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U.S. Geological Survey Digital Line Graphs (DLGs)

Digital Line Graphs (DLGs)

 

CONTACT INFO
CTRE
ISU Research Park
2901 S. Loop Dr.
Suite 3100
Ames, IA 50010

Phone
515-294-8103
Fax
515-294-0467

Webmaster:
mregenol@iastate.edu

 

 

 

U.S. Geological Survey Digital Line Graphs (DLGs)

Overview

(http://edcwww.cr.usgs.gov/glis/hyper/guide/usgs_dlg)

General description

The U.S. Geological Survey's (USGS) digital line graph (DLG) files are digital vector representations of cartographic information. Data files of topographic and planimetric map features are derived from either aerial photographs or from cartographic source materials using manual and automated digitizing methods.

Geographic coverage

Large Scale

These DLGs are produced from the largest scale topographic quadrangle maps available, which are usually the USGS 7.5-minute, 1:24,000-scale topographic maps for the contiguous United States, Hawaii, and the Virgin Islands. Large-scale DLGs also are produced from 1:25,000- and 1:63,360-scale maps for Alaska and 1:30,000-scale maps for Puerto Rico.

The DLG data are being collected for all 50 States, and the 1:24,000-scale series eventually will provide complete national coverage. A status graphic is available through the Product Status Information web page (see 7.5' DLG Coverage).

Intermediate Scale

The 1:100,000-scale DLG data are being collected for the contiguous United States and Hawaii. The hydrography and transportation categories are complete, and the series will eventually provide complete national coverage for all categories. A status graphic is available through the Product Status Information web page (see 1:100,000-scale DLG Coverage).

Small Scale

The 1:2,000,000-scale DLG data files organized by State include the conterminous United States and Hawaii. The data files are available via Graphic Map.

The 1:2,000,000-scale DLG data files, organized by section, include all 50 States. Alaska is available only in this sectional format.

Who collects it

The USGS collects and revises DLG data files from stable-base manuscripts, from stable-base composites of original map separates, and from aerial photography, using semiautomatic (e.g., digitizers); and automatic (e.g., scanners) digitizing systems.

How often

As needed

What is in it generally

Large Scale

The large-scale DLG data primarily are derived from USGS 7.5-minute topographic quadrangle maps at 1:24,000 and 1:25,000 scales (1:25,000 and 1:63,360 scales for Alaska).

Intermediate Scale

Intermediate or 100,000-scale DLG data are derived from USGS 1:100,000-scale, 30- by 60-minute quadranglemaps. If these maps are not available, Bureau of Land Management (BLM) planimetric maps at a scale of 1:100,000 are used.

Small Scale

Small or 1:2,000,000-scale DLG data are organized two ways (by section or by State) and contain information onplanimetric base categories, including transportation, hydrography, and boundaries for all 50 States. The Section DLG data files are historical files dating between 1973 and 1980 that are organized by sections of the United States (e.g., northeastern States). The State data files are recent files dating between 1990 and 1994 that are organized by State.

How to access

To place orders and to obtain additional information regarding technical details and pricing schedules, contact: Customer Services, EROS Data Center Earth Science Information Centers (ESICs)

A Data Users Guide is provided with each mailed order. Data Users Guides and Technical Instructions may be ordered separately for a nominal fee. The Data Users Guides for large-, intermediate-, and small-scale DLG data also are available online (see References). Additional information on retrieving DLG data via anonymous File Transfer Protocol (FTP) is available through the US GeoData web page.

Online requests for these data can be placed via the USGS Global Land Information System (GLIS) interactive query system. The GLIS system contains metadata and online samples of Earth science data. With GLIS, you may review metadata, determine product availability, and place online requests for products.

The large-scale DLG data are available in the optional format on 8-mm cassettes, 3480 cartridges, CD-recordables, and through a semi-anonymous FTP at the EROS Data Center. The large-scale DLG data also are available in SDTS format on CD-recordables and through anonymous FTP.

