Because the features of the topographic maps are of interest for so many purposes, some people have converted them to digital form in various ways. Digitizing maps is not a simple matter (nor is it a standardized process), and scanning these map sheets in their full size requires equipment most of us don't have access to.
Find and Purchase DRGs
Other Digital Versions of Topo Maps
What is a Digital Raster Graphic (DRG)?
Find and Purchase Digital Topo Maps
To view a status graphic and a list of DRG quadrangles available from USGS visit: http://geography.usgs.gov/www/products/status.html .
To find the DRG quad name you want, consult the list of USGS topographic maps at: http://topomaps.usgs.gov/
The USGS has private-sector business partners who sell USGS maps and digital data. Buying from these companies is usually the fastest way to obtain USGS products. Also noteworthy, the USGS does not endorse software, but is interested in systems that displays or uses DRGs.
The USGS distributes DRGs on Compact Disc-Recordable (CD-R). DRG orders are filled on demand. There is a base charge of $45.00 per order, plus $5.00 shipping, plus $1.00 for each DRG quadrangle purchased. Sale of DRGs in fixed 1-degree blocks was discontinued on October 1, 1998. The current policy allows any combination of quadrangles to be ordered. Phone USGS at 888-ASK-USGS, or order DRGs online from USGS Earth Explorer.
In Wisconsin, the DNR produced an enhanced DRG product. This non-copyrighted product is created by reprojecting the DRGs into both WTM 27 and WTM 83 (1991) projections, as well as recoding the collar part of the mapsheet to make it easier to join adjacent digital mapsheets when using certain map viewing software, such as ArcView. The WTM 83 files are available from the DNR, either as individual files to be downloaded or purchased as block coverage on CD-ROM.
Digital Topo Map Providers and Related VendorsTerraserver
TopoZone
MapFocus GeoTIFF
GIS Data Depot
Color Raster Topographic Maps
EarthWatch
OMNI Resources
MPN Components
Sure !Maps Raster
US Terrain Series with G-REF
TOPO! Interactive Maps
mytopo
Other Digital Versions of Topo Maps
Cartographic digital data are the electronic equivalent of a map. The data are in the form of files containing a set of coordinate points and information about what is located at those points. Because cartographic digital data use a large volume of computer memory and different users have difference applications, the data are categorized by type into several files.
Digital Elevation Models (DEMs)
Digital Elevation Models (DEMs) contain computerized terrain elevation information. Data is organized as a grid across the landscape and consists of elevations for ground points at regularly spaced horizontal intervals.
USGS produces five different digital elevation products. Although all are identical in the manner the data are structured, each varies in sampling interval, geographic reference system, areas of coverage, and accuracy, with the primary differing characteristic being the spacing, or sampling interval, of the data.
Digital Line Graphics (DLGs)
Digital Line Graphs (DLGs) are computerized versions of planimetric information. USGS created the term and produced DLGs from three of its map series: 1:2,000,000, 1:100,000, and 1:24,000. Separate DLG files are produced from a map sheet, each representing a different theme (hydrography, boundaries, transportation, etc.). National Map Accuracy Standards do not apply to the DLG series.
1:2,000,000-Scale DLGs The data source for this series is the 1970 National Atlas of the United States of America. The Northern Great lakes States sectional map covers Wisconsin, Michigan, and Minnesota. There are three categories of data: boundaries (political and administrative), hydrography (streams and water bodies), and transportation (roads, trails, railroads, and airports).
1:100,000-Scale DLGs The data source for the 1:100,000 DLGs is the 100,000-scale (30 x 60 minute) map series. Wisconsin coverage is complete for hydrography, the Public Land Survey and transportation. Additional data categories of boundaries and/or contours are available for many quads.
1:24,000-Scale DLGs The data sources for the 1:24,000 DLGs are the 7.5-minute and 15-minute map series. Coverage in Wisconsin is spotty and includes the data categories of Boundaries, Survey Control, misc. Culture, Hydrography, Hypsography, Public Land Survey System, Transportaion, Vegetation, and Nonvegetation.
Digitizing maps is not a simple matter (nor is it a standardized process), and scanning these map sheets in their full size requires equipment that most of us don't have access to.
People have digitized selected features from USGS maps for some time, but usually not over large areas nor to particular standards. Much of this work relates to development of data for geographic information systems (GIS) or computer-aided drafting (CAD)—not something easily mastered by people who simply want to see a topo map on their computer screen. The USGS has done some of this work, creating formats such as DLG and DEM.
