DIGITAL LINE GRAPHS - USGS
Description:
USGS DLGs are digital representations of program-quadrangle format and sectional maps. All DLG data distributed by the United States Geological Survey (USGS) are DLG-Level 3 (DLG-3), which means the data contain a full range of attribute codes, have full topological structuring, and have passed certain quality-control checks.
Areas of product coverage for series are indicated in a National Index for 7.5- and 15-minute (large scale) digital line graphs (DLGs). On-line status graphics are available for the 7.5-minute and 1:100,000-scale DLGs. A National Index is also available for 1:100,000-scale maps (intermediate) as a guide to locating 1:100,000 DLGs. Small Scale DLGs cover areas in the United States that correspond to the sectional National Atlas maps at 1:2 million scale.
Large-scale DLG data files are available for the U.S. Public Land Survey, boundaries, transportation, hydrography and in some areas, hypsography. They are produced in 7.5-minute by 7.5-minute units which correspond to USGS 1:20,000-,1:24,000-, 1:25,000-scale topographic quadrangle maps. The unit sizes of 1:63,360-scale Alaska quadrangles vary depending on the latitudinal location of the unit.
Intermediate (1:100,000 scale) DLG data files that cover transportation and hydrography, are available for all States except Alaska. Intermediate scale DLG data are sold in 30-minute by 30-minute units which correspond to the east half 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 subdivided into four 7.5-minute cells.
Abstract continued in the comments field.
Record Details:
Record Type:DATA SET
Product Published Date:05/13/1997
Record Last Revised:12/10/2002
Record ID:
3751
Quality Assurance:
Title
:DIGITAL LINE GRAPHS - USGS
Data Use & Constraints
:Acknowledgement of the U.S. Geological Survey would be appreciated in products derived from these data.
1:24,000 and 1:100,000-scale Digital Line Graph (DLG) accuracy is estimated to be 98.5 percent (accuracy percent not quoted for 1:2,000,000-scale DLG). Attribute accuracy was tested by one or more of the following methods: 1) manual comparison of the source with hard copy plots, 2) symbolized display of the digital line graph on an interactive computer graphic system, or 3) selected attributes that could not be visually verified on plots or on screen, were interactively queried and verified on screen.
In addition, U.S. Geological Survey (USGS) Production System (PROSYS) software tested the attributes against a master set of valid attributes for the category; it also checked for selected valid attribute combinations, and for valid attributes relative to topology and dimensionality. All attribute data conform to the attribute codes current as of the date of digitizing.
Logical_Consistency_Report: Topological requirements include: lines must begin and end at nodes, lines must connect to each other at nodes, lines do not extend through nodes, left and right areas are defined for each line element and are consistent throughout the file, and the lines representing the limits of the file (neatline) are free of gaps.
The tests of logical consistency were performed by the USGS PROSYS program. The neatline was generated by connecting the four corners of the digital file, as established during initialization of the digital file. All data outside the enclosed region were ignored and all data crossing these geographically straight lines were clipped at the neatline. Data within a specified tolerance of the neatline were snapped to the neatline. Neatline straightening aligned the digitized edges of the digital data with the generated neatline, that is, with the longitude/latitude lines in geographic coordinates. All internal areas were tested for closure using PROSYS.
