You used to be able to access 1:2,000,000 DLGs on-line at the USGS' National Atlas of the United States, but the National Atlas has recently been removed from service. Large- and Intermediate -scale DLGs are available for download through EarthExplorer system. The files are in the public domain, and can be used for any purpose without restriction. At 1:24,000 (7.5 minutes by 7.5 minutes), coverage remains spotty. At the intermediate scale, 1:100,000 (30 minutes by 60 minutes), all hydrography and transportation files are available for the entire U.S., and complete national coverage is planned. Not all DLG layers are available for all areas at all three scales. For example, a line segment associated with the attribute code "050 0412" represents a hydrographic feature (050), specifically, a stream (0412). Each node, line segment, and area is associated with two-part integer attribute codes. The basic elements of DLG files are nodes (positions), line segments that connect two nodes, and areas formed by three or more line segments. Elevations are referenced either to NGVD 29 or NAVD 88 (USGS, 2006a).
Some are based on NAD 27, others on NAD 83. Some use UTM coordinates, others State Plane Coordinates. Spatial Reference InformationĭLGs are heterogenous. Similar to the Census Bureau's TIGER/Line, line segments in DLGs must begin and end at point features (nodes), and line segments must be bounded on both sides by area features (polygons). In addition, however, DLGs are tested for the logical consistency of the topological relationships among data elements. Like other USGS data products, DLGs conform to National Map Accuracy Standards. Not all layers available for all quadrangles (USGS, 2006). Layers and contents of large-scale Digital Line Graph files. Horizontal and vertical monuments (third order or better)Ĭultural features, such as building, not collected in other data categories Roads and trails, railroads, pipelines and transmission linesįlowing water, standing water, and wetlandsĬontours and supplementary spot elevations State, county, city, and other national and State lands such as forests and parks Table 7.1 Description of Digital Line Graph Layers Heading 1 Large-scale DLGs are produced in tiles that correspond to the 7.5-minute topographic quadrangles from which they were derived.
Individual feature sets (outlined in the table below) are encoded in separate digital files.
Digital Line Graphs (DLGs) are vector representations of most of the features and attributes shown on USGS topographic maps.
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