So tell me... what's a DRG?
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 topographic map:
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 scanner, and
4.software to correct distortion and reference the scanned raster image to ground
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 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 DRG.
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 fewer files.
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 map collar.
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.
Source: United States Geological Survey (USGS)