DLG, DRG and DOQ File Types
The USGS has been a driving force in GIS. Some of the USGS formats date back to the 1980s – just when Geographic Information Systems were emerging from its roots.
What you’ll discover in this article are three USGS formats used to store GIS data.
These three USGS file types are:
- Digital Line Graph (DLG) are vector files generated on traditional paper topographic maps such as township & ranges, contour lines, rivers, lakes, roads, railroads and towns.
- Digital Raster Graphic (DRG) are scanned images of a U.S. Geological Survey (USGS) standard series topographic map georeferenced to the Universal Transverse Mercator projection.
- Digital Orthophotos Quadrangle (DOQ) are geometrically-corrected orthophotos photograph at a scale of 1:40,000.
Let’s explore USGS formats in a bit more detail:
What is a Digital Line Graph (DLG)?
Digital Line Graph (DLG) is vector data format developed and distributed by the United States Geological Survey (USGS).
DLGs depict information about geographic features about terrain, administrative, hydrography, transportation, man-made features and more. DLGs are prepared at medium to large scales as part of the National Mapping Program. All DLG data distributed by the USGS have a full structure of attribute codes, topology and have passed a level of quality-control checks.
How do you download Digital Line Graph GIS files?
Digital Line Graphs can be downloaded on USGS Earth Explorer. Under the data sets tab, click the “Digital Line Graphs” option. You have the option of DLG 1:100K and DLG Large Scale. Here’s a quick tutorial how to download USGS Earth Explorer data.
What is a Digital Orthophotos Quadrangle (DOQ)?
Digital Orthophotos Quadrangle (DOQ) is an orthophotos photograph produced by the USGS. Orthoimages are geometrically corrected and have an area of interest with a scale of 1:40,000. The spatial resolution is about 1 meter pixels.
As the computerization of geographic data accelerated in the 1980s, there was a need to produce DOQs for a GIS base layer and as a point of reference. DOQs began production in 1991 by the USGS Western Mapping Center but was short-lived as efforts of DOQ creation shifted to the private section in 2000.
How do you download Digital Orthophotos Quadrangle GIS files?
Digital Orthophotos Quadrangle can be downloaded on USGS Earth Explorer. Under the data sets tab, click the “Aerial Imagery” option and select the DOQ option. Here’s a quick tutorial how to download USGS Earth Explorer data.
What is a Digital Raster Graphic?
Digital Raster Graphics (DRG) are digital version maps of USGS fine to medium scale topographic maps.
DRGs have These topographic maps are based on the Universal Transverse Mercator projection using the North American Datum 1983. Features in these topographic maps include imagery (NAIP), roads (Interstate, route, ramp and local roads), toponymy names, hydrography (rivers, lakes), elevation contours and boundaries.
Here’s how to download Digital Raster Graphic GIS data.
The History of USGS Formats
TIGER has been releasing TIGER/Line files for over 20 years. In the 1980s, the US Census Bureau pioneered the creation of GBF/DIME coverage files. Dual Independent Map Encoding (DIME) was the encoding structure to store the geographic data. The file format developed for storing the DIME-encoded data was known as Geographic Base Files (GBF).
During this same era, the USGS was developing their own GIS data called Digital Line Graphs (DLGs). DLGs are at a scale of 1:100,000 which include transportation, hydrography and United States boundaries. The two federal agencies collaborated to build a national geospatial database merging their data. DLG features were included as part of GBF/DIME coverage files.
This is when the US Census Bureau replaced the data format with Topologically Integrated Geographic Encoding and Referencing (TIGER) in 1990.