The intermediate-scale DLG data are available in the optional format on 8-mm cassettes, 3480 cartridges, and CD-recordables. The intermediate-scale DLG data are available in the SDTS format on CD-recordables and through anonymous FTP.

The small-scale DLG sectional data are available in standard, optional, and graphic formats on CD-ROM. All three formats are included on one CD-ROM, allowing the user to choose the format best tailored to their needs.

The small-scale DLG State data are available in the optional format on CD-ROM. The small-scale DLG State data are available in SDTS format on CD-ROM and through anonymous FTP.

SDTS FTP Instructions

To access the account for 1:100,000-scale DLG data:

  1. FTP to edcftp.cr.usgs.gov
  2. Enter "anonymous" at the Name prompt.
  3. Enter your complete e-mail address at the Password prompt.
  4. Change (cd) to the "pub/data/DLG/100K" subdirectory.
  5. Set the file transfer mode to binary by typing the word "binary".
  6. Use the "get" or "mget" commands to download the 00README and data files.
  7. The 00README file located in the "pub/data/DLG/100K" subdirectory contains an explanation of the directory structure.

To access the account for 1:2,000,000-scale DLG data:

  1. FTP to edcftp.cr.usgs.gov
  2. Enter "anonymous" at the Name prompt.
  3. Enter your complete e-mail address at the Password prompt.
  4. Change (cd) to the "pub/data/DLG/2M" subdirectory.
  5. Set the file transfer mode to binary by typing the word "binary".
  6. Use the "get" or "mget" commands to download the 00README and data files.
  7. The 00README file located in the "pub/data/DLG/2M" subdirectory contains an explanation of the directory structure.

Data Dictionary

Data Characteristics

The large-, intermediate-, and small-scale DLG data distributed by the USGS are in the DLG Level-3 (DLG-3) format. All DLG data in the NDCDB are level-3 data, which have the full range of attribute codes and have a full topological structure.

A DLG-3 file is composed of node, line, and area-identifier elements. Nodes define the location of the endpoints of every line. A single node may mark the start or the end of one or more lines; therefore, nodes occur at intersections of linear features and at other places on linear features where the feature is subdivided into separate line segments. A line is an ordered set of points (vertices) that describes the position and shape of a linear feature on a map. Each line starts and ends at a node, thus, having an explicit order from start to stop and a left-to-right connotation. Lines may not cross over themselves or any other lines except at a node. An area is a continuous unbroken region of a map bounded by lines. Every DLG data file identifies at least two areas (one representing an area covered by the file and the other representing the area outside the coverage of the file).

Attribute codes are used to describe the physical and cultural characteristics of DLG node, line, and area elements. The codes are based on cartographic features symbolized on source maps. Each DLG element has one or more attribute codes composed of a three-digit major code and a four-digit minor code. The attribute scheme is open-ended so that additional codes may be added as needed. 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 based on relationships to adjacent elements.

Large Scale

The large-scale DLG data files are produced in 7.5-minute units that correspond to 1:20,000-, 1:24,000-, and 1:25,000-scale topographic quadrangle maps (1:25,000- and 1:63,360-scale maps for Alaska). The unit sizes in Alaska vary depending on latitude.

The DLG data files derived from the 1:24,000-scale maps contain selected base categories of cartographic data in digital form. These categories include: (1) political boundaries (e.g., State, county, city, and other municipal boundaries) and administrative boundaries (e.g., National and State forest boundaries); (2) hydrography, including all flowing water, standing water, and wetlands; (3) Public Land Survey System (PLSS) data describing the rectangular system of land surveys administered by the BLM, representing or referencing property boundaries (e.g., township, range, and park information); (4) transportation data, including major transportation systems collected in three separate overlays (roads and trails; railroads; and pipelines, transmission lines, and miscellaneous transportation features); (5) other significant manmade structures, including miscellaneous cultural features not in the other major data categories (e.g., schools, churches, hospitals); (6) hypsography; (7) surface cover, including information about vegetative surface cover (e.g., woods, scrub, orchards, and vineyards); (8) nonvegetative surface features, including information about the natural surface of the Earth (e.g., lava and sand); and (9) survey control and markers (i.e., information about the points of established position and third-order or better elevations that are used as fixed references in positioning and correlating map features).

