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 DRG's 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 DRG's 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 DRG's 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 (573)308-3500 Facsimile (573)308-3615 E-mail to mcmcesic@usgs.gov World Wide Web: http://mcmcweb.er.usgs.gov/drg/
Each raster entity or pixel contains a color index from 0 through 12 referencing a color palette of RGB values from 0 through 255 in which the standard colors used in the DRG are defined.USGS DRG Color Palette
Digital Number Color Red Green Blue
0 Black 0 0 0 1 White 255 255 255 2 Blue 0 151 164 3 Red 203 0 23 4 Brown 131 66 37 5 Green 201 234 157 6 Purple 137 51 128 7 Yellow 255 234 0 8 Light Blue 167 226 226 9 Light Red 255 184 184 10 Light Purple 218 179 214 11 Light Grey 209 209 209 12 Light Brown 207 164 142