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Digital Line Graph Data
If you have no idea how to work with DLG data or are totally lost at how to get the data into ArcView GIS then please see the following resources:
DLG-3 SDTS README file: This file contains the DLG-3 Master Data Dictionary (MDD).
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.
Data Sources, File Content, and File Extent
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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