Digital Raster Graphic (DRG)
A Digital Raster Graphic (DRG) is a digital image (scanned version) of a USGS topographic map. DRGs are produced from USGS 1:24,000-, 1:24,000/1:25,000-, 1:63,360- (Alaska), 1:100,000-, and 1:250,000-scale topographic map series. The image inside the map neatline is georeferenced to the surface of the Earth and fit to the Universal Transverse Mercator (UTM) projection. The horizontal positional accuracy and datum of the DRG matches the accuracy and datum of the source map.
DRGs for Lake Tahoe are available here as TIFF and MrSID images of USGS
7.5-minute topographic maps, along with its associated world file in text.
The world file can be used by some software to place the DRGs in digital
space. DRGs in MrSID format can be viewed with MrSID viewers from 
LizardTech, which
are available for free from their website.
For more information about DRGs, please visit
http://craterlake.wr.usgs.gov/drg_brief.html.
DRG Download
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DRGs are in UTM NAD27 zone 10 projection. Raw (original) data are in the following projections: data east of 120 degrees longitude are in UTM zone 11 and data west of 120 degrees longitude are in UTM zone 10. It is not possible to combine different projections in Geographic Information System (GIS), therefore, all data have been reprojected into one standard projection, UTM NAD27 zone 10 projection, for the Lake Tahoe Basin. The following DRG data have been mosaicked together from the 18 7.5-minute quadrangles which cover the Lake Tahoe Basin. The mosaicked files are more convenient for looking at areas throughout the basin or areas on the edge of two quadrangles. The mosaic is a very large file and has been compressed in two versions: TIFF and MrSID. These two files are available with its associated world files. DRG mosaic of the Lake Tahoe Basin:
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