Intermediate Scale

The 100,000-scale DLG files are derived from USGS topographic maps published as 30- by 60-minute quadrangles at 1:100,000 scale. Editions of BLM 1:100,000-scale maps or archival compilation materials are used when the USGS maps are not available.

The DLG data files derived from the 1:100,000-scale maps contain selected base categories of cartographic data in digital form. These categories include: (1) hydrography, including flowing water, standing water, and wetlands; (2) transportation collected in three different categories (roads and trails; railroads; and pipelines, transmission lines, and miscellaneous transportation); (3) hypsography, including contours and supplementary spot elevations; (4) boundaries, including State, county, city, and other National and State lands such as forests and parks; and (5) PLSS, including township, range, and section information.

The DLGs are distributed in groups of files that make up 30- by 30-minute areas of coverage representing the selected category of information in the east half or west half of a 1:100,000-scale source map. Each 30-minute area consists of a varying number of DLG files depending on the category and the feature density. The normal distribution group is four 15-minute files per 30-minute area. If the feature density in an area is such that the file size exceeds the limitations of the processing programs, then the 30-minute area is covered by sixteen 7.5-minute files. A further breakdown of the distribution of files by category can be found in the Appendix.

Small Scale

The Section DLG files were created by manually digitizing the 1:2,000,000-scale sectional maps of the National Atlas of the United States of America. Originally, three levels of DLG data (DLG-1, DLG-2, and DLG-3) were defined. The DLG-3 data have the most accurate positional control and have been topologically structured for use in mapping and geographic information system (GIS) applications. Of the three file levels, the DLG-3 files are the only files that are distributed to the user community.

The U.S. State data are broken into 52 rectangular cells. Cell corners are represented in even degrees of latitude or longitude, and cell size is determined by the area necessary to encompass a given State (except for Alaska, which is divided among three cells along arcs of longitude and latitude). The registration points for each cell are the cell corners.

The data for each cell are encoded in multiple categories including: (1) boundaries; (2) hydrography; (3) manmade features; (4) pipelines, transmission lines, and miscellaneous transportation features; (5) railroads; roads and trails; and (6) PLSS. There is one file per category. The files for each cell are vertically registered and the State boundaries are edge matched to each other.

Spatial Resolution

Large and Intermediate Scales

The data are either manually digitized using equipment with a resolution of 0.001 inch and an absolute accuracy from 0.003 to 0.005 inch or the data are scanned on an automatic device with a resolution of 0.0013 inch (30 points per millimeter).

Small Scale

The 1:2,000,000-scale DLG files were digitized using the National Atlas of the United States of America as a base map. The National Map Accuracy Standards states that 90 percent of well-defined points on the map must be within .02 inch at scale. At 1:2,000,000 scale, this corresponds to a maximum displacement of 3333.33 feet or 1,016 meters.

Data Organization

The DLG data are organized in graphic, optional, standard, or Spatial Data Transfer Standard (SDTS) formats. A comparison of the formats can be found in (http://edcwww.cr.usgs.gov/glis/hyper/guide/usgs_dlg).

SDTS

The SDTS format differs from the graphic and optional formats. The character set consists of a binary data file that contains non-ASCII characters. The characters are formatted according to International Organization for Standardization (ISO) standards 646 and 2022. The binary floating point is determined according to the ANSI/IEEE 754-1985 standard, and binary integers are determined according to the ISO/IEC 8632-3 standard. The logical record length is variable as defined in ISO 8211 and the block size is media dependent. The coordinate system is geographic (latitude/longitude) for the small-scale DLG data and planimetric for the large- and intermediate-scale DLG data; the topological linkages are only in lines (though SDTS supports all objects, DLG to SDTS supports only in lines).

