U.S. Geological 
ugh contracts 
mers. The 
lized to remove 
es in Tagged 
combined with 
"ital ortho- 
es vector data 
methods. DRG's 
ic data. A 
/scopic vector 
lection and 
e entire country. 
irtography 
t new. Some of 
lere simple raster 
nbols on line 
not very useful. 
were not visually 
seful for 
tor models 
)blems. Vector 
ounts of useful 
nall enough to be 
iters of the 
cartography 
fore, emphasized 
er visualization 
ght up with 
Interest in 
an to grow again 
as computers powerful enough to display 
map images became relatively common 
and affordable. 
Today, desktop workstations are easily 
powerful enough to manipulate raster 
images of large maps. Today's geographic 
information systems (GIS) can use both 
vector and raster data. 
1.2 Background of USGS Raster Graphic 
Product 
Various Federal agencies have experi- 
mented with, and in some cases sold, 
different kinds of raster maps in the past 
few years. Several private companies offer 
similar products. 
The U.S. Geological Survey (USGS) 
proposed a digital raster graphics program 
for USGS standard quadrangles in early 
1994. The product name digital raster 
graphic was already in use in the 
cartographic community. The USGS 
adopted this name rather than cause 
confusion by establishing a new one. 
Through the first part of 1994, the USGS 
produced a few dozen DRG’s of its 
1:24,000-scale topographic quadrangles. 
These test maps were used for demon- 
strations to other Federal and State 
government agencies. This helped to 
refine the requirements for a DRG 
product. 
In October 1994, a "Product Development 
Report" on compact discs-read only 
memory (CD-ROM) was released to the 
public. This report solicited comments 
from the users on the proposed product. 
The CD contained several dozen standard 
USGS quadrangles at 1:24,000,1:100,000, 
and 1:250,000 scale, as well as docu- 
mentation, metadata, and examples of 
derivative products. 
27 
2. CHARACTERISTICS OF USGS 
DRG's 
DRG's are true cartographic products, not 
simple document scans. They have correct 
scale and orientation, as well as a precisely 
defined relationship with ground 
coordinates. 
DRG's are less complex than digital 
orthophotos or topological vectors, but 
they are not trivial to produce. As with 
other kinds of maps, there are many 
different ways to define and produce a 
DRG. The desirable characteristics of the 
final product depend on how the data will 
be used. 
The objectives of USGS DRG specifi- 
cations are to (1) preserve the positional 
accuracy, feature content, and general 
appearance of a published map, (2) make 
the data available to a wide variety of 
users and application software throughout 
the public domain and (3) allow flexibility 
by permitting a reasonably broad range of 
technical specifications. 
Some of the important technical 
characteristics of USGS DRG's follow. 
o  USGS DRG's are produced by 
scanning published paper maps. 
The scanned image includes all map 
collar information. Scanning map 
separates was considered but ruled 
out as too slow and expensive. 
Scanning paper maps does not 
measurably affect the positional 
accuracy of a DRG. 
©  J Maps are scanned at 500 dots per 
inch (dpi). The data are resampled 
during processing to a final 
resolution of 250 dpi. The DRG 
product specification will allow a 
broad range of resolutions.