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USGS Digital Raster Graphics


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Overview of the USGS Digital Raster Graphic (DRG) Program

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HISTORY

Digital raster graphics (DRG) were produced from 1995 to 1998 by the U.S. Geological Survey (USGS) through an innovative partnership agreement with the Land Information Technology Company, Ltd., of Aurora, Colorado. The objective of the program was to scan all USGS standard quadrangle maps quickly and at low cost.

More than 60,000 maps were scanned during the original program. The series includes DRGs of USGS standard quadrangle maps of the United States and its trusts and territories.

Since 1999 about 1,000 new DRGs have been made per year. In May 2001 a complete revision to the Standards For Digital Raster Graphics was approved. Starting in October 2001, all new DRGs are produced to the revised standard. The new standard is backwardly compatible with the original standard: all original coverage DRGs also conform to the revised standard. The most noticeable change in the new "second generation" DRGs is an increase in scan resolution from 250 dots per inch (dpi) to 500 dpi. For more information about the revised standard, see http://topomaps.usgs.gov/drg/drg_standard_change.html

AREAS OF COVERAGE

DRGs have been made for all quadrangles in the USGS standard topographic map series for the United States, its territories and trusts.  These maps include:

There are two significant holes in USGS DRG coverage: most of California and areas around Tennessee.  Both result from agreements with other agencies to make and distribute DRGs for their areas of interest. These agreements were made early in the DRG program, in part to avoid duplication of work by multiple Government agencies.  See the DRG status graphic for approximate areas covered by these two agencies, and refer to their web sites for product and ordering information. DRGs produced by these organizations conform to slightly different technical specifications than USGS DRGs.

DESCRIPTION OF A DRG

Most DRGs are made by scanning published paper maps on high-resolution scanners. The raster image is georeferenced and fit to the Universal Transverse Mercator (UTM) projection. Colors are usually standardized to duplicate the line-drawing character of the published map. The average data set size of a 7.5-minute DRG is about 8 megabytes in Tagged Image File Format (TIFF) with PackBits compression.

The DRG product standard can be found at http://rockyweb.cr.usgs.gov/nmpstds/drgstds.html.

PHYSICAL FILE FORMATS AND NAMES

USGS DRG images are stored in TIFF version 6.0. PackBits compression (run-length encoding) is used according to the TIFF standard. Georeferencing information is contained in the TIFF file according to the GeoTIFF standard, version 1.0.

The TIFF 6.0 standard is copyrighted by Adobe Systems Incorporated. Copies of the standard can be found at http://partners.adobe.com/public/developer/tiff/.

The GeoTIFF Web page is at http://www.remotesensing.org/geotiff/geotiff.html.

The USGS DRG product consists of two physical files: the TIFF image and a text file of metadata. A third file, an Arc/Info world file, is also currently included, though this is not required by the product standard.

The base names of all files follow a convention designed to allow standard USGS quadrangle locations to be uniquely coded within the old DOS 8.3 file name restrictions. The names are based on the map reference code (MRC), and are a legacy of the mid-1990s, before long file names were practical on desktop PCs. For details about this naming convention, see http://topomaps.usgs.gov/drg/drg_name.html.

Relating DRG file names to quadrangle names is easy for individual datasets -- simply look at the DRG image or the metadata file to find the quadrangle name. But quickly relating file names to quadrangle names for large numbers of datasets is more of a problem. One 7.5-minute cell can have many products, and each product can have many instances. MRC codes, cell names, and product names are text strings, not database keys. Names are neither unique nor permanent, so there is no simple and clean way to relate many DRG file names to the corresponding map or cell names. GIS developers may find the file referenced at the end of this paragraph helpful. The file is a plain-text, delimited listing of all USGS standard geographic cells with their map reference codes, suitable for importing into spreadsheets or databases. Documentation about the meanings of the fields is included at the top of the file. Download gzip'd text file.

GEOREFERENCING PROCEDURES

The map image is fit to the theoretical UTM coordinate positions of the published map's graticule ticks. The number of fixed tick marks used for control varies with map series:

Map Series

1:24,000

1:25,000 (7.5 x 7.5)

1:25,000 (7.5 x 15)

1:100,000

1:250,000

1:63,360 (Alaska)

Number of gratitude ticks

16

16

28

15

45

varies with latitude

The exact transformation used to fit the image to the control points is not dictated by the DRG product standard. Transformations implemented in a variety of commercial GIS packages have been tested and found to produce identical results (that is, the differences between them are less than the size of 1 DRG pixel).

