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Enhancements to DLG Geographic Features in DLG Sample Maps Drawn with DLG Geographic Coverage of DLG |
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A digital map of the United States, developed by the U.S. Geological
Survey, which contains detailed maps of all 50 states. The most
recent data in the database dates from 1980. Data in the database
were derived by digitizing the National Atlas of the United States at a
scale of 1:2,000,000. Successive points in the database are never
more than 2 miles (3 km) apart.
Enhancements made to USGS DLG for use with Versamap:
USGS DLG has 72 geographic features in the
following 7 major categories:
Sample maps drawn using USGS DLG:
The following maps were exported from Versamap 3 as Windows Bitmaps
(BMP) and as Windows Metafiles (WMF). To reduce file size,
each BMP image was converted to portable network graphics (PNG) format
with Microsoft Paint. WMF images are vector graphics files and
can be enlarged and reduced without loss of detail, whereas BMP and PNG
images are bitmapped files and do not enlarge or reduce well.
The following map (WMF, 6098 KB)
of the United States was drawn on an Albers equal-area conic
projection. The map shows the coastline; islands; lakes; and
state and county boundaries. Most geographic features, such as
roads; railroads; canals; national parks; rivers; and intermittent
rivers and lakes are not shown.
The following map (WMF, 804 KB)
of the Southwestern United States was drawn on a Cylindrical equal-area
projection. The map shows the coastline; islands; perennial
rivers and lakes; and state boundaries. Most geographic features,
such as roads; railroads; canals; national parks; intermittent rivers
and lakes; and county boundaries are not shown.
The following map (WMF, 1142 KB)
of Cape Hatteras was drawn on a Equirectangular projection. The
map shows the coastline; islands; perennial rivers and lakes; and state
and county boundaries. Most geographic features, such as roads;
railroads; canals; national parks; and intermittent rivers and lakes
are not shown.
Geographic coverage of USGS DLG files:
The USGS DLG database contains detailed maps of all 50 states.
The database divides the United States into the following 21 sections.
The USGS DLG database consists of the following 22 digital
maps. The first 21 files (S01.grv through S21.grv) cover the 21
individual sections of the United States shown in the above
figure. Usgs_all.grv, covers the entire country, and was formed
by concatenating the 21 sectional files.
| Filename | Size (bytes) | Section |
| Snn.grv |
There is 1 .GRV file for each of
the 21 USGS DLG sections, where nn is the section number. See
the above figure for the states in each section. |
|
| Usgs_all.grv | 18,075,424 | Entire country. |
As an additional example of the detail available in the DLG data, the
file UsaEast.grv, which contains all data in the database for the east
coast of the United States near Cape Hatteras, is included with the
shareware version
Versamap.
The binary file format used to store the USGS DLG .GRV files was
developed by Fred Pospeschil to store World Data Bank II in a compact,
binary form.