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 Universal Transverse Mercator (UTM) coordinates of the 16 2.5-minute grid ticks, which are obtained using the in-house produced program COORDAT and stored in a ground control file. Those 16 2.5-minute ticks are interactive and assigned 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 dots per inch.
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 geometrical consistency 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 latitudinal and longitudinal 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.