Streets, Water Features, Transportation
The USGS Digital Line Graph (DLG) 100K and 24K data, plus the Census Bureau TIGER/LINE
files were used for the basic planimetric data for TopoDepot.
We have used three different sources for the planimetric data, because each source
has it's advantages and disadvantages.
USGS 24K Digital Line Graph (DLG) Data
This data from the USGS is exceptional! It has excellent quality street centerlines
and water feature accuracy. Unfortunately, the USGS has only completed part of the
USA in this new digital series. Also, it has no street names. To get around the
lack of street names, we have taken the TIGER street names, and combined them with
the 24K linework.
Accuracy: The 24K DLG files have a stated accuracy of better than +- 33 feet.
Currency: The 24K DLG files have been digitized starting around 1996, using
the most up-to-date quad sheets available at the time of digitization.
Coverage: Only part of the USA has been completed.
Click here for the current status map showing what has been published by
the USGS.
USGS 100K Digital Line Graph (DLG) Data
The 100K DLG data was completed by the USGS in the mid 1980s. It is remarkably good
considering its age. We only use this data in urban areas where the TIGER road centerlines
are poor. Also, it has no street names. To get around the lack of street names,
we have taken the TIGER street names, and combined them with the 100K linework.
Accuracy: The 100K DLG files have a stated accuracy of better than +- 166
feet.
Currency: The 100K DLG files have been digitized starting around 1985, using
the most up-to-date 100K quad sheets available at the time of digitization.
Coverage: 100% of the USA has been completed.
TIGER/LINE Data
These files are the most up-to-date digital maps currently available from the US
Government. They are identical to the USGS 100K DLG feature data in rural areas,
but are more up-to-date. They also have excellent attributes such as street
names, zip codes, and street addresses. In urban areas, the positional accuracy
of the TIGER files is quite poor. Recently, we improved the TIGER data centerline
accuracy in urban areas by allowing the substitution of 100K or 24K DLG (USGS Digital
Line Graph) for the TIGER centerlines, while keeping the TIGER street names (since
DLG has no street names!).
Accuracy: Map accuracy for these maps varies by feature and by geographical
area. Each type corresponds to National Map Accuracy standards for the original
data from the US Government. The TIGER maps were derived from the USGS 1:100,000
DLG data in rural areas. The 100K DLG files have a stated accuracy of approximately
±166 feet. In urban areas, the Census Bureau DIME data were used; they have no accuracy
specifications, but generally are quite poor.
Currency: The TIGER files are quite up-to-date, with major renovations occurring
with the 2000 Census.
Coverage: 100% of the USA has been completed.
The TopoDepot Decision Tree for determining the default selection is:
 |
If 24K DLG is available,
use this, enhanced with TIGER street names. |
|
|
Else, if a Rural area, use TIGER/LINE
data |
|
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Else, if an Urban area, use
100K DLG, enhanced with TIGER street names |
