Process 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 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 he corresponding image points in the DRG.