About California DRGs
=====================

Abstract
--------

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.


Printed Map Sources
-------------------

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.


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.

    * 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:

          o The color index values of each DRG are checked to ensure the
            RGB combinations are consistent with the standardized color
            palette.

          o All DRG files are inspected to ensure that they are
            geometrically consistent with normal map presentation.

          o 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.

          o 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.

          o 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.


California Specific Image Processing Notes
------------------------------------------

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.


Additional Information
----------------------

Further Information can be found at the following web sites:

    * USGS

          o "Digital Raster Graphics":http://topomaps.usgs.gov/drg/

          o "Standardized Data Set Names For DRG Products":http://topomaps.usgs.gov/drg/drg_name.html

          o "Revised DRG Product Standard":http://topomaps.usgs.gov/drg/drg_standard_change.html

    * California State Government

          o "Califonia Geospatial Information Library":http://www.gis.ca.gov/

          o "CA DFG - USGS Digital Raster Graphics":http://maphost.dfg.ca.gov/HTML_Sites/drg.htm

          o "Teale Data Center":http://www.teale.ca.gov/