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DLG CODING
DEVELOPING A FEATURE ATTRIBUTE CODING SYSTEM The USGS digitizes maps and encodes the resulting Digital Line Graph Data (DLGs) using a unique 7-digit attribution code system, adding another level of interpretation. Since DLG collection is a map interpretation process, it is not possible to provide absolute rules and guidelines to account for all possible situations. A standard such as the one the USGS has developed (detailed below), however, will help you provide information that will allow reasonable decisions to be made about how to encode most of the information shown on maps. Each attribute code identifies the general data category to which a feature belongs, as well as the specific nature of the feature. Codes also provide additional descriptive information. Many features are uniquely described by a single attribute code. Others may require two or more attribute codes for a complete description. If multiple attributes are needed to describe a feature, their ordering is significant only if ordering is necessary to indicate a relationship between two codes and avoid ambiguity. General Coding Structure Each attribute coding description typically contains the following information:
Major Codes The third digit of the major code is used to designate the interpretation of the minor code in two ways: if it is zero, the minor code numbers represent a description or classification of a specific feature, for example, 050 0300, which identifies a spring. If it is not a zero, the minor code numbers have special interpretations as a parameter. The interpretation of each parameter code is given in each category. For example, major code 055 is used to encode the value of a river mile mark.
Minor Codes The first digit of the minor code of a nonparameter code is zero. The remaining three digits of a nonparameter code are used to classify specific features. The type of element described by a particular code can generally be determined from the value of these digits:
General DLG Collection Criteria Collection criteria should be set to define category standards for features (i.e. minimum size and area, where to digitize.) Size Criteria If a feature is questionable, it may be necessary to measure it on the map to determine if it meets the criteria, and if it should be collected as points, lines, or areas. When measuring features for USGS DLG extraction, the following size collection guidelines apply:
USGS Criteria for Where to Digitize Features Lines are digitized as centerlines. For many features, the symbol is a single line on the map, and the digitized centerline and the symbol are, for all practical purposes, the same. However, for most roads, multiple track railroads, and other features that are symbolized with casings, like racetracks and boardwalks, the centerline must be digitized by determining the center of the symbolized feature. If the edge of an area feature, which is not shown by tint, is formed by another linear feature, the centerline of the linear feature is digitized as the outline of the area feature. Degenerate lines and symbolized nodes are digitized in the center of the symbol, unless the coding description provides additional information. Symbol Hierarchy and Symbol Suppression on the Map Some symbols are suppressed on USGS maps because of legibility constraints, but the features must still be collected in the DLG. It is important to set rules for when symbols can be suppressed on maps, since this affects how features are coded. There is likely to be a hierarchy of boundary line symbolization. And there may symbol size reductions in special cases. For instance, boundary lines often follow the centerlines of roads and the shorelines of lakes, streams, and oceans. Boundary lines generally are not suppressed, however in some cases the boundary line may be dropped (i.e. for roads, the lineweight of the boundary line symbol is reduced by one-half, so it may be seen.) Example List of Hypsography (020) Attribute Codes Area attribute codes
Line attribute codes
Single-point attribute codes
Descriptive attribute codes
Parameter attribute codes
Example List of Hydrography (050) Attribute Codes In the real world, hydrographic features vary considerably. It is necessary, when collecting hydrographic features from DLGs, to pay attention to such things as shape or form, context within the map, names, labels, and marginal information, some or all of which may be important aids to classification. The names and labels of water features do not always convey the proper classification, and care must be taken not to use names and labels alone in determining how to classify them. For example, many features that are known as "Reservoirs" or labeled on the graphic as "Reservoir" may in fact, be lakes or ponds. Further, hydrographic features form networks where large and small rivers, streams, lakes, ponds, and so on, flow in and out of each other. On the map, these networks appear as blue areas with no dividing lines. However, in collecting digital data, each part of the network must be classified as the appropriate feature and assigned a code. What the feature is and where it begins and ends is not always clear. Finally, the direction of flow is indicated on many maps by the contours and the overall pattern of the drainage network. Flow codes are only used when flow can readily and confidently be determined from the topography and overall pattern of the drainage network as shown on the map. Information on the flow of water bodies is collected in a GIS to support data users who model network flow on hydrographic phenomena for scientific studies. Flow is coded in one of two ways in the DLG data. For double-line features, the descriptive attribute codes for right bank and left bank are applied. The shore of any land area between the right and left banks is collected as shoreline. For single-line features, the node attribute codes for upper origin, upper origin at water body, sink, entering water body and exiting water body are applied. Node attribute codes
Area attribute codes
Line attribute codes
Single-point attribute codes
General purpose attribute codes
Descriptive attribute codes
Parameter attribute codes
Example List of USGS Vegetative Surface Cover (070) Codes Area attribute codes
Line attribute codes
EXAMPLE LIST OF NON-VEGETATIVE (080) FEATURE CODES Features digitized in this category portray information about the naturally occurring, non-vegetated surface of the Earth; such as lava, sand, and gravel features. While numerous other non-vegetative features exist, they are digitized in other categories that are more applicable. For example, features that are the result of human activity, such as mines, are digitized in Manmade Features and features associated with water, such as glaciers, are digitized in Hydrography. Area Attribute Codes
Line attribute codes
Parameter attribute codes
EXAMPLE LIST OF BOUNDARIES (090) CODES Boundary examples include international boundaries, state boundaries, counties or county equivalents; incorporated places include cities, towns, and villages. Incorporated places can be within, exclusive of, or coextensive with, the county or minor civil division in which they are located. Where boundaries of two or more units of different rank coincide, a hierarchy of symbols exists to decide which one unit boundary is shown on the map. In the USGS system, boundaries are ranked in the following order: national; State, commonwealth, and territory; county, parish, borough, and municipalities; township or minor civil division; city or incorporated place; Federal reservation; other reservation; large park and small park. Node attribute codes
Area attribute codes
Line attribute codes
Single-point attribute codes
Parameter attribute codes
EXAMPLE LIST OF SURVEY CONTROL AND MARKERS (150) CODES The purpose of collecting Survey Control and Markers category data is to capture information about established horizontal and vertical positions which are used as fixed references in positioning and correlating map features. Single-point attribute codes
Descriptive attribute codes
Parameter attribute codes
EXAMPLE LIST OF ROADS AND TRAILS (170) CODES Transportation data are collected in three separate categories: Roads and Trails; Railroads; and Pipelines, Transmission Lines, and Misc. Transportation Features. If multiple attributes are needed to describe a feature the ordering is not significant. An exception occurs in coding highway route numbers. Node attribute codes
Area attribute codes
Line attribute codes
General purpose attribute codes
Descriptive attribute codes
Parameter attribute codes
EXAMPLE LIST OF Railroads (180) CODES Node attribute codes
Line attribute codes
General purpose attribute codes
Descriptive attribute codes
Parameter attribute codes
EXAMPLE LIST OF PIPELINES, TRANSMISSION LINES, and TRANSPORTATION FEATURES (190) CODES Line attribute codes
General purpose attribute codes
Descriptive attribute codes
EXAMPLE LIST OF MANMADE FEATURES (200) CODES Area attribute codes
Line attribute codes
Single-point attribute codes
General purpose attribute codes
Descriptive attribute codes Ø 200 0605 OilØ 200 0606 GasØ 200 0610 SandØ 200 0611 ClayReference: http://rockyweb.cr.usgs.gov/nmpstds/acrodocs/dlg-3/3dlg0798.pdf |