Point location at which one or more observations are made, or samples are collected.
BoreholeCollar Point location at which a borehole section intersects a map horizon (typically the Earth surface).

 

Notes on Schema Notation and Implementation

The schema included in this paper use a UML profile defined by ESRI to build UML models for geodatabase physical implementation using CASE tools. Classes with names in italics represent abstract classes, i.e. there is no feature class in the geodatabase with the same name. Attributes of abstract classes are included in all subtype classes. UML subtype links have an open triangle at the parent class end of the link. Attributes in the UML classes represent fields in the geodatabase tables. In the models here, attribute names that end in 'GUID' represent database fields populated by 36-character string GUIDs (Globally Unique IDentifiers). ESRI geodatabase subtypes are linked to their parent class (which represents the physical table implemented in the database) using dashed lines. ESRI subtypes represent subsets of records in a particular relational database table that are differentiated based on an integer value in one of the fields identified as the 'subtype' field. In the NGMDB implementation, ESRI subtype field names always begin with a prefix "ESRISubtype". The undecorated solid lines linking classes in the diagrams represent associations implemented as ESRI relationship classes in the geodatabase. These associations are navigable in the ArcMap attribute browser.

 

Top level features in the NGMDB implementation

Figure 1. Top level features in the NGMDB implementation. GeologicUnitOutcrop polygons are associated with bounding OutcropBoundaryTrace arcs (LPoly and RPoly of Arc/Info coverages) explicitly through the OutcropBoundary correlation table. PrincipalUnitOutcrop and SuperimposedUnitOutcrop are ESRI subtypes of GeologicUnitOutcrop, discriminated using the ESRISubtype_GeologicUnitOutcrop field. See Figure 3 for some other MappedOccurrence subtypes.

 

Figure 1 shows the top level hierarchy of feature classes in the geodatabase implementation model. The ESRIClasses::Feature and NGMDBFeature classes define fields shared by all spatial data classes (see Richard et al., 2004 for discussion of standard NGMDB fields). The feature classes are divided into broad groups represented by abstract classes beneath NGMDBFeature in Figure 1. ObservationLocation represents features located based on human, observation factors—e.g. where access is possible, what part of a mountain could actually be seen, where the airplane flew. Their location is typically related to geologic phenomena of interest, but their location is not determined by the location of the phenomena. MappedOccurrence includes features whose location is determined by phenomena inherent in the earth—contacts between rock bodies, fault zones, fold hinges, etc. GeologicRoute features aggregate MappedOccurrences that are interpreted to represent the traces of extended geologic surfaces identified based on multiple observations. AnnotationFeatures position annotation in map displays. The location of these features is related to a MappedOccurrence or ObservationLocation, but the actual positioning is determined by cartographic considerations. BoreHoleCollar is in a sense a sort of MappedOccurrence, but because the properties of interest are different, they have been implemented as a distinct feature class. The following discussion first treats the subtypes of MappedOccurrence, followed by GeologicRoutes, ObservationLocations, and AnnotationFeatures.

Surface Traces

Surface traces are the lines in a GIS that represent intersection of 3-D surfaces with a map horizon. Thus a surface trace always has an explicit or implicit defining map horizon (MapHorizon in MappedOccurrence, Figure 1), and a classifier that specifies the kind of surface that intersects the map horizon to form the trace. The surface trace has associated location uncertainty related to how discretely the mapped surface may be located (e.g. sharp or gradational contact), how precisely the location can be determined (good or poor exposure), and how accurately that location can be specified in map coordinates (Figure 2).

 

Geologic surface trace feature classes

Figure 2. Geologic surface trace feature classes. These feature classes represent the intersection of geologic surfaces with a map horizon. The OutcropBoundaryTrace features participate in polygon topology with the GeologicUnitOutcrop.PrimaryUnitOutcrop features (Figure 1).