Large and Intermediate Scales

Large-scale DLG files are available in optional and SDTS formats. Intermediate-scale DLG files are available in optional and SDTS formats.

Data in the optional format are organized into 11 distinct record types (4 header records and 7 data records). The four header record types are file identification and description; accuracy (not currently used); control point identification; and data category identification. The seven data record types are node and area identification; node-to-line linkage; area-to-line linkage; line identification (also contains line-to-node and line-to-area linkages); coordinate string; attribute code; and text (not currently used).

A more detailed description of the DLG optional format can be found in the following USGS publications: "Digital Line Graphs from 1:24,000-Scale Maps--Data Users Guide 1" and "Digital Line Graphs from 1:100,000-Scale Maps--Data Users Guide 2." More information on the SDTS format is available from the USGS' Mid-Continent Mapping Center (see the Appendix).

Small Scale -- 1:2,000,000-Scale DLG Sectional Data (1973-1980)

Each of the 21 sections of the 1:2,000,000-scale DLG file has one or multiple layers. There are three layers for each section:

  • Boundaries - States, counties, and Federal lands
  • Hydrography - Streams and water bodies
  • Transportation - Roads and Trails (e.g., Interstate and primary State highways)

Railroads (e.g., Main line and branch line railroads)

Miscellaneous transportation features (e.g., Airports and pipelines)

  • All sections are contained on one CD-ROM.

Small Scale -- 1:2,000,000-Scale DLG State Data (1990-1994)

The 1:2,000,000-scale DLG State data are available in five categories:

  • Boundaries - States, counties, and Federal lands
  • Hydrography - Streams and water bodies
  • Transportation - Roads and Trails (e.g., Interstate and primary State highways)

Railroads (e.g., Main line and branch line railroads)

Miscellaneous transportation features (e.g., Airports and pipelines)

  • Manmade features - Built-up areas, capitols, county seats, populated places, and population range
  • U.S. Public Lands Survey System (collected only for the 30 public land States)

Land grants, township, range, and subdivisions of public lands

The data are available in optional and SDTS formats.

A more detailed description of DLG optional and graphic formats can be found in the following USGS publication: "Digital Line Graphs from 1:2,000,000-Scale Maps--Data Users Guide 3." More information on the SDTS format is available from the USGS' Mid-Continent Mapping Center (see the Appendix).

In addition to the DLG data, there are two associated data files in ASCII format for each State which contain airport names and populated place names with population figures. There are also two national files. One file contains land grant names and identifiers for the entire United States. The other file contains the Federal Information Processing Standard (FIPS) five-digit codes for all States, State equivalents, counties, and county equivalents. Records in these data files can be associated with cartographic features contained in the DLG files.

Large and Intermediate Scales

The DLG data do not carry quantified accuracy statements. However, the following procedures are used prior to data release for distribution: codes are checked by software against a table of valid codes to ensure that each attribute in a file is valid for the category and element type to which it is assigned. A visual validation may be involved in the correlation of formatted listings with proof plots. File Fidelity and Completeness -- The data are either manually digitized using equipment with a resolution of 0.001 inch and an absolute accuracy of from 0.003 to 0.005 inch or are scanned on an automatic device with a resolution of 0.0013 inch (30 points per millimeter). For general information on accuracy, see Horizontal, Positional Accuracy and Vertical Positional Accuracy.

Attribute Accuracy -- DLG attribute

Topological Fidelity -- The topological structure of each DLG file is fully validated by software. Lines are checked to ensure that all intersections have nodes present. Validation of DLG data is performed for each category within a file.

Edge Matching -- Validation software provides for checking the edges of each quadrangle against the edges of the four adjacent quadrangles. Each edge of a DLG-3 is checked for positional and attribute matching along the neatlines of the adjoining DLG-3 cells, provided that the surrounding data cells are available at the time the DLG-3 is entered into the National Digital Cartographic Data Base (NDCDB).

