Information about the Digital Maps from Global Positions
More than 60,000 maps were scanned as part of USGS’s desire to create digital versions of the standard paper maps. The series includes the USGS standard quadrangle maps of the United States and its trusts and territories. The first wave of digital maps were produced from 1995 to 1998 by the U.S. Geological Survey (USGS) through an innovative partnership agreement with a few private contractors. The objective of the program was to scan all USGS standard quadrangle maps quickly. These individual digital maps are in the public domain.
Since 1999 about 1,000 new digital maps have been made per year. In May 2001 a complete revision to the Standards For the digital maps was approved. Starting in October 2001, all new digital maps were produced to the revised standard. The new standard is backwardly compatible with the original standard: all original coverage maps also conform to the revised standard. The most noticeable change in the new "second generation" digital maps is an increase in scan resolution from 250 dots per inch (dpi) to 500 dpi.
The digital maps 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 electronic map is about 8 megabytes in Tagged Image File Format (TIFF) with PackBits compression.
USGS digital 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 digital quadrangle maps have been made for all quadrangles in the USGS standard topographic map series for the United States, its territories and trusts. These maps include:
Digital quad maps made before October 2001 have scan resolutions of 250 dots per inch (dpi). Most digital maps 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 Digital Quad Map 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
250 dpi was originally selected as the standard resolution because:
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 digital maps use a color model designed to duplicate the line-drawing nature of the map. The original 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 image. The colors are always indexed in the same order, with the same RGB values. See the digital image product standard for details.
This color model was selected for several reasons, some of which were more important in 1994 than they are now.
CURRENT PRODUCTION METHODS
The Digital Quad Map program from 1995 to1998 was designed to be "one-pass" data production. Digital Quad Maps were made of some 60,000 USGS maps during this time. In almost all cases, the Digital Quad Map 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 Digital Quad Maps are now made from several different data sources. The resulting Digital Quad Maps conform to the same product standard, but the images may look significantly different.
Digital Quad Maps can also be made by scanning map feature separates instead of paper. The scans are then digitally combined to make a Digital Quad Map. This is much more expensive than scanning a paper map; therefore, Digital Quad Maps currently are not produced by this method except when there are other reasons for scanning the original map materials.
All USGS Digital Quad Maps 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 Digital Quad Map production is about 1,000 quadrangles per year. At this time, the USGS has no plans to systematically replace all Digital Quad Maps with higher resolution scans.
Differences in map revision and Digital Quad Map production methods have no effect on either the positional accuracy or the content completeness of the Digital Quad Map. These reflect the characteristics of the published map in all cases.