General Information About Map Files

The following information is provided by the USGS MCMC. Please visit their web site for additional information.

What is a DRG?

A DRG is a raster image of a published map. DRG's 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 standardized to remove scanner limitations and artifacts. The average data set size is about 8 megabytes in Tagged Image File Format (TIFF) with PackBits compression. DRG's can be easily combined with other digital cartographic products such as digital elevation models (DEM) and digital orthophoto quadrangles (DOQ).

DRG viewing softwareUSGS DRG Viewer Web Page
It is recommended that users of the DRG map information visit the above page to obtain more detailed information about the DRG viewing software and information on latest releases. Click on image to visit the DRG viewer site!

Latest USGS Map Viewer, Version 2, self-extracting zip file available on this server

The Windows 95 MSVCIRT.DLL file may be missing from some systems and need to be installed in the C:\windows\system subdirectory. Please download this file if the viewer program indicates that it is missing. Most browsers will let you use your right mouse button to "click" on the file link and "save link as" to your computer.

What is a TIFF File?

An industry-wide standard for specifying cartographic information in TIFF tags, referred to as "GeoTIFF," has been developed by several organizations in the GIS community. These organizations include SPOT Image Corporation, NASA's Jet Propulsion Laboratory, Intergraph Corporation, Environmental Systems Research Institute (ESRI), and the USGS, among others. Geographic information is embedded in the TIFF data file in the form of descriptive tags. For detailed information about TIFF, GeoTIFF, and PackBits compression, refer to GEOTIFF.TXT text file or the MCMC web site. The most recent versions of TIFF and GeoTIFF specifications are available via World Wide Web at http://mcmcweb.er.usgs.gov/

What is a World File?

World files, required for image georeferencing by the viewing software applications on this disc, contain the following information:

Map Projection

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 credit note on the image collar.

Accuracy

The DRG maintains the horizontal positional accuracy of the 7.5-minute source map that meets National Map Accuracy Standards (NMAS). Because it is fit to the theoretical quadrangle corners based on the UTM projection, the DRG may have distortions outside the map projection lines.

Product Liability

The DRG files were prepared by an agency of the U.S. Government. Neither the U.S. Government nor any agency thereof nor any of their employees make any warranty, expressed or implied, or assume any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed herein or represent that its use would not infringe privately owned rights. Reference therein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. Government or any agency thereof.

Although all data published associated with the DRG files have been used by the USGS, no warranty, expressed or implied, is made by the USGS as to the accuracy of the data and related materials. The act of distribution does not constitute any such warranty, and no responsibility is assumed by the USGS in the use of these data or related materials.

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