Quality Control Flags -- Information in the header of the DLG-3 indicates the status of the file with respect to the edge matching described above. Twelve bytes at the end of record 3 in the optional distribution format of the ASCII file are set aside for quality control flags. A more detailed description of the codes for large- and intermediate-scale DLGs can be found in the following USGS publications:

    • "Digital Line Graphs from 1:24,000-Scale Maps--Data Users Guide 1"
    • "Digital Line Graphs from 1:100,000-Scale Maps--Data Users Guide 2"

 

Small Scale

The following validation procedures apply to 1:2,000,000-scale DLG Sectional and DLG State data:

File Fidelity and Completeness -- The positional accuracy and completeness of the file are checked by visually comparing proof plots with the original stable-base source material used in the digitizing process. All points on the edit plot should be within one line weight (usually .005 inch) of the original line at the same plotted scale. The digitizing and plotting equipment have resolution to .001 inch and absolute accuracy from .003 to .005 inch.

Attribute Accuracy -- Code validation is done manually involving the correlation of formatted listings with proof plots.

Topological Fidelity -- The topological structure of each DLG file is fully validated by software. Lines are checked to assure that all intersections have nodes present.

The following two procedures apply only to the 1:2,000,000-scale DLG State data:

Edge Matching -- The data for each State are edge matched to the adjacent States. The files are checked for both positional and attribute consistency. Software exists to perform digital edge matching along straight cell edges (i.e., neatlines). However, because the 1:2,000,000-scale data do not touch the cell edges, and because it is the irregular State edges that need to be checked, the edge matching must be interactively completed for these data.

Quality Control Flags -- The quality control flags in the DLG header record were designed for use with digital edge-matching software; therefore, the flags do not provide information on graphic edge matches. However, DLG data cannot be entered into the NDCDB unless the quality control flags are set. For the 1:2,000,000-scale DLG, the quality control flags are set to indicate that edge matching was not performed even though a graphic edge check was done. A more detailed description of the codes for small-scale DLGs can be found in the following USGS publication: "Digital Line Graphs from 1:2,000,000-Scale Maps--Data Users Guide 3"

Applications and Related Data Sets

Large-, intermediate-, and small-scale DLG data are useful for the production of cartographic products (e.g., base maps). Also, the data are structured to support GIS technologies. A typical use of base category digital cartographic data is to combine them with other geographically referenced data, enabling the conduct of various automated spatial analyses.

Other digital cartographic data sets include:

    • 1:24,000-scale DLG files
    • 7.5-minute Digital Elevation Models (DEM)
    • 1:63,360-scale DLGs
    • 15-minute DEM
    • 1:100,000-scale DLGs
    • 30-minute DEM
    • Land Use/Land Cover Data
    • 1-degree DEM
    • Alaska Interim Land Cover
    • Geographic Names Information System

Further information regarding the above data sets can be obtained by contacting:

Customer Services, EROS Data Center

Earth Science Information Centers (ESICs)