DATA RESOLUTION

DRGs made before October 2001 have scan resolutions of 250 dots per inch (dpi). Most DRGs made after October 2001 have scan resolutions of 500 dpi. The ground resolution of the image is directly related to map scale. For 250 dpi images, this relationship is:

Map Scale                        DRG ground resolution

1:24,000                                    8 ft = 2.438 meters

1:25,000                               8.33 ft = 2.54 meters

1:100,000                           33.33 ft = 10.16 meters

1:250,000                           83.33 ft = 25.4 meters

1:63,360 (Alaska)              21.12 ft = 6.44 meters

Both the scan and the ground resolutions of a DRG are simply measures of pixel size. Unlike a digital aerial photograph, the resolution of a DRG is not a measure of the detail that can be seen on the map.

250 dpi was originally selected as the standard DRG resolution because:

The 2001 revision of the DRG product standard permits scan resolutions of up to 1000 dpi. Starting in October 2001, all new USGS DRGs have scan resolutions of 500 dpi.

PROJECTIONS AND DATUMS

To be consistent with other USGS digital data, the image is cast on the UTM projection. Therefore, the projection of the DRG will not always be consistent with the map credit note that is part of the scanned image. For a detailed discussion of the DRG use of the UTM, see http://topomaps.usgs.gov/drg/mercproj/index.html.

DRGs usually retain the datum of the source map. USGS quadrangles are defined by lines of latitude and longitude. Changing the map datum moves these lines relative to the real surface of the Earth. Changing the datum of a quadrangle has one of two effects: either the geographic extent of the map changes, or the edges of the map no longer fall on even 7.5-minute graticule lines. Both cases are confusing and are avoided by retaining the datum of the published map in the DRG.

POSITIONAL ACCURACY

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 and 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 they were derived from.

COLORS

USGS standard topographic maps are printed with no more than seven colors (including white). Up to six additional colors are simulated with lithographic screens. Most DRGs use a color model designed to duplicate the line-drawing nature of the map. The original DRG product standard defines a standard color palette of 13 colors. The colors are indexed according to the TIFF standard, with the additional requirement that the TIFF color look-up table be exactly the same for every DRG. The colors are always indexed in the same order, with the same RGB values. See the DRG product standard for details.

This color model was selected for several reasons, some of which were more important in 1994 than they are now.

This color model does not generalize well to maps with more colors than the "normal" USGS six-color topographic quadrangle. The revised DRG standard of May 2001 permits a second color model of up to 256 non-standard colors. DRGs made with this color model typically have higher visual quality, but may be less suitable for automatic data manipulation and GIS applications. USGS DRGs will continue to use the original model of 13 standard colors whenever the source materials make it technically feasible.

METADATA

DRG metadata files contain information about content, quality, and other data characteristics. Metadata are compliant with the Federal Geographic Data Committee (FGDC) Content Standard for Digital Geospatial Metadata (June 8, 1994). The content and composition of the DRG metadata files are described in the Metadata Template, Appendix 2-A of the DRG product standard.

The FGDC standard is a content standard. It does not specify any physical file format for cartographic metadata. DRGs implement the metadata standard by putting metadata in a separate physical file in flat ASCII text. There is exactly one metadata file for each TIFF image file.

Metadata files made before 1999 do not conform to any formal standard for physical format. The files are simple ASCII text, and the information is organized to mirror the FGDC content standard.  DRGs made since 1999 use a similar but more stable format based on conformance with the "metadata parser" (mp) software tools. This format is also ASCII text but has more rigorous rules for text arrangement.  See http://www.fgdc.gov/metadata/geospatial-metadata-tools for more information.

The FGDC home page is at http://www.fgdc.gov. A copy of the metadata content standard can be found at http://www.fgdc.gov/metadata/geospatial-metadata-standards.

PRODUCT DISTRIBUTION

USGS DRGs are sold by the USGS on Compact Disc-Recordable (CD-R) media. DRGs are also sold by USGS business partners and data partners. See the DRG home page at http://topomaps.usgs.gov/drg for details.

GEOREFERENCING INFORMATION

A "georeferenced" dataset contains information needed to relate image coordinates to a ground coordinate system. DRGs are georeferenced to the plane ground coordinates of the Universal Transverse Mercator (UTM) coordinate system. The DRG product contains georeferencing information in three places:

GEOTIFF

The GeoTIFF standard was developed in the mid-1990's by a consortium of private companies and Government agencies. The effort was led by the SPOT corporation, and the first versions of the standard were written primarily by Niles Ritter, then with SPOT. The USGS did not play a significant role in developing the standard, but the USGS DRG program was one of the first large implementations of GeoTIFF.

Like the base TIFF standard, GeoTIFF is conceptually simple, but the details are complex and technical. For a copy of the standard and other information, see the GeoTIFF home page at http://www.remotesensing.org/geotiff/geotiff.html. The revised DRG standard of May 2001 defines specific GeoTIFF requirements and restrictions for USGS DRGs.

METADATA FILE

Each TIFF image file has exactly one companion file (*.fgd) containing metadata. (See metadata above for references.)

