USGS DLG DATA
DLG data files contain selected base catagories of cartographic data in digital form. Each element in the file is represented as a node, an area, or a line. Nodes define the location of the endpoints of every line. An area is a portion of the map bounded by lines. A line is an ordered set of points that describes the position and shape of a linear feature on the map. Each element is attributed, when applicable, and the attribute descriptions can be found in technical instructions available from the Earth Science Information Center at the USGS. Each map can have as many as eleven overlays, as described below. 
TRANSPORTATION
This catagory of data includes major transportation systems collected in three separate overlays: (1) Roads and Trails (2) Railroads (3) Pipelines, Transmission Lines, and Miscellaneous Transportation Features.
HYDROGRAPHY
This catagory consists of all flowing water, standing water, and wetlands.PUBLIC LAND SURVEY SYSTEM
Includes the rectangular system of land surveys that is administered by the U.S. Bureau of Land Management. PLSS data are only collected for areas falling solely, or in part, within the states that were formed from the public domain. The PLSS subdivides the public domain and represents property boundaries or references to property boundaries. These DLG data are not intended to be official or authoritative. They are presented as cartographic reference information.
BOUNDARIES
Consists of political boundaries that identify States, counties, cities, and other municipalities, and administrative boundaries that identify areas such as National and State forests. Political and administrative boundaries are always collected as a single data set.
HYPSOGRAPHY
Consists of information on topographic relief (primarily contour data).
VEGETATIVE SURFACE COVER
Consists of information about vegetative surface cover such as woods, scrub, orchards, and vinyards.
NON-VEGETATIVE SURFACE COVER
Consists of information about the natural surface of the Earth as symbolized on the map such as lava, sand, and gravel features.
MANMADE FEATURES
SURVEY CONTROL AND MARKERS
Consists of 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.
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