References

  • Monmonier, M.S., 1982, Computer-assisted cartography--principles and prospects: Englewood Cliffs, New Jersey, Prentice-Hall, 214 p.
  • Thompson, Morris, 1987, Maps for America: U.S. Geological Survey, 265 p.
  • Thompson, M.M., 1988, Maps for America (3d ed.): Washington, D.C., U.S. Government Printing Office, 265 p.
  • U.S. Geological Survey, 1988a, Standards for digital line graphs--part 2, specifications, change 4, May 1994: [Reston, Virginia], U.S. Geological Survey technical instructions, [variously paged]. [Information on Standards is available online.]
  • U.S. Geological Survey, 1988b, Research, investigations, and technical developments--National Mapping Program, 1985-86: U.S. Geological Survey Open-File Report 87-315, 139 p.
  • U.S. Geological Survey, 1989, Digital line graphs from 1:100,000-scale maps--data users guide 2: Reston, Virginia, U.S. Geological Survey, 88 p. [This document is available online.]
  • U.S. Geological Survey, 1990a, Digital line graphs from 1:24,000-scale maps--data users guide 1: Reston, Virginia, U.S. Geological Survey, 107 p. [This document is available online.]
  • U.S. Geological Survey, 1990b, Digital line graphs from 1:2,000,000-scale maps--data users guide 3: Reston, Virginia, U.S. Geological Survey, 70 p.
  • U.S. Geological Survey, 1991, 1:2,000,000-scale digital line graph data on CD-ROM, factsheet: [Reston, Virginia], U.S. Geological Survey, 1 p.
  • U.S. Geological Survey, 1992a, US GeoData, [brochure]: [Reston, Virginia], U.S. Geological Survey, 2 p. [fold out, oversized]
  • U.S. Geological Survey, 1992b, US GeoData data users guides and associated documentation, order form: [Reston, Virginia], U.S. Geological Survey, 2 p.
  • U.S. Geological Survey, 1993, US GeoData digital line graphs, factsheet: [Reston, Virginia], U.S. Geological Survey, 2p.
  • U.S. Geological Survey, 1994a, Cartographic technical instructions, price list: [Reston, Virginia], U.S. Geological Survey, 2 p.
  • U.S. Geological Survey, 1994b, 1:2,000,000-scale digital line graphs--data users guide 3: Reston, Virginia, U.S. Geological Survey, 85 p. [This document is available online.]

Appendix

Attribute Codes

An attribute code is comprised of a three-digit major code and a four-digit minor code. The first two digits of the major code uniquely identify the category to which the described element belongs. The third digit of the major code is used to modify the minor code in two ways:

If zero, the minor code represents a description or classification of the element.

If nonzero, the minor code which follows is a parameter requiring special interpretation according to instructions given in the attribute coding standards for each category.

The first digit of the minor code is normally zero. If nonzero, the digit is used as a modifier to provide additional information. The last three digits of the minor code (and occasionally all four digits) also may be used as a parameter code. Parameters are used when a minor code can legitimately assume a range of values.

For more information and examples of attribute codes, see the following three USGS publications:

"Digital Line Graphs from 1:24,000-Scale Maps--Data Users Guide 1"

"Digital Line Graphs from 1:100,000-Scale Maps--Data Users Guide 2"

"Digital Line Graphs from 1:2,000,000-Scale Maps--Data Users Guide 3"

Distribution Formats

Standard --The standard format DLG data are encoded using an internal file coordinate system to minimize storage requirements.

Graphic* --The graphic format was designed to be compatible with the Geological Survey – CartographicAutomatic Mapping (GS-CAM) software. The GS-CAM software provides for plotting elements to a variety of map projections, scales, and graphic symbologies. Each line record has been reformatted into two record types (a one-line identifier record and multiple latitude-longitude records). Supplemental coordinate pairs (i.e., linedensification) have been added to assure that the distance between any two points is not greater than .01 inch.This minimizes distortion associated with reprojection. The graphic files are organized by feature type andgeographic coordinates are expressed in radians. These files are not topologically structured. The graphic formatapplies only to the 1:2,000,000-scale DLG sectional data.

Optional --The optional distribution format was designed to facilitate data usage; therefore, topological linkages are explicitly encoded for node, area, and line elements. The files are comprised of 8-bit ASCII characters organized into an 80-byte logical record length. A total of 11 distinct record types are defined in this format.

Coordinates are expressed as integer meters in varying map projections (i.e., Universal Transverse Mercator projection for large- and intermediate-scale data and Albers Equal Area Conic projection for small-scale data). SDTS --The Spatial Data Transfer Standard (SDTS) distribution format is designed as a mechanism for th transfer of spatial data between various computer systems. The SDTS format is designed to transfer data with complete content transfer (no loss of information). Data quality reports provide complete metadata and documentation of processing. The SDTS is a FIPS standard intended to work between various platforms and various data models. There is limited software available at this time which can accommodate the SDTS-formatted data. For more in-depth information on SDTS, refer to the SDTS technical documentation (available online). *The graphic format applies only to the 1:2,000,000-scale DLG sectional data.

 

 

 

 


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