LIBRARY       : <NONE>
  LAYER NAME    : <NONE>
  NAME          : DRG - Digital Raster Graphic 
                  USGS 7.5' Quad Images (trimmed)
  LOCATION      : /drg

  COVERAGE DESCRIPTION:
Trimmed Digital Raster Graphics are PackBit-compressed tiff images of USGS 
7.5 minute quadrangle maps that have been trimmmed down to their neat line,
or in the case of irregular DRGs, to their outer map extent, removing the
map collar that contains legends and descriptive text.  This enables side
by side display without map areas being overlapped by adjacent map collars.
There are 2,851 images covering the entire state.  Trimmed DRGs are 
directly derived from the full sheet DRGs that Teale Data Center scanned
on behalf of the USGS.

  BRIEF METHODOLOGY:
 - Image is scanned at 400 dpi, and colors normalized to 13 RGB values.
 - Image is converted to Arc Info grid file
 - Grid file undergoes descreening to eliminate scatter and enhance solid 
   fills in areas such as water bodies and forests.  
 - Grid file is registered to albers projection.
 - Grid file is trimmed to its neatline or outer map extent.
 - Trimmed grid file is converted to .tif image with PackBit compression 
   option.


 VITAL STATISTICS:
  
  Resolution:                  400 d.p.i.
  Projection:                  Albers
  Units:                       Meters
  1st Std. Parallel:           34 00 00 (34.0 degrees N)
  2nd Std. Parallel:           40 30 00 (40.5 degrees N)
  Longitude of Origin:         -120 00 00 (120.0 degrees W)
  Latitude of Origin:          00 00 00 (0.0 degrees)
  False Easting (X shift):     0
  False Northing (Y shift):    -4,000,000
  Datum:                       NAD 27
  Spheroid:                    Clarke 1866
  Source:                      U.S.G.S.
  Source Media:                Map sheets
  Source Projection:           vary
  Source Units:                Meters
  Source Scale:                1:24,000, 11 DRGs are 1:25,000
  Capture Method:              Scanned
  Conversion Software:         ARC/INFO rev. 7.2.1
  Data Structure:              Raster
  Size of DRGs:                Range 2 - 40 MB, 13 MB on average per image
                               40 Gigabytes statewide
  Use limitation:              Value added product for purchaser's use only
  Data Updated:                December 1998



  FILE NAMES

There are 64 DRGs in each 1 degree block of longitude and latitute, filling
8 rows and 8 columns. DRG files are named according to USGS 'ocode' 
designation:

       example: t35116a7.tif
                t   denotes trimmed version of DRG
                35  degree of latitude
                116 degree of longitude
                a   row  (a-h south to north)
                7   column (1-8 east to west)  

Some quad maps of islands and coastal areas are irregularly sized and/or 
positioned.  File names for these images are determined by which quad their
south east corner falls in.  In three instances this creates a discrepancy with
the USGS code:  

Filename   Map Name              USGS ocode
o40124a2   SHUBRICK PEAK         o40124b2 
o33118f2   LONG BEACH            o33118g2 
o33118d8   SANTA BARBARA ISLAND  o33119d1



  TFW FILES

Each DRG has an accompanying 'world' file with the same file name, but with
a .tfw extension, that contains geo-referencing information:

       example: t35116a7.tfw

         1.52786215012960   pixel size (meters)
         0.00000000000000   rotation factor 
         0.00000000000000   rotation factor
        -1.52786215012960   pixel size
    284543.57694197021192   image min x (meters)  (NW
   -316411.17369714472443   image max y (meters)   Corner)

The min x and min y coordinates relate to the most upper left pixel
in the image.  Coordinates are in albers projection.