 

Surface traces that are in the depicted map horizon

A geologic map is assumed to portray surface traces and outcrops on some particular map horizon. The depicted map horizon may be different in different parts of the map. For example the current Earth surface may be the map horizon except in the area of a large mine, where the pre-mining surface may be depicted. All of these surface trace types may have elevation values (Z values in geodatabase) associated with vertices along the trace; these elevations represent the elevation of the map horizon.

 

Other mapped occurrence feature classes

Figure 3. Other mapped occurrence feature classes. These are less frequently encountered mapped occurrences not shown in Figure 1.

 

Surface traces not in the depicted map horizon

In either case the topology rules are similar: may not self intersect, no dangles. Concealed GeneticSurfaceTraces are not required to be covered by the boundary of a GeologicUnitOutcrop—they do not participate in geologic unit polygon topology on the map horizon depicted. Concealed GeneticSurfaceTraces may intersect other GeneticSurfaceTraces (e.g. boundaries of mapped covering geologic unit outcrops), but may not intersect FaultTrace, DikeTrace, VeinTrace or Concealed GeneticSurfaceTrace, Concealed FaultTrace, Concealed DikeTrace or Concealed VeinTrace.

Outcrop

Outcrop is used here in the very specific sense of the intersection between a geologic unit and a map horizon, whether or not the geologic unit is exposed on that horizon. The term exposure is used to refer to places where the geologic unit is visible on the map horizon. Outcrops are represented by polygons in the GIS. An outcrop always has an explicit or implicit defining map horizon, and a classifier that specifies the geologic unit that intersects the map horizon to form the outcrop.

Routes

Collections of surface traces classified to belong to a single 'broader' classification entity. The individual surface trace instances in the SurfaceTrace feature classes are differentiated based on their classification (depositional contact, thrust fault, intrusive contact, facies boundary . . .), and observation-related properties (classification confidence, location confidence, observer, depiction scale, observation method . . .). These may be considered together to represent the trace of some geologic entity. These aggregated features are treated separately from the MappedOccurrences because they are fundamentally interpretive, and have different metadata properties.

 

Geologic route feature classes

Figure 4. Geologic route feature classes. These feature classes are used to aggregate line segments (arcs) differentiated based on observation-related properties (information source, locatability, location uncertainty) into traces of surfaces that have identity and are bounded by intersection with other surfaces (Fault, Fold). These may then be aggregated into systems that represent compound structures that include multiple faults or folds.

 

Observation Location Features

Observation location features have geometry that records the location at which some data or sample was collected. The NGMDB implementation allows structural measurements, text notes, images, and samples to be associated with observation locations. Stations are point features that locate observation sites, SectionLine lines and AreaOfInterest polygons represent extended observation sites. SectionLines may represent traverses, which are observation sites located in the depicted MapHorizon. BoreHoleProjection and FlightlineGroundTracks represent observation sections that do not lie in the depicted map horizon; they are projected into the map plane to depict in a 2-D image. The implementation revolves around the concept of an observation section as a linear analog of a map horizon surface. The observation section is a line in three dimensional space that provides a reference frame in which three dimensional points may be located using a single coordinate, measured along the observation section line from some defined origin (e.g. depth below kelly bushing in a bore hole). In the NGMDB implementation, this reference frame is described informally (as text) in the CoordinateReferenceSystem field (ObservationSection, Figure 5).

 

Observation location features

Figure 5. Observation location features. These (the classes on the left side of the diagram) are mapped features that have geometry that records the location at which some data or sample was collected. Stations are point features that locate observation sites. AreaOfInterest polygons represent an extended observation site in some map horizon. SectionLine lines are the projection of 3-D observation sections into a map horizon.

 

Borehole Representation

Figure 6 shows the principal geodatabase object classes associated with BoreHoleCollar points to construct a representation of subsurface geologic information derived from boreholes. This model uses the same pattern to represent data from surface traverses (e.g. measured sections), and flight lines (or ship tracks . . .), but only the treatment of borehole sections is discussed here.

