The image inside the map neatline 
is oriented, scaled and geo- 
referenced by rubber-sheeting to the 
true values of the published map’s 
geographic coordinate tick marks. 
All 16 2.5-minute tick marks on a 
USGS 1:24,000-scale quadrangle are 
used for this control. The map is 
reprojected to Universal Transverse 
Mercator (UTM). The geodetic 
datum of the source map is 
preserved in the DRG. 
The horizontal positional accuracy 
of the DRG matches the accuracy 
of the published source map; 
therefore, the 7.5-minute DRG 
complies with the National Map 
Accuracy Standards for the 
7.5-minute topographic map. The 
distributed 1:24,000-scale DRG at 
250 dpi will have a ground sample 
distance of 2.4 meters. 
DRG colors are fit to a 12-color 
palette that will be constant for all 
USGS standard quadrangles. Some 
scanning systems are accurate 
enough to reduce the color palette 
further, to the precise number of 
ink colors on the original map 
(seven, including white, on a USGS 
1:24,000-scale quadrangle). For 
some applications, however, it may 
be desirable to scan more colors. 
The DRG product specification 
will allow a range of color 
characteristics. 
DRG's are distributed in the 
Tagged Image File Format (TIFF) 
using packbit compression. TIFF is 
a widely accepted raster file format 
and can be read directly by many 
graphics systems and GIS's. This 
compatibility with existing appli- 
cation software is critical for 
DRG's. A 1:24,000-scale DRG at 
250 dpi contains about 60 million 
28 
pixels, so full decompression is not 
practical on most computers. 
Packbit compression reduces the 
size of a 1:24,000-scale DRG to 
between 5 and 15 megabytes. 
DRG's will also be available in 
Spatial Data Transfer Standard 
(SDTS) format in 1996. 
© Complete metadata, compliant with 
the Federal Geographic Data 
Committee (FGDC) standard, will 
be part of each data set. Metadata 
will be stored in a separate physical 
file in ASCII text. 
3. GIS APPLICATIONS OF DRG's 
Since DRG's are relatively new, it is 
difficult to predict how, or for how long, 
they will be useful to the GIS community. 
One important use is certainly to provide a 
spatially accurate, visually appealing 
backdrop for thematic vector data. Like 
the standard quadrangles from which they 
are derived, DRG's can furnish consistent 
base data for the entire country. 
3.1 Derivative Products 
DRG data can be combined with other 
kinds of digital cartographic data. Shaded 
relief maps can be made quickly and 
cheaply by combining DRG's with digital 
elevation models (DEM's). Digital 
orthophoto maps can be made by 
removing the white background and area 
tints from DRG's and then combining the 
data with digital orthophotos. 
3.2 Vector Collection and Revision 
Base-category vector data can be digitized 
directly from a DRG on a computer 
screen. This is not cost effective for such 
data categories as contours. For most 
categories, however, on-screen digitizing 
provides a cheap and easy alternative to 
scanning or 
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