  COLORS
Colors on the DRGs have been normalized to 13 RGB values:

  0   0   0  black
255 255 255  white
 91 159 230  dark blue
230  45  30  red
162  96  71  dark brown
210 255 177  green
197 101 197  dark magenta
255 240   0  yellow
202 225 245  light blue
255 202 197  light red
250 202 250  light magenta
230 230 230  light grey
222 167 146  light brown



  TRIMxx.DBF - dbase file that contains metadata for each quad on a CD.
  TRIMxx.DBF fields:

 COL  ITEM NAME         WDTH  OPUT  TYP  N.DEC  

    1  FILENAME            8     8    C    -
    9  MAPNAME            30    30    C    -
   39  MAPNAME25          20    20    C    -
   59  CD                  2     2    I    - 
   61  FILESIZEMB          4     5    F    2
   65  MAPSCALE            8     8    C    -
   73  FORMAT             20    20    C    -
   93  REF_CODE           25    25    C    -
  118  YEAR                4     4    C    -
  122  YRORIGMAP           4     4    C    -
  126  YRREVISION          4     4    C    -
  130  YRAIRPHOTO          4     4    C    -
  134  YRFIELDCK           4     4    C    -
  138  YRPHTOTOPO          4     4    C    -
  142  YRREVSOURC          4     4    C    -
  146  YRPHTOINSP          4     4    C    -
  150  SOUTH               8    10    F    5
  158  NORTH               8    10    F    5
  166  EAST                8    10    F    5
  174  WEST                8    10    F    5
  182  SE_COR_LON          9     9    C    -
  191  SE_COR_LAT          9     9    C    -         
  200  CONTOURINT         10    10    C    -        
  210  CONINTRMIT         10    10    C    -       
  220  VERT_DATUM         15    15    C    -      
  235  STATE1              2     2    C    -     
  237  STATE2              2     2    C    -    
  239  TYPE                1     1    C    -   
  240  PROJECTION         80    80    C    -  
  320  NOTES             175   175    C    -


  FILENAME     USGS coding convention for 7.5 X 7.5 minute quads.               
  MAPNAME      Name of 7.5 X 7.5 minute quad map            

  MAPNAME25    Name of 7.5 X 15 minute quad map  (see note below)        

  CD           CD number - index to which CD the image is on

  FILESIZEMB   Size of tiff image file in megabytes      

  MAPSCALE     Scale of scanned map        

  FORMAT       Dimension in Minutes of map, N-S X E-W             

  REF_CODE     USGS reference code      

  YEAR         Most recent year that appears on the map.       

  YRORIGMAP    Year of original map       

  YRREVISION   Year of map revision       

  YRAIRPHOTO   Year of aerial photography used for revision       

  YRFIELDCK    Year of field check       

  YRPHTOTOPO   Year of aerial photography used for topography base       

  YRREVSOURC   Year of source data for revision       

  YRPHTOINSP   Year of photo inspection

  SOUTH        Southern extent of map area, decimal degrees      

  NORTH        Northern extent of map area, decimal degrees 

  EAST         Eastern extent of map area, decimal degrees 

  WEST         Western extent of map area, decimal degrees

  SE_COR_LON   Longitude at south-east corner, degrees minutes seconds

  SE_COR_LAT   Latitude at south-east corner, degrees minutes seconds

  CONTOURINT   Contour interval

  CONINTRMIT   Intermittant contour interval

  VERT_DATUM   Vertical Datum

  STATE1       State primarily covered in the image

  STATE2       State secondarily covered in the image

  TYPE         Type of map, to identify non standard maps
               S  Standard    7.5 X  7.5 Minute
               T  Tall       >7.5 X  7.5 Minute
               W  Wide        7.5 X >7.5 Minute
               O  Oversize   >7.5 X >7.5 Minute
               2  Double      7.5 X 15.0 Minute
               I  Island      vary

  PROJECTION   Projection of map sheet

  NOTES        Notations



  1:25,000 SCALE 7.5' x 15' USGS TOPO MAPS

21 trimmed DRGs are derived from 1:25,000 scale 7.5' x 15' USGS topo maps.
These DRGs have been trimmed to their 1:24,000 scale 7.5' X 7.5' neatline.
These maps occur in portions of Trinity, Shasta and Tehama counties, and
at one location on the California-Nevada border.