One or more boreholes may be associated with a single BoreHoleCollar point, because a collar may be reentered to drill a splay or side track, which is considered a separate borehole. Individual boreholes (or any observation sections) have collections of associated intervals (SectionInterval table) and intercepts (SectionIntercept table) that may have associated description data. Intervals have a top and bottom, and represent rock volumes, while intercepts have a single coordinate location in the observation section, and represent the intersection of a surface with the section, or a point location in the earth. The NGMDB implementation allows classification of an interval (or an intercept, link not shown in Figure 6) through an ObservationRelationship instance, analogous to mapped occurrence alternative classification (Figure 7). This allows assignment of intervals to geologic units, lithology classes, or any other sort of classification (e.g. aquifer), and association of intercepts with multiple surface classifications. StructureObservation (orientation) data, free text descriptions, and samples may also be associated with intervals or intercepts.

 

Observation sections: borehole implementation

Figure 6. Observation sections: borehole implementation. BoreHoleCollar is a point feature class in the geodatabase that represents the intersection of a borehole with a map horizon. Borehole is a geodatabase object class. The ShapeSysGUID field in Borehole is an NGMDB-defined link to a three dimensional description of the borehole geometry. This is a hook to provide compatibility with three-dimensional models. Borehole is a kind of ObservationSection (Figure 5, see also https://www.seegrid.csiro.au/twiki/bin/view/Xmml/BoreHole, and discussion in text here).

 

 

Observation relationship links for alternative classification of mapped features

Figure 7. Observation relationship links for alternative classification of mapped features. Default (primary) classification is assigned by ClassifierTermGUID/ClassifierEntityGUID attribute tuple in the feature class table. The primary classification records the original intention with which the mapped feature was delineated. Alternative classifications are used for derivative maps produced by grouping the primary classifiers to define a different classification scheme. The ObservationRelationship correlation table uses sysGUID/entityGUID pairs to identify the type of feature class and actual data instance for the source and the type of classifier and particular classifier term for the target of the relationship. ObservationRelationship also has properties on the relationship (classification) instance to assign a confidence and other metadata information.

 

Classification and Descriptions Associated with Features

The NGMDB implementation includes a link to a classifier term in each geodatabase feature class for MappedOccurrences (Figure 1). This primary classification records the original intention with which the mapped feature was delineated, and is thus considered to inhere more strongly with the geometric description than other possible classifications. For GeologicUnitOutcrops, the default or primary classifier will be a GeologicUnit term. Outcrop trace and observation features will typically be classified using terms from a science vocabulary (ScienceLanguage table). Alternative classifications are used for derivative maps or analytical processes, and are produced by grouping the primary classifiers to define a different classification scheme. Such derivative or alternative classifications are represented using classification-typed data instances in the ObservationRelationship correlation table (Figure 7).

Any of the various kinds of descriptions implemented in the NGMDB database may have an associated feature that specifies a geographic extent over which the properties in the description are asserted to hold (Figures 8-10). The feature assigned as the extent for a description may be an ObservationLocation feature or any of the various MappedOccurrences.

Geographic extents associated with geologic unit description Geographic extents associated with geologic structure description Structure observation description extent

Figure 8. Geographic extents associated with geologic unit description. The extent associated with a description indicates the geographic region within which the description is asserted to be valid. The associations shown are implemented as geodatabase relationship classes, allowing navigation from mapped features (e.g. GeologicUnitOutcrop) to associated descriptions in the ArcMap interface. The foreign key from GeologicUnitDescription to the mapped feature is DescriptionExtentSysGUID, which links to the primary key (SysGUID) in the mapped feature. Details of GeologicUnitDescription are discussed in Richard et al. (2004).

