USGS Digital Line Graph (DLG)
Table of Contents
Background
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.
- 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 quadrangle maps. 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 on planimetric 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.
Extent of 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 1:2,000,000-scale DLG data files, organized by section, include all 50 States.
Alaska is available only in this sectional format.
Acquisition
Processing Steps
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.
- Data Validation -- 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:
- 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 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.
- 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:
- Data Validation -- 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"
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 divided 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 follows:
Comparison of Graphic, Standard and SDTS Formats
| Format Criteria |
Graphic* |
Optional |
Standard |
| Character Set |
8 - Bit ASCII |
8 - Bit ASCII |
8 - Bit ASCII |
| Logical Record Length |
20 Bytes |
80 Bytes |
144 Bytes |
| Block Size |
Multiples of 20 |
Multiples of 80 |
Multiples of 144 |
| Coordinate System |
Geographic Latitude and Longitude |
Planimetric** |
Cartesian |
| Topological Linkages |
None |
All Elements |
Line Elements |
*Applies to 1:2,000,000-scale sectional data only.
**Coordinate system varies (e.g., 1:24,000 Scale--Universal Transverse
Mercator; 1:100,000-Scale--Universal Transverse Mercator;
1:2,000,000 Scale--Albers Equal-Area Conic).
The Spatial Data Transfer Standard (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
- 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.
The data are available in standard, optional, and graphic formats. The DLG standard distribution format was designed to minimize storage requirements. Explicit topological linkages are contained only in the line elements. The DLG optional
format is topologically structured and is widely used with GIS technologies.
The graphic format is commonly used in the generation of maps and plots
by using the Geological Survey's Cartographic Automatic Mapping (GS-CAM)
software.
- 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
- 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.
Procedures for Obtaining Data
Information on DLG products and services can be found at:
http://edc.usgs.gov/products/map/dlg.html
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.
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].
- 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.
- 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.
- 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, 2 p.
- 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.
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:
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 - Cartographic Automatic 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., line densification) 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 and geographic coordinates are
expressed in radians. These files are
not topologically structured. The graphic format applies 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 the 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 and standard format applies only to the 1:2,000,000-scale DLG sectional data.
Intermediate-Scale DLG File Distribution by Category
| CATEGORY |
NUMBER OF FILES |
SIZE |
| Hydrography |
|
|
| Normal |
4 |
15 Minute |
| Very Dense |
16 |
7.5 Minute |
| Transportation |
|
|
| Normal |
|
|
| Roads and Trails |
4 |
15 Minute |
| Railroads |
4 |
15 Minute |
Pipelines, Transmission Lines and
Miscellaneous Transportation |
4 |
15 Minute |
| Very Dense |
|
|
| Roads and Trails |
16 |
7.5 Minute |
| Railroads |
4 |
15 Minute |
Pipelines, Transmission Lines and
Miscellaneous Transportation |
4 |
15 Minute |
| Hypsography |
|
|
| Normal |
4 |
15 Minute |
| Very Dense |
16 |
7.5 Minute |
| Boundary* |
|
|
| Normal |
4 |
15 Minute |
| PLSS |
|
|
| Normal |
4 |
15 Minute |
*Boundary and PLSS overlays are by nature less dense and
will always be contained within four 15-minute files.
Small-Scale DLG Data Source Information
- Small-Scale 1:2,000,000-Scale DLG Section Data (1973-1980)
- The National Atlas of the United States of America 1:2,000,000-scale maps
were completely revised in 1973. The DLG files were digitized from these
maps and from additional sources as noted below:
| CATEGORY |
SOURCE(S) |
REVISION DATE |
| Boundaries |
Bureau of the Census Administering Federal Agency |
1979 |
| Hydrography |
NOAA Aeronautical Charts |
1979 - 1980 |
| Transportation |
Federal Aviation Administration
Interstate Commerce Commission |
1979 - 1980 |
| Alaska Sections |
USGS Maps and Landsat |
1979 - 1980 |
- Small-Scale 1:2,000,000-Scale DLG State Data (1990-1994)
- The digital files were revised and expanded in 1993 and 1994 from a
variety of Federal graphic and digital sources. Data source and currency
information is listed below.
| CATEGORY |
SOURCE(S) |
REVISION DATE |
| Boundaries |
Bureau of the Census Administering Federal Agency |
1990 |
| Hydrography |
Various |
1990 |
| Manmade Features |
Geographic Names Information System (GNIS)
Bureau of the Census |
1990
1990 |
Pipelines, Transmission Lines and
Miscellaneous Transportation |
Department of Transportation
USGS Maps - Alaska Pipeline |
1993
Various |
| Railroads |
Department of Transportation |
1993 |
| Roads and Trails |
Department of Transportation
USGS 1:100,00 Scale Quadrangles |
1994
Various |
| U.S. Public Land Survey System |
Bureau of Land Management
Alaska Department of Natural Resources |
1990
1993 |
