![]() Large-Scale (7.5-minute) DLGs correspond to the USGS 1:20,000-, 1:24,000-, and 1:25,000-scale topographic quadrangle maps. ![]() Data layer projections for DLG data vary from state to state. The disadvantages of the DLG format are that importation of the data to a GIS requires more effort than importing a DRG raster image, and that the DLG data are quite large, sometimes slowing down the screen refresh rate of the GIS software, and that not all data layers are available for all topographic map quadrangles. The advantage to using DLGs in a GIS setting is that the vector features can be rendered at any scale, such that zooming in and out on the map does not change the quality of the topographic data representation. Image courtesy of United States Geological Survey.ĭigital line graphs (PDF fact sheet) are files that contain the point, line, and polygon data found on a traditional paper topographic map, including the Public Land Survey System (township and range boundaries), contour lines, rivers, lakes, roads, railroads, towns, land cover, survey control points, etc. Example of a large-scale Digital Line Graph.
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