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The DRG is a faithfully reproduced digital image of the original source map. Some differences may be detected between the source graphic used and the DRG due to the RGB values assigned that particular color. The intent is to recreate those colors as near as possible. Data completeness for DRG files reflect content of the source graphic. Features may have been eliminated or generalized on the source graphic due to scale and legibility constraints. For information on collection and inclusion criteria, see U.S. Geological Survey, 1994, Standards for 1:24,000-Scale Digital Line Graphs and Quadrangle Maps: National Mapping Program Technical Instructions and U.S. Geological Survey, 1994, Standards for Digital Line Graphs: National Mapping Program Technical Instructions.
Topographic maps accurately represent the natural and man-made features of the land. USGS topographic maps are compiled to National Map Accuracy Standards using modern mapping techniques. The shape and elevation of terrain are portrayed by contour lines and specific features such as roads, towns, water areas, and vegetation are portrayed by map symbols and colors.
Some of these DRG images were acquired indirectly from the United States Geological Survey (USGS) through the North Coast Geographic Information Cooperative (NCGIC). The remaining DRG images were acquired directly from the USGS EROS Data Center via FTP order. These images represent the topographic 1:250,000 series - the entire state of California is available with the exception of the Santa Rosa Island quad. This data is provided "as-is" - the California Department of Fish and Game makes no warranty as to the suitability of this data for any purpose.
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.
- 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.
- Removal of screens (descreening) and color quantization to reduce the number of colors also takes place during the scanning phase.
- 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.
- An output resolution of 2.4 meters (8.2 feet) is chosen in order to resample the file to 250 dpi.
- The image file is converted to a TIFF and further reduced by converting the file to a run length encoding Packbits compression(type 32773).
- 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.
- Prior to archiving the DRG undergoes the following quality assurance procedures:
- The color index values of each DRG are checked to ensure the RGB combinations are consistent with the standardized color palette.
- All DRG files are inspected to ensure that they are geometrically consistent with normal map presentation.
- 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.
- 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.
- 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.
Source images were in UTM projection, zone 10 or 11. All images were converted to ARC/INFO grids, reprojected to Albers conic equal-area (standard Teale parameters) while retaining source data resolution, and converted back to TIFF image format using PackBits compression. Some grid reclassification was performed to transform reprojected NODATA areas to white pixels in the output images. Two sets of statewide images are available, standard or trimmed. The standard DRG images include map collar information as displayed on a regular USGS paper map. Trimmed DRG images have been clipped to the neatline of the quad, allowing for easier image mosaicking and edgematching. Clipping was performed using an automated routine in ARC/INFO.
Further Information can be found at the following web sites:
- USGS
- California State Government
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