Figure 9. Geographic extents associated with geologic structure description. The extent associated with a description indicates the geographic region within which the description is asserted to be valid. The associations shown are implemented as geodatabase relationship classes, allowing navigation from mapped features (e.g. FaultTrace) to associated descriptions in the ArcMap interface. The foreign key from GeologicStructureDescription to the mapped feature is DescriptionExtentSysGUID, which links to the primary key (SysGUID) in the mapped feature. Details of GeologicStructureDescription are discussed in Richard et al. (2004).

Figure 10. Structure observation description extent. Structure observation is a geodatabase 'object class' that is used to record orientation measurements for geologic structures. The extent associated with a structure observation indicates the geographic region within which the orientation measurement is asserted to be valid. The associations shown are implemented as geodatabase relationship classes, allowing navigation from mapped features (e.g. Station) to associated orientation measurements in the ArcMap interface. The foreign key from StructureObservation to the mapped feature is LocationSysGUID, which links to the primary key (SysGUID) in the mapped feature. Details of StructureObservation attributes are discussed in Richard et al. (2004). ObservationRelationship links to StructureObservation are used to correlate related observations (e.g. foliation in lineation, cleavage axial planar to fold).

Annotation

These are locations in a map view used to position graphical elements for cartographic display. The location of the symbol is related to some geologic feature, but its actual positioning is based on cartographic consideration. In the ArcMap v.9 implementation, annotation that is a text string used to provide supplemental information for spatial objects (point, line, polygon) is best represented using feature-linked annotation feature classes that are built into the Geodatabase. For symbols that are located by points, and identified by a symbol identifier (CartoObjID), the implementation includes a PointAnnotationFeature class. This is subtyped for different kinds of annotation based on relationships to other feature classes.

 

Geoland demonstration map

Figure 11. Geoland demonstration map. Original is in color, converted to grayscale for display in print.

 

Orientation Data

Structure observation is a geodatabase 'object class' that is used to record orientation measurements for geologic structures. The extent associated with a structure observation indicates the geographic region within which the orientation measurement is asserted to be valid (Figure 10). The most common application is the association of common bedding or foliation measurements with a Station feature. This model makes a clear separation between the observation location—a station, and the data acquired at the location (a structure observation). For cartographic purposes, for example to display a strike and dip symbol, a MapAnnotationPoint is created using the station location and the azimuth value to rotate the symbol and text label for the symbol are determined from the related structure observation data. The symbol can be repositioned for cartographic purposes without losing information about the actual location at which the measurement was made.

EXAMPLE MAP

Figure 11 is a geologic map of an imaginary study area, based on a demonstration geodatabase designed to exercise various capabilities of the NGMDB design. Many of the geologic unit names and general character are based on units found in Arizona, but this is just a convenience to make easier the generation of descriptions. Figure 12 is the ArcMap table of contents for layers displayed in Figure 11; each layer is described below:

ArcMap table of contents showing layers displayed in map shown in Figure 11 Map annotation points, StructureObservation events, and MapAnnotationPointAnno layers

Figure 12. ArcMap table of contents showing layers displayed in map shown in Figure 11.

Figure 13. Map annotation points, StructureObservation events, and MapAnnotationPointAnno layers.

GeologicUnitOutcrop and GeologicUnitOutcropAnno layers Geomorphic feature trace (fault scarp), hinge surface trace, and fault route layers

Figure 14. GeologicUnitOutcrop and GeologicUnitOutcropAnno layers.

Figure 15. Geomorphic feature trace (fault scarp), hinge surface trace, and fault route layers. The fault route is used to place the triangular decorations along the thrust fault so that the spacing between triangles is regular, because the decorated line has no pseudonodes.

Superimposed geologic unit polygon and concealed boundary trace layers Outcrop boundary trace layer

Figure 16. Superimposed geologic unit polygon and concealed boundary trace layers.

Figure 17. Outcrop boundary trace layer.