FILENAME 1:24,000 MAP NAME              1:25,000 MAP NAME (trimmed from)

t39119h8 SEVEN LAKES MOUNTAIN           DOGSKIN MTN
t40122a7 SOUTH YOLLA BOLLY MOUNTAINS    SOUTH YOLLA BOLLY
t40122a8 SOLOMON PEAK                   SOUTH YOLLA BOLLY
t40122b7 TOMHEAD MOUNTAIN               NORTH YOLLA BOLLY
t40122b8 NORTH YOLLA BOLLY              NORTH YOLLA BOLLY
t40122c5 CHICKABALLY MTN EAST           CHICKABALLY MTN
t40122c6 CHICKABALLY MTN WEST           CHICKABALLY MTN
t40122c7 BEEGUM                         BEEGUM
t40122c8 PLATINA                        BEEGUM
t40122d5 ONO EAST                       ONO
t40122d6 ONO WEST                       ONO
t40122d7 ARBUCKLE MOUNTAIN              CHANCHELULLA PEAK
t40122d8 CHANCHELULLA PEAK              CHANCHELULLA PEAK
t40123a1 WRIGHTS RIDGE                  FOUR CORNERS ROCK
t40123a2 FOUR CORNERS ROCK              FOUR CORNERS ROCK
t40123b1 BLACK ROCK MOUNTAIN            BLACK ROCK MTN
t40123b2 SWIM RIDGE                     BLACK ROCK MTN
t40123c1 PONY BUCK PEAK                 PONY BUCK PEAK
t40123c2 SMOKY CREEK                    PONY BUCK PEAK
t40123d1 WILDWOOD                       DUBAKELLA MTN
t40123d2 DUBAKELLA MOUNTAIN             DUBAKELLA MTN


7 DRGs were scanned from quad maps that have NAD83 corners:
t38121e3 CARMICHAEL
t38121e6 DAVIS
t38121e7 MERRITT
t38121f3 CITRUS HTS
t38121f4 RIO LINDA
t38121g3 ROSEVILLE
o38121h4 SHERIDAN
These images have been shifted back to NAD27 position, and
minor edits have been made to map features at the neat lines:
 


      DATAFILE NAME: DRGINDEX.PAT
      RECORD LENGTH:     70
  
COLUMN   ITEM NAME        WIDTH OUTPUT  TYPE N.DEC  
  -------------------------------------------------------------
    1  AREA                   8    18     F      5 
    9  PERIMETER              8    18     F      5
   17  DRGINDEX#              4     5     B      -  
   21  DRGINDEX-ID            4     5     B      - 
   25  OCODE                  8     8     C      -
   33  MAPNAME               31    31     C      -
   64  USGS100                3     3     C      - 
   67  USGS250                2     2     C      -
   69  CD                     2     2     C      -

  AREA:        The area of the polygon in square coverage units.

  PERIMETER:   The length of the polygon perimeter of the polygon
               in coverage units.

  DRGINDEX#:   The software-assigned unique integer identification 
               number.

  DRGINDEX-ID: A user-assigned identifier number.

  OCODE:       USGS coding convention for 7.5 X 7.5 minute quads.

  MAPNAME:     Name of 7.5 X 7.5 minute quad map.

  USGS100:     Three character USGS code for 1 degree by 30 minute (100K)
               quad containing the polygon.

  USGS250:     Two character USGS code for 2 degree by 1 degree (250K) 
               quad containing the polygon.

  CD:          CD number - index to which CD the image is on.



  DISPLAYING ADJACENT QUADS
Though the quads have been trimmed to 7.5 minute neat lines,
adjacent quads will still overlap due to "filler" pixels along the
sides.  These filler pixels are required because in an albers-based
coordinate system the Y axis is parallel with true north only
along the central meridian.  Therefore, away from the central
meridian the quads are canted with respect to the raster data
structure and areas between the neatline and the edge of the data
structure contain filler cells that overlap adjacent quads.

Fortunately these filler cells have a unique color index of "0"
that, depending on the display software being used can be set to
transparent, allowing colors from adjacent quads to show through.
Also, a raster-based GIS can be used to merge adjacent quads
employing rules to ensure that index "0" never overwrites any
other index.


  DATA CONTACT:
  
  Contact Name:      Roger Ewers - Teale GIS
  Contact's Phone:   916-263-1488
  

  DOCUMENTATION DATE: 12/15/1998