More DRG Scanned Map Information |
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| What are DRGs? |
| Digital Raster Graphics (DRGs) are digital images of paper USGS topographic quadrangle maps. These digital images were produced by the USGS who scanned and registered the topographic quadrangles. They scanned the 7.5-minute topo maps at 250 dot-per-inch minimum resolution. DRGs are available of all three scales of statewide USGS topo maps: 1:24,000, 1:100,000 (except county series), and 1:250,000. |
| How DRG's Are Made |
| A DRG is made by scanning a printed map at a minimum of 250 dots per inch (dpi) on a high-resolution scanner. The raster image is georeferenced and put into the Universal Transverse Mercator (UTM) projection. Colors are standardized to remove scanner limitations. The image is compressed to reduce its size. The final result is a compressed TIFF file that ranges from 5 to 15 megabytes. |
| The horizontal positional accuracy of the DRG matches the accuracy of the source map. The 1:24,000-scale DRG at 250 dpi will have a ground sample distance of 2.4 meters (8 feet). Colors are usually standardized to duplicate the line-drawing character of the published map. The average data set size of a 7.5-minute DRG is about 8 megabytes in Tagged Image File Format (TIFF) with PackBits compression. |
| GeoTIFF is a public domain extension of the TIFF standard. GeoTIFF files conform to the TIFF 6.0 base standard, but contain additional information that allows image coordinates to be transformed to ground coordinates. TIFF readers that do not recognize GeoTIFF tags can still read GeoTIFF files as ordinary TIFF data. |
| View a sample 7.5-minute DRG below: |
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Note that the resolution of images on the web is significantly lower than the scanned resolution of DRGs. |
| How are DRGs different from my own scanned maps? |
| Geo-referencing information is added to the DRGs to associate the maps with their true position on the ground. This allows for some viewing software to disclose coordinate positions, automatically calculate map scale or distances, and perform other spatial manipulations of the maps. Also, other geo-referenced digital information (e.g., Digital Line Graphs or Digital Orthophoto Quadrangles) may be overlaid more easily by matching coordinates. |
| What kind of software do I need to view a DRG? |
| Our topo DRGs on CD-ROM come with the freeware "dlgv32" viewing software. Other graphics programs that handle large TIFF files will work but all may not take advantage of the geo-referencing information available with the image. |
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