File: DRG_PKG.TXT
Date: August 29, 1995

****************************************************************************

PACKAGING SPECIFICATIONS FOR DRG PRODUCTS ON CD-ROM           

Digital Raster Graphic (DRG) images of USGS standard series quadrangles cover
systematically subdivided areas of latitude and longitude with formats
ranging from 7.5- by 7.5-minutes to 1- by 2-degrees.  DRG's of standard
quadrangles range in scale from 1:20,000 to 1:250,000.  DRG coverages include
the conterminous United States, Alaska, Hawaii, Puerto Rico and the Virgin
Islands, and the Pacific Islands.


Optimal Packaging Scheme

Each CD-ROM in this series normally contain DRG's covering a 1- by 1-degree
geographic area (secondary cell).  These cells are further subdivided into
7.5- by 7.5-minute quadrangle grids (primary cells) which represent the
location of individual maps covering the geographic area of each secondary
cell. The number, series, scale, and class of the DRG products contained in
each secondary cell may vary depending on the type of map coverage available
for any particular geographic location, although the standard distributable
compact disc contains the following number and types of DRG's:

     64 - 1:24,000-scale, 7.5- by 7.5-minute quadrangle maps.
      2 - 1:100,000-scale, 30- by 60-minute quadrangle maps.
      1 - 1:250,000-scale, 1- by 2-degree quadrangle map.   


Packaging Variations

To maximize the data storage capabilities of CD-ROM media, variances to the
optimal packaging scheme of one secondary cell containing 67 DRG's per disc
occur throughout this series.  Adjacent cells along the shorelines of the
Great Lakes, coastal margins of the conterminous United States and Alaska,
and island groups may contain large areas of water with limited amounts of
mapped surface area.  In such instances, additional adjacent cells of DRG
data may be included on one disc. Conversely, cells of DRG's covering areas
of extensive urbanization may exceed the 650 Mb data storage capacity of the 
CD-ROM, necessitating multiple discs to store a single cell of DRG data.  

Secondary cells in Alaska are normally configured in 1- by 2-degree and  1-
by 3-degree converges and are identified by the general area of interest,
such as Fairbanks.  Undersize, oversize, and shifted cells occur throughout
the Alaska 1:63,360-scale series.  The number of quadrangles per cell vary
along coastal areas and the position of each cell along the north
longitudinal axis.  Because of the relatively small file size of each
1:63,360-scale DRG, several cells of data are included on each distributable
CD-ROM depending on the northerly trend of the cell.    

****************************************************************************