OUTSTANDING ISSUES

Major implementation questions that must be addressed in developing a multiple map database, of the sort envisioned for a geological survey enterprise archive, include:

One approach that we are experimenting with would group features into different feature datasets to represent different map horizons or resolution (generalization). Related, but loosely coupled geologic features like alteration map units would be represented using subtypes of the GeologicUnitOutcrop polygons, perhaps with a separate set of topologic rules from the principal unit outcrop boundary traces and outcrop polygons.

REFERENCES

Brodaric, B., and Hastings, J., 2001, Evolution of an Object-Oriented, NADM-Based Data Model Prototype for the USGS National Geologic Map Database Project [web page, abstract]: Annual Conference of the International Association for Mathematical Geology, IAMG2001, Cancun, Mexico, accessed at http://www.kgs.ku.edu/Conferences/IAMG/Sessions/I/brodaric.html.

Brodaric, Boyan, and Hastings, Jordan, 2002, An object model for geologic map information, in Richardson, D., and van Oosterom, P., eds;, Advances in Spatial Data Handling, 10th International Symposium on Spatial Data Handling: Heidelberg, Germany, Springer-Verlag.

Brodaric, B., Journeay, M., Talwar, S., and others, June 18, 1999, CordLink Digital Library Geologic Map Data Model Version 5.2 (Web Page), available at http://cordlink.gsc.nrcan.gc.ca/cordlink1/info_pages/English/dm52.pdf, accessed June 13, 2001.

Cox, Simon, Daisey, Paul, Lake, Ron, Portele, Clemens, Whiteside, Arliss, eds., 2004, OpenGIS® Geography Markup Language (GML) v. 3.1.0, Implementation Specification: OpenGIS® Recommendation Paper, Document OGC 03-105r1, ISO/TC 211/WG 4 Document 19136, 02-07-2004, 601 p., accessed at http://www.opengeospatial.org/specs/?page=recommendation.

Johnson, B.R., Brodaric, B., and Raines, G.L., 1998, Digital Geologic Maps Data Model, V. 4.3 (Web Page): unpublished report by AASG/USGS Geologic Map Data Model Working Group, accessed at http://www.nadm-geo.org/dmdt/.

NADMSC (North American Data Model Steering Committee), 2004, NADM conceptual model 1.0, A conceptual model for geologic map information: U.S. Geological Survey Open-File Repot 2004-1334, 60 pages text, 1 Adobe Acrobat (pdf) file, accessed at http://pubs.usgs.gov/of/2004/1334/.

Richard, S.M., 2003, Geologic map database implementation in the ESRI Geodatabase environment, in Soller, D.R., ed., Digital Mapping Techniques '03—Workshop Proceedings, U.S. Geological Survey Open-File Report 03-471, p. 169-183, accessed at http://pubs.usgs.gov/of/2003/of03-471/richard2/index.html.

Richard, S.M., and Orr, T.R., 2001, Data structure for the Arizona Geological Survey Geologic Information System-Basic Geologic Map Data, in Soller, D.R., ed., Digital Mapping Techniques '01—Workshop Proceedings, U.S. Geological Survey Open-File Report 01-223, p. 167-188, accessed at http://pubs.usgs.gov/of/2001/of01-223/richard2.html.

Richard, S.M., Craigue, J., Soller, D.R., 2004, Implementing NADM C1 for the National Geologic Map Database, in Soller, D.R., Editor, Digital Mapping Techniques '04—Workshop Proceedings, U.S. Geological Survey Open-file Report 2004-1451, p. 111-144, accessed at http://pubs.usgs.gov/of/2004/1451/pdf/richard.pdf.

Soller, D.R., Brodaric, Boyan, Hastings, J.T., Wahl, Ron, and Weisenfluh, G.A., 2002, The central Kentucky prototype: An object-oriented geologic map data model for the National Geologic Map Database: U.S. Geological Survey Open-File Report 02-202, 39 p., available at http://pubs.usgs.gov/of/2002/of02-202/.


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