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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.