Most of the data in the metadata file, especially information about the source graphic map, come from USGS databases. These databases originally were populated by transcribing information from printed maps and therefore contain some errors. In cases of conflict between different metadata sources, the map collar usually is the most reliable. Map projection is an important exception to this rule, as nearly all DRGs are cast to the UTM projection, regardless of the projection of the published map.

The metadata file contains complete georeferencing information. However, the metadata file format is not directly useable by standard GIS software, so georeference values must be transcribed from this file to a GIS by hand.

WORLD FILE

World files (*.tfw) are not required by the DRG product standard, but to date they have been included with all product distributions as a service to users of ESRI software and other commercial software that uses ESRI's world files.

The world file contains the ground coordinates of the upper left pixel of the TIFF image (the (1,1) or (0,0) pixel, depending on convention), and scale and rotation information.  World files do not contain projection and datum information. More information about world files can be found at http://support.esri.com/

UPDATES AND PRODUCT MAINTENANCE

Replacement DRGs are made from the same map as an existing DRG to correct technical errors. New-version DRGs are made of new maps. These terms and associated policies are explained in more detail below.

List of new DRGs, ordered by month from the most recent.

A plain-text list of all DRGs currently available for sale can be retrieved from ftp://edcftp.cr.usgs.gov/pub/metadata/DRG/

REPLACEMENT DRGs - ERROR CORRECTION

When an error is found in the TIFF image file or the FGDC metadata file, both files are replaced. The original files are then no longer available for sale.

The rate of error discovery has fallen steadily in recent years but still averages about one instance per month. When a user report is confirmed to be an actual data error, the error is fixed. If a user outside the USGS reported the error, a new copy of the data is sent to that person.

For some types of errors, any user who purchased a precorrection dataset may request a free replacement. DRGs replaced to correct georeferencing or serious image quality errors will be replaced free of charge for one year after the error is found and corrected. To report new errors or request replacements, send email to mcmcesic@usgs.gov.  Please include the quadrangle name, the scale, and the approximate date the data were purchased.  Free replacements are distributed only through FTP; replacements on CD must be purchased like new data.  This offer applies only to data purchased directly from the USGS.  If the DRG was obtained somewhere else, please contact the actual data provider.

NEW VERSION DRGs - UPDATED MAPS

The USGS has several programs for revising and updating its topographic maps. When a new version of a map is produced, a new DRG is made. The previous version remains available for sale as an earlier version of the map.

New DRGs are not made from "as-is reprints." Maps that are out of stock are sometimes reprinted without modification. In most cases, the only thing that changes on such a map is the date of printing.

There is a significant production backlog of U.S. Department of Agriculture Forest Service (FS)  "single-edition" maps -- The USGS and the FS have an interagency agreement governing the update of maps that contain national forest lands. The purpose of this agreement is to reduce duplicate mapping by the two agencies. Maps revised by the FS conform to slightly different graphic standards than regular USGS quadrangles. One significant difference is that single edition maps contain additional colors that do not map well to the standard 13 DRG colors. The revised DRG standard of May 2001 permits high quality scans of these maps, and the USGS has started working on this backlog. For more information and sample data, see http://topomaps.usgs.gov/drg/drg_standard_change.html.

CURRENT PRODUCTION METHODS

The DRG program from 1995 to1998 was designed to be "one-pass" data production. DRGs were made of some 60,000 USGS maps during this time. In almost all cases, the DRG was made by scanning a copy of the published paper map. Beginning in early 1999, the program began moving into a data maintenance phase. New DRGs are now made from several different data sources. The resulting DRGs conform to the same product standard, but the images may look significantly different.

DRGs can also be made by scanning map feature separates instead of paper. The scans are then digitally combined to make a DRG. This is much more expensive than scanning a paper map; therefore, DRGs currently are not produced by this method except when there are other reasons for scanning the original map materials.

All USGS DRGs made after October 1, 2001 have scan resolutions of 500 dpi. Maps other than the standard six-color topographic quadrangles may have a nonstandard color palette with up to 256 colors. The current rate of DRG production is about 1,000 quadrangles per year. At this time, the USGS has no plans to systematically replace all DRGs with higher resolution scans.

Differences in map revision and DRG production methods have no effect on either the positional accuracy or the content completeness of the DRG. These reflect the characteristics of the published map in all cases.

DRGs FROM NON-USGS SOURCES

The term "digital raster graphic" was not invented by the USGS and is not owned by any data producer. A USGS DRG conforms to particular technical specifications, which other data producers are not obligated to follow. The USGS DRG specification represents an attempt to produce one standard product that will be appropriate for a relatively wide variety of applications. The USGS makes no claim that a USGS DRG is superior to other products made to different specifications.

Other Government agencies and many private companies make their own products from scanned images of USGS maps. Please contact the original data provider for assistance with non-USGS products.

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