Definition of DLG-E Features
The DLG-E data model has a variety of components for representing a feature. However, before describing how those components are used to represent any given feature, the domain of features (that is, the data reality) to be described by the model should be discussed. The report “Proposed Definition of Cartographic Features for Digital Line Graph Enhanced (DLG-E)” (April 1988), by the Committee Investigating Cartographic Entities, Definitions, and Standards, defined an initial domain of features. A synopsis of that work is incorporated in this report. In preparing the set of DLG-E features, the committee reviewed related efforts including those of the Digital Cartographic Data Standards Task Force (1988), the Defense Mapping Agency (1987), the Canadian Council of Surveying and Mapping (1982), the Bureau of Land Management (1978), and the South African Natural Research Institute for Mathematical Sciences (1987).
Methodology for Identifying Features
Persons reading a map perceive features according to their needs and experiences. For example, one may be interested in the extent of a given named feature, such as the Potomac River or the Blue Ridge Mountains. Others may have a different interest and employ a different set of criteria. They may be interested in bicycling from Mount Vernon, Va., to Leesburg, Va., and would be concerned with a connected network of roads and trails between the two locations. In a digital environment, one attempts to explicitly encode information that best satisfies most users. The designers are faced with the problem of specifying a set of information in a form that can be perceived unambiguously by the majority of users. This specification process can be viewed as a process of classification. An expandable method of classification is necessary when defining features.
Classification involves the selection and grouping of phenomena into classes on the basis of common properties or relationships. The selection of a classification system has implications beyond the definition of common names for things. It is the classification scheme that defines the nature of the generalizations that are made about the phenomenon under study (Abler and others, 1971). Note the linkage between the classification of map features and the concept of the map as a model. As stated by Board (1967), “it is important to realize that they [maps] are also conceptual models containing the essence of some generalization about reality.” In the map model, the characteristics of the classification scheme and the resulting features specify this essence of the generalization of reality.
In the DLG-E feature classification process, an approach was taken that initially subdivided the world of geographic phenomena into five classes, termed views, that correspond to the major facets of a model of geographic reality as described by Geological Survey requirements and specifications. A view is defined as a systematic classification of a set of entities in which all members of the set possess a common defining characteristic. This characteristic is specified explicitly in the definition of the view. Views are similar to the geographical concepts of region and spatial system (James and Jones, 1954; James, 1972) and “geographic matrix” (Berry and Marble, 1968). Common cartographic terms such as category and overlay are related to views in derivation but lack the rigor of the defining characteristic.
While views provide a methodology for classifying features, it is the classification scheme that defines the nature of the generalizations about the phenomenon under study. These generalizations are reflected in feature definitions. For example, a road is an entity that covers a part of the Earth’s surface. In the DLG-E methodology, it is classified in a view called “cover,” a view with the defining characteristic of material at a location on or near the surface of the Earth. Other features in the view “cover” include “building,” “bridge,” “railway,” and “grassland.” A “county” is not a feature in the view “cover” because it is not covering material on the Earth’s surface. A county is in the view “division” because it is a political entity independent of the actual material on or near the Earth’s surface. Other examples of features in the view “division” include “state,” “city,” “reservation,” and “census block.”
New views are introduced in the methodology when an entity cannot be appropriately classified in existing views. For example, a contour line is an entity that does not fit the views “cover” or “division.” A contour line reflects measurement data about the Earth’s surface. The feature “contour” requires a view in which measurement data are the focus. That same view is needed to classify features such as “control station” and “spot elevation.” Thus, a new view called “geoposition” that reflects entities of measurement was created.
In an approach similar to creating views, finer divisions of a view, called subviews, may be created to further refine the concepts included in the view. The view and subview approach allows a user to fit unclassified entities into the appropriate part of the schema. It also provides a framework for comparison to other sets of features as defined by other organizations.
While the approach uses the concept of multiple views of the world and a hierarchy of subviews within each view, neither the views nor the hierarchy need be stored with feature data. Nor is it necessary to group the features into categories. The views and subviews are used only to define the domain of features. Alternative views will yield different features, and features within a single view can be modified or expanded. Users can apply other views or hierarchies to add new features to the list. Use of features in the domain and use of the data require limited information concerning the approach methodology.
