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So tell me... what's a DLG?

Source: USGS
Digital line graph (DLG) data are digital representations of cartographic information. DLG's of map features are digital vectors converted from maps and related sources. The U.S. Geological Survey (USGS) DLG data are classified as large, intermediate, and small scale.

Large-Scale DLG's

Large-scale DLG's are derived from the USGS 1:20,000-, 1:24,000-, and 1:25,000-scale 7.5-minute topographic quadrangle maps. When large-scale DLG's are revised, a recent digital orthophoto quadrangle is usually used to update those categories of DLG data that can be detected from an aerial photograph.

Large-scale DLG's are produced in 7.5-minute units that correspond to USGS 1:20,000-, 1:24,000-, and 1:25,000-scale topographic quadrangle maps. However, some older units in the western United States cover 15-minute areas and correspond to maps at 1:62,500 scale. The unit sizes in Alaska vary depending on latitude. Units south of 59 degrees N., cover 15- by 20-minute areas; between 59 and 62 degrees N., 15- by 22.5-minute areas; between 62 and 68 degrees N., 15- by 35-minute areas (all values are latitude and longitude, respectively).

Large-scale DLG's are available in nine categories or units: (1) Public Land Survey System, including township, range, and section line information; (2) boundaries, including State, county, city, and other national and State lands such as forests and parks; (3) transportation, including roads and trails, railroads, pipelines, and transmission lines; (4) hydrography, including flowing water, standing water, and wetlands; (5) hypsography, including contours and supplementary spot elevations; (6) nonvegetative features, including lava, sand, and gravel; (7) survey control and markers, including horizontal and vertical positions (third order or better); (8) manmade features, including cultural features not collected in other data categories such as buildings; and (9) vegetative surface cover, including woods, scrub, orchards, vineyards, and vegetative features associated with wetlands.

All nonstandard quadrangles with neatlines that extend beyond the standard unit size to accommodate overedge boundaries are collected as multiples of the standard unit size. Data covering a 7.5- by 8.5-minute area would, therefore, be sold as two 7.5-minute units.

Intermediate-Scale DLG's

Intermediate-scale DLG's are derived from USGS 1:100,000-scale 30- by 60- minute quadrangle maps. If these maps are not available, Bureau of Land Management planimetric maps at 1:100,000 scale are used, followed by archival compilation materials.

Intermediate-scale DLG's are sold in 30- by 30-minute units that correspond to the east or west half of USGS 30- by 60-minute 1:100,000-scale topographic quadrangle maps. Each 30-minute unit is produced and distributed as four 15- by 15-minute cells, except in high-density areas, where the 15-minute cells may be divided into four 7.5-minute cells.

Intermediate-scale hydrography and transportation DLG's are sold on compact disc-read only memory (CD-ROM). Each disc contains all the 15- by 15-minute cells within the 1:100,000-scale quadrangles that cover a State or States. Fourteen sectional regions in the United States covering the conterminous 48 States and Hawaii are available.

Presently, intermediate-scale DLG's are available in five categories or units: (1) Public Land Survey System; (2) boundaries; (3) transportation; (4) hydrography; and (5) hypsography. Small-Scale DLG's

Small-scale DLG's are derived from the USGS 1:2,000,000-scale sectional maps of the National Atlas of the United States of America. Small-scale DLG's were revised from 1990-95 sources.

Small-scale DLG's are sold in State units. At present, the 48 conterminous States and Hawaii have been revised. The District of Columbia is contained within the Maryland unit. Puerto Rico, the U.S. Virgin Islands, and Alaska will be revised by the end of 1996.

Small-scale DLG's are available in five categories or units: (1) boundaries; (2) transportation, including roads and trails, railroads, pipelines, and airports; (3) hydrography; (4) manmade features, including built-up areas, capitals, county seats, populated places, and population range; and (5) Public Land Survey System, including land grants, township, range, and subdivisions of the public lands.

Attribute Codes

Attribute codes are used with vector data to describe the physical and cultural characteristics of node, line, and area elements. Each attribute code identifies the major category or unit to which a data element belongs, as well as the specific nature of the element. Codes also may provide additional descriptive information, numerical values, or identifiers. Many elements are uniquely described by a single attribute code, but some may require two or more codes for a complete description. Allowing for a variable number of attribute codes creates an open-ended structure to which information can be added at any time. It is not necessary for each element to have associated attributes; in general, attribute codes are not assigned to an element if the attributes can be derived on the basis of relationships to adjacent elements.

Data Distribution Formats

Large-scale DLG's are available in optional format. The optional format is easy to use with an 80-byte logical record length, a ground planimetric coordinate system (Universal Transverse Mercator), and topological linkages contained in node, line, and area elements. Large-scale DLG's will also be available in the Spatial Data Transfer Standard (SDTS) format as each State's data are converted.

Intermediate-scale DLG's are available in optional format. The hydrography and transportation categories of intermediate-scale DLG's are also available in the SDTS format. All categories of intermediate-scale DLG's are available in SDTS format.

Small-scale DLG's are available in optional and SDTS formats. Small-scale DLG's in optional format use the ground planimetric coordinate system of the Albers Equal-Area Conic projection. Small-scale DLG's in SDTS format use the geographic coordinate system of latitude and longitude.

Spatial Data Transfer Standard

The SDTS is a mechanism for transferring of spatial data between dissimilar computer systems. The SDTS specifies exchange constructs, addressing formats, structure, and content for spatially referenced vector and raster data. Advantages of the SDTS include data and cost sharing, flexibility, and improved quality, all with no loss of information.

Source: United States Geological Survey (USGS) Earth Science Information Center

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