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:

• 7.5-minute maps of the conterminous United States, Hawaii, and limited areas of Alaska at 1:24,000 and 1:25,000 scale.
• 7.5 x 15-minute maps in limited areas of the conterminous United States at 1:25,000 scale.
• Pacific Island maps at 1:20,000, 1:24,000, and 1:25,000 scales.
• Puerto Rico and the Virgin Islands at 1:20,000 scale.
• Culebra, its adjacent islands, and the Island of Vieques at 1:30,000 scale.
• Alaska at 1:63,360 scale.
• 30 x 60-minute maps of the conterminous United States at 1:100,000 scale.
• 1 x 2-degree maps of the United States at 1:250,000 scale.

DATA RESOLUTION

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:

• it is high enough to preserve all map information,
• it is low enough to keep file sizes reasonable for large maps (a more important consideration in 1994 than in 2000), and
• it results in an even ground resolution of 8 feet square per pixel for 1:24,000-scale maps.

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 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.

• A limited color palette makes the data more compressible with run-length-encoding.
• The original map is a line drawing, so this color model preserves the nature of the source data.
• USGS map colors correspond roughly to feature types, so it is useful to GIS applications to have the colors of the digital product standardized. For example, it is useful to be able to "turn off" the green timber tint and white background.
• At the time the product was designed, we believed that variations in colors between maps caused by the age of the paper, different brands of ink, and so on, could be removed and the digital product could be made more uniform than the paper product. Though technically possible, such procedures proved to be prohibitively expensive in most cases.

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.

• Data from digital map revision. Some topographic maps are now revised using all-digital methods. Copies of the map's feature separates are scanned, and these scans are revised with digital aerial photographs and raster editing software. The primary outputs of this revision process are digital feature separates used to print the new map. These digital separates also can be combined to make a new Digital Quad Map. Digital Quad Maps made this way are extremely clean, without the color "noise" that is characteristic of a paper scan. Only a few hundred maps per year are currently revised digitally, but this number is expected to increase as map revision contractors convert to digital methods.
• Data from traditional map revision. If a map is revised by traditional analog methods, the new Digital Quad Map is made by scanning the new paper map. All minor revisions are done with traditional methods, and some USGS contractors use traditional methods for other types of revision.

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.