This approach allows for expansion by creating a completely different view, adding new subviews, or augmenting the features within the existing five views. To classify a new entity, one
would first determine whether an existing view, subview, feature, attribute, or attribute value is appropriate by comparing the characteristics of the new entity to existing definitions in the domain. After making this comparison, the new entity would be placed in the correct level of the hierarchy either by creating new views or subviews, or by expanding the list of features, attributes, or attribute values. Some examples are given below.
Large-scale mapping of base category information considers entities not currently listed in the domain as features. A specific instance is a “curb.” Examining the views, a curb would be classified as cover. Within the view “cover,” the curb is considered to be in the subview “built-up land” and a “structure.” In the subview “structure,” entities are differentiated into features based on form, examining the list of features under “structure,” a curb does not fit any of the existing feature definitions, and, thus, a new feature must be created.
A second example from a large-scale product is a single housetrailer with a permanent foundation. A housetrailer is a type of cover, built-up land, and a structure. The feature “building” encompasses house- trailers, and the attribute “text” with a value of “house- trailer” is appropriate. An alternative would be to institute a new attribute such as “building construction type” with one possible value being “housetrailer.”
Expanding the number of views can be used to encompass features such as those that occur on geologic or other thematic maps. A geologic or other thematic view would require complete generation of the hierarchy within the view and the development of a list of features and attributes for that view. The resulting set of features then becomes a part of the total list of features independent of the approach methodology and completely usable with the other features in the total set.
The current DLG-E model classifies selected entities into five views on the basis of common defining characteristics. The five views are cover, division, ecosystem, geoposition, and morphology. The views are exclusive; each view reflects a self-contained analytical approach to world features. Because each view is independent, a single point on the surface of the Earth can be represented under multiple views, and features within any one view may coexist with features in the same or any other view.
For example, examine the entity “boundary point.” If a monument exists at the location of the boundary point, then two features will be recorded in the data. The feature “boundary point,” referring to the monumented location on a boundary, is in the view “division” while the second feature, “point monument,” denoting the structure on a boundary line, is in the view “cover.” Brief definitions of the five views follow:
Cover: Reflects physical or material features at a location on or near the surface of the Earth. While this view is based on form, at the lowest level features it may be differentiated by function.
Division: Reflects cultural demarcations of the Earth’s surface for a particular purpose or for separations resulting from human activity.
Ecosystem: Based on climate, vegetation, soils, and other controlling environmental factors that result in unique ecological units.
Geoposition: Reflects measurement data about the Earth’s surface and contains points or lines on the Earth or its representation for which the location, relative to a particular datum, is well known.
Morphology: Based on the form of the land. While a strict geomorphological interpretation of landforms based on process is inherent in the contour information on Geological Survey maps, the actual features must be interpreted. However, some of the features carry names and those features must be coded in the digital data. Thus, morphological features are those landform features that are named, labeled, or symbolized as distinct entities on current map products.
Discussion of Views
The following section offers a more detailed discussion of each view and includes definitions of subviews.
The view of cover reflects physical or material features at a location on or near the surface of the Earth. This view contains a mixture of land use and land cover information. Multiple features derived from this view may occupy the same location.
Because of the diversity in the features defined by this view, additional subviews were added to clarify the distinctions between features. There are five subviews, based on the land use and land cover terms and definitions recommended by the Department of the Interior Land Use Land and Cover Common Terminology Work Group (Department of the Interior, 1985).
· Barren land: A surface composed of exposed bare rock, other earthen material, or ice with little or no vegetation.
· Built-up land: Structures and areas associated with intensive land use. This subview is further divided into network, structure, and complex.
o Network--An interconnected set of constructions used for transportation or communication.
o Structure--A construction having a unique form.
o Complex--Cover of intensive use with much of the land covered by constructions.
Agricultural. A group of associated structures functioning as a
unit used predominantly for the production of food and fiber, such as livestock holding areas, fish hatcheries, and other developed land.
Commercial. A group of associated structures functioning as a unit used predominantly for wholesale and (or) retail sale of goods and services.
Entertainment/Recreational/Memorial. An area or group of associated structures functioning as a unit used predominantly (1) for leisure activities, (2) for athletic or artistic events, (3) as archaeological or historic sites, or (4) for burial of the dead.
