SDTS format is available by 7.5' quadrangle name via File Transfer Protocol (FTP). Optional format is available for download through the USGS Earth Explorer. These data are available for download in two file formats, Optional and SDTS. 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 of 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. Digital Line GraphsĮxample of a large-scale Digital Line Graph. Availability of each are described below. The vector topographic data are called Digital Line Graphs (DLGs) while the raster topographic data are called Digital Raster Graphics (DRGs). Digital Topographic Map Dataĭigital topographic map data exists for the United States in both vector (points, lines and polygons) and raster (image) formats. In addition to the national sources listed below, our state-by-state list provides links to state digital topographic data and digital elevation models. These data are commonly called Digital Elevation Models or DEMs. The second are raster data sets that include an elevation for each pixel. These are typically called digital topographic maps or digital orthoquads. The first are data that represent the information typically found on a topographic quadrangle map, such as contour lines, roads, streams, railroads, towns, etc. Topographic data provide information about the elevation of the surface of the Earth.
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