Beginning in 1995, the USGS began a program to scan paper copies of currently maintained topo map series into Digital Raster Graphic (DRG) files (raster TIFF images). This work was completed by 1997 with 50% state coordinated costsharing, and further funds underwrote enhancements that help make the images more usable in a GIS context. Neither the paper maps, the standard DRGs, nor the enhanced products are copyrighted.
The 7.5-minute topo maps were scanned at 250 dot-per-inch minimum resolution. DRGs are available for all three scales of statewide USGS topo maps: 1:24,000, 1:100,000 (except county series), and 1:250,000. However, the DRG product standard had a complete revision in May 2001, and implemented in October 2001. This revision included two significant changes:
• original coverage DRGs contain only 13 colors that model the line-drawing nature of a printed map. An additional color model was added to the standard to allow DRGs to be made with a color palatte of up to 256 colors.• original coverage DRGs have scan resolutions of 250 dots per inch (dpi). The new standard allows scan resolutions anywhere in the range of 250 dpi to 1000 dpi.
DRGs are not "smart" GIS images, but simply scanned images. However, whether used with a GIS or more simply, these images have many uses.
Similar products have been developed from private sector firms, and while they are copyrighted (as is the viewing software that's included in some cases), these alternatives to the DRG may meet your needs more fully than the public product. For additional information pertaining to the USGS DRGs, visit http://www.umesc.usgs.gov/data_library/maps_quads_figs/drg.html.
Format, viewing and modifying
DRGs are in TIFF format. These "raster" (gridded) images can be viewed and printed at various scales with a variety of computer graphics programs and equipment. In the simplest use, a DRG can be printed at the same scale as the original map. DRGs can also be clipped, edited, and enhanced in various ways. File size is roughly 5-9MB per map sheet. A free viewing program called "dlgv32 Pro" is available to download from the USGS. For more information about the "dlgv32 Pro" software visit: http://mcmcweb.er.usgs.gov/drc/dlgv32pro.
GIS and CAD viewer software typically can display TIFF files.
From other sources, scanned topo map files may be in formats other than TIFF. Some commercial products include quite convenient specialized viewing software, some of which allows draping of the image over a terrain model.
Georeferencing and overlay
A "georeferenced" dataset contains information needed to relate image coordinates to a ground coordinate system., or put more simply, georeferencing information is added to the DRGs to associate the maps with their true position on the ground. This allows for some viewing software to disclose coordinate positions, automatically calculate map scale or distance, and perform other spatial manipulations of the maps.
The DRGs are referenced to the UTM coordinate system, but can be modified to fit others by using specialized software. Wisconsin DRGs have been converted to both WTM 27 and WTM 83 (91 adjustment) coordinate systems.
The DRG product contains georeferencing information in three places:
• in GeoTIFF tags of the TIFF image file (*.tif)
• in a companion metadata file (*.fgd)
• a "world file" (*.tfw) contains partial georeferencing information
Other georeferenced digital information (e.g., Digital Line Graphs or Digital Orthophoto Quadrangles) may be overlaid more easily by matching coordinates. DRGs referenced to a common coordinate system can be joined edge-to-edge to form a continuous map coverage, provided that each map's "collar" can be hidden. The Wisconsin Department of Natural Resources (DNR) has processed all state DRGs in this way.
Visual quality and resolution
The grid pattern of a 1:24,000-scale (7.5-minute) DRG represents cells with a ground size of 8 feet. At original map scale this is .004 inches which captures topo map symbols and text well. Color consistency is good but when enlarged several times usually doesn't equal the original paper map.
Accuracy and Scale
The absolute spatial accuracy of a feature visible on a DRG is based on the accuracy of the original printed map with some additional error introduced through scanning and further processing. For DRGs from 7.5-minute quadrangle maps, accuracy is probably better than 60 feet in most cases. Relative accuracy between nearby features is usually better than the absolute accuracy. And, like any computerized map file, a DRG can be viewed or plotted at any size; however, it does not become more accurate simply because it is enlarged.
USGS maps are designed to meet the National Map Accuracy Standard. The DRG production process is designed to retain this accuracy in the digital version of the product, but not to improve it. DRG georeferencing reduces map error caused by paper shrinking an stretching, but it also introduces error during the manual matching of image and control points. The overall accuracy of DRGs is approximately the same as the accuracy of the paper maps there were derived from.
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