DRG data is now available for download from the GISDataDepot only via the account mechanism. In order to access these data you will need to setup a data download account.
See Also http://www.gisdatadepot.com/sdts/
A digital raster graphic (DRG) is a scanned image of a U.S. Geological Survey
(USGS) topographic map. The scanned image includes all map collar information.
The image inside the map neatline is georeferenced to the surface of the
Earth. The DRG can be used to collect, review, and revise other digital data,
especially digital line graphs (DLG). When the DRG is combined with other
digital products, such as digital orthophoto quadrangles (DOQ) or digital
elevation models (DEM), the resulting image provides additional visual
information for the extraction and revision of base cartographic information.
The USGS is producing DRG's of the 1:24,000-, 1:24,000/1:25,000-, 1:63,360-
(Alaska), 1:100,000-, and 1:250,000-scale topographic map series.
Data production is through an innovative partnership agreement with Land
Information Technologies, Ltd., of Aurora, Colo. The order and rate of
production will be determined by user requirements and by partnerships with
Federal and State agencies. Full coverage of the contiguous United States is
expected to be completed in late 1997.
Producing a DRG
Four items are needed to produce a DRG of a 7.5-minute
1.a USGS topographic map,
2.the Universal Transverse Mercator (UTM) coordinates of the sixteen
2.5-minute grid ticks for georeferencing and rectification,
3.a digital image produced by scanning a USGS map on a high-resolution
4.software to correct distortion and reference the scanned raster image to
At the USGS, the first step in the process is to scan a paper 7.5-minute
topographic map at 250 dots per inch. The position of each of the sixteen
2.5-minute grid ticks on the image is collected. Software uses these
coordinates to rectify and georeference the image to the UTM ground
coordinates. A bilinear transformation completes the georeferencing.
The image is compressed using lossless compression to reduce the size of the
data set. The final result is a compressed TIFF 6.0 file. The file size range
is between 5 to 15 megabytes.
The standard USGS 7.5-minute DRG has the following specifications:
The source material for a DRG is a paper USGS topographic map.
The USGS DRG's are in TIFF 6.0 and employ GeoTIFF 0.2 specifications to
define a set of TIFF tags. These tags describe all cartographic information
associated with the file.
The map is scanned at a minimum resolution of 250 dpi.
The digital image is georeferenced to the true ground coordinates of the
2.5-minute grid ticks and projected to the UTM for projection consistency with
USGS DOQ's and DLG's. The datum of the source materials is preserved in the
If scanned at a finer resolution, the image is resampled to 250 dpi. The
image is converted to an 8-bit color image in a compressed TIFF file.
Color values are standard between DRG quadrangles. The USGS employs up to
13 colors on each DRG. Color values are contained in each TIFF file.
The digital image is accompanied by a metadata file that complies with
the Federal Geographic Data Committee's Content Standards for Digital
Geospatial Metadata (June 8, 1994).
The DRG's will be available on Compact Disc-Recordable (CD-R); each CD-R
includes the USGS topographic maps for a 1-degree cell. For the contiguous
States plus Hawaii, the cells will usually contain sixty-four 1:24,000-scale
files, two 1:100,000-scale files and one 1:250,000-scale file. Variability
in the number of files and area covered occurs over irregularly-shaped land
masses; for example, 1-degree cells that encompass coastal areas may include
Attribute and Positional Accuracy Requirements
The DRG uses a standard palette to ensure uniform color throughout a
particular map series. The RGB values for a particular color, therefore, will
remain consistent throughout that DRG series. Although the color values of the
DRG may sometimes match those of the paper source map, a user will usually
notice small differences between the colors on the digital image and on the
paper map. Also, the quality of the user's monitor affects the DRG color
displayed. Although the DRG generally contains the complete content of the
source map, features may occasionally be blurred because of substandard source
materials.The DRG also may contain misclassified pixels (color noise).
The horizontal positional accuracy of the DRG matches the accuracy of the
published source map. To be consistent with other USGS digital data, the image
is cast on the UTM projection and will, therefore, not always be consistent
with the credit note on the image collar. Only the area inside the map
neatline is georeferenced, so minor distortion of the text may occur in the
The distributed 1:24,000-scale DRG at 250 dpi will have a ground sample
distance of 2.4 meters (8 feet).
Uses of a DRG
The DRG is useful as a backdrop onto which other digital data can be overlaid.
At the USGS, the DRG is used for validating DLG's and for DLG data collection
and revision. The DRG can help assess the completeness of digital data from
other mapping agencies. It can also be used to produce "hybrid" products.
These include combined DRG's and DOQ's for revising and collecting digital
data, DRG's and DEM's for creating shaded-relief DRG's, and combinations of
DRG, DOQ, and DLG data. Although a standard DRG is an effective mapping tool,
its full potential for digital production is realized in combination with
other digital data.
United States Geological Survey (USGS)