New York State Office of Cyber Security
Geographic Information Systems Clearinghouse
USGS Digital Raster Graphics (DRGs)
A digital raster graphic (DRG) is a scanned image of a U.S. Geological Survey (USGS) standard series topographic map, including all map collar information. The image inside the map neatline is georeferenced and fit to the Universal Transverse Mercator projection. The horizontal positional accuracy and datum of the DRG matches the accuracy and datum of the source map. The map is scanned at a minimum resolution of 250 dots per inch.
A digital image is produced by scanning a USGS map on a high-resolution scanner at a resolution of 500 dots per inch (dpi), resulting in a red-green-blue (RGB) file of up to 300 megabytes. The RGB file is then viewed, and the position of each of the sixteen 2.5-minute grid ticks on the image is collected. Image processing software uses these coordinates to rectify and georeference the image to the UTM ground coordinates. A piecewise linear rubber sheet transformation completes the georeferencing. The image is resampled to 250 dpi and compressed to reduce the size of the data set. The final result is a packbit-compressed file in the tagged image file format (TIFF). The file size is between 5 and 15 megabytes.
The NYS GIS Clearinghouse has made the DRGs (1:24,000) for New York State quadrangles are available from this site. Users may download a zipped file that contains the DRG (a TIFF image), a .tab file (for MapInfo users) and a .tfw file (for ESRI users). GIF images of the DRGs are also available for online viewing; however, the average file size is about one MB, so it may take some time for the images to load, particularly if you are accessing the Web via a modem.
If you are unfamiliar with DRGs, a sample image is included below. This is just one small section of the DRG for a quad in central New York; the actual DRGs are much larger.
For more information about DRGs, see the USGS DRG information page.
Sample Section of a DRG