Disposal. A designated area where refuse is dumped or exists.
Extraction. An excavation or a group of excavations or drillings in the Earth for the purpose of removing earth materials.
High density building area. A congested, built-up area where all buildings cannot be represented on the map because of map scale.
Industrial. A group of associated structures functioning as a unit used predominantly for manufacturing, testing, processing, or storage.
Institutional. A group of associated structures functioning as a unit used predominantly for educational, correctional, governmental, medical, or religious purposes.
Residential. A collection of structures used for human habitation.
Transition. Area in change from one land use activity to another and characterized by a lack of information to about future or use.
Transportation. An area or group of structures that function as a unit associated with travel or conveyance of people and (or) goods, together with the necessary adjacent facilities.
Utility. An area or group of structures that function as a unit to provide a public service and are used for the generation and (or) transportation of communications, water, gas, oil, or electricity.
· Cultivated cropland:Areas characterized by function that are tilled and dominated by vegetation growth for the production of food and (or) fiber. Cultivated cropland includes fallow land, land in any stage of annual crop production, and land being regularly cultivated for production of crops from perennial plants.
· Vegetation: An area that is extensively covered with plant life.
o Grassland--An extensive noncultivated area where vegetation is dominated by grasses or grass-like plants.
o Shrubland--Areas covered with low-growing or stunted perennial vegetation, such as cactus, mesquite, or sagebrush, common to arid regions and usually not mixed with trees.
o Forestland--Areas on which vegetation is dominated by woody perennial plants having a single, usually elongated main stem and generally few or no branches on its lower part.
· Water: Cover composed of flowing or standing water, impounded or naturally occurring, with channels or basins that are largely naturally occurring.
This view includes the cultural demarcations of the Earth’s water and land surfaces. Two types of features exist in this view: areal divisions and boundaries. Boundaries may either delimit the areal divisions or, for historical reasons, occur as independent features. The subviews of the division view are defined as follows:
· Administrative: A division under the jurisdiction of a common group for purposes such as preservation or exploitation of cultural or natural resources.
· Boundary: Part or all of a bounding or separating line on the Earth’s surface having current or past significance.
· Census: Divisions of the Earth’s surface established by the Bureau of the Census for enumerating and reporting the population of the United States.
· Hydrologic unit: Divisions of the Earth’s surface established by the U.S. Geological Survey based on properties, distribution, and circulation of water.
· Land parcel: Divisions of the Earth’s surface based upon land ownership.
· Locale: A named place not otherwise categorized.
· Maritime: Divisions of the Earth’s water surface identified for the purpose of navigation or control of ship traffic.
· Political: Divisions of the Earth’s surface based upon governmental jurisdiction and activities such as voting and taxation.
· Survey system: Divisions of the Earth’s surface to determine and delineate the form, extent, and position of land tracts by taking linear and angular measurements.
Ecosystem is a view based on climate, vegetation, soils, and other controlling environmental factors that result in unique entities. These entities are often of large extent and mapped as land use/land cover classes or as named places. Tundra, desert, and wetland are included in this view.
Geoposition contains features associated with the measurement of the size and shape of the Earth. The view reflects points or lines on the Earth or its representation for which the location, relative to a particular datum, is well known.
Morphology is a view based on the form of the land surface. A domain of features has been developed that reflects morphology as presented on Survey map products. Although morphologic features appear on map graphics through hypsography, within the scale limitations of the products, only those features that are named, labeled, or otherwise symbolized as unique entities appear in the feature list. While these features may require interpretation of boundary limits, the text and symbol information distinguish them.
Attempts were made to organize morphologic features according to classical geomorphological treatments including processes such as erosion and deposition and generating agents such as glaciers, streams, wind, or volcanoes. While these approaches yield consistent features and correlate directly with geomorphological theory and practice, the information required to make these types of feature distinctions is not available on map source materials without significant effort in the interpretation of hypsography. The level of knowledge necessary to perform this interpretation requires a skilled geomorphologist. Therefore an alternate approach was taken whereby the morphology of the land surface was organized directly into a domain of features that can be readily obtained from map graphics. The feature list reflects the named and symbolized entities organized and grouped to account for aliases such as cliff and bluff, or valley and hollow.