Topographic Maps

Abstract: The Digital Raster Graphic (DRG) is a raster image of a scanned 1:24000 USGS topographic quadrangle map. The DRG's accompanying this metadata have been enhanced by the Wisconsin Dept. of Natural Resources (WiDNR) and converted from the original UTM grid as provided by USGS, to the Wisconsin Transverse Mercator (WTM) coordinate system based on the 1991 adjustment to the North American Datum of 1983 (NAD83/91).

Purpose Description: A DRG is useful as a source or background layer in a GIS, as a means to perform quality assurance on other digital products, and as a source for the collection and revision of DLG data. DRGs can also be merged with other digital data, e.g. DEMs or DOQs, to produce a hybrid digital file.

Access Constraints: None

Use Constraints: Acknowledgment of the USGS would be appreciated in products derived from these data.

Currentness Reference: Ground condition

Geographic Extent: Statewide

Native Data Environment: TIFF image files

Maintenance Update Frequency: Irregular

Progress Type: Complete

Beginning Date: 1990

Ending Date: Present

Process Step Description: The production procedures, instrumentation, hardware, and software used in the collection of standard DRG products vary depending on systems used at the contract, cooperator or USGS production sites. The majority of DRG data sets are acquired through government contract. The process step describes, in general, the process used in the production of standard DRG data sets. 1. Production of a DRG begins with the scanning of a paper 7.5-minute topographic map (map1) on a high-resolution scanner. Scanning resolutions range from 500 1000 dpi with the output file running between 160-300 mb. 2. Removal of screens (descreening) and color quantization to reduce the number of colors also takes place during the scanning phase. 3. The raw scan file is then transformed and georeferenced using UTM coordinates of the sixteen 2.5-minute grid ticks, which are obtained using the in-house produced program COORDAT and stored in a ground control file. Those sixteen 2.5-minute ticks are interactively visited and assigned their respective UTM coordinates. USGS program XSHAPES4 then performs a piecewise linear rubber sheet transformation. 4. An output resolution of 2.4 meters (8.2 feet) is chosen in order to resample the file to 250 dpi. 5. The image file is converted to a TIFF and further reduced by converting the file to a run length encoding Packbits compression(type 32773). 6. The color palette of the compressed DRG is then standardized by replacing the original RGB values assigned during the scanning process with standard RGB value combinations using the in-house produced TIFFREMAP program. 7. Prior to archiving the DRG undergoes the following quality assurance procedures: a. The color index values of each DRG are checked to ensure the RGB combinations are consistent with the standardized color palette. b. All DRG files are inspected to ensure that they are geometrically consistent with normal map presentation. c. Selected DRGs are checked to ensure that data elements in the DRG metadata file correspond to the map collar information and to the information in the associated image file. d. Selected DRGs are checked for georeferencing accuracy by comparing the book value of latitude and longitude tick marks with corresponding tick intersections in the DRG image. e. Transformations are checked on selected DRGs by comparing the positions of well defined points, such as UTM grid intersections in the graphic product, with the corresponding image points in the DRG. USGS DRG production specifications are available on request from the National Mapping Division and Mid-Continent Mapping Center by contacting: Rolla-ESIC U.S. Geological Survey 1400 Independence Rd., MS231 Rolla, MO 65401-2602 Phone (314)341-0851 Facsimile (314)341-9375 E-mail to esic@mcdgs91.cr.usgs.gov World Wide Web: URL:http://mcmcweb.er.usgs.gov/drg/ [The preceding Lineage information is from the original USGS metadata. The following paragraphs describe subsequent processing carried out by the WiDNR Geographic Services Section after receiving copies of the USGS DRG product described above.] 8. The collar surrounding the map portion of each DRG containing ancillary information about the printed map, such as scale bars, legend and attribution, was reclassified by WiDNR to a distinct range of pixel values in the scanned image (values 20 through 32). This enables ArcView users to optionally turn off the DRG map collar for display purposes. 9. The DRGs were converted to the Wisconsin Transverse Mercator (WTM) coordinate system, based on the 1991 adjustment to the North American Datum of 1983 (NAD83/91).

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