USGS Digital Line Graph Files - User Information

    OVERVIEW
    ---------
    The USGS provides digitized base map data in "line graph" form.  It is
    available on either 9-track magtape and on CD-ROM.  The CD-ROM contains
    the data for DLG files at a 1:2,000,000 scale.

    Golden Software, Inc. has taken these files and converted them to the
    MapViewer [.GSB] boundary file format.  

    DLG FILE ORGANIZATION
    ---------------------
    For the 1:2,000,000-scale files, the USGS has broken the U.S. into 21
    sections.  Please open SECTMAP.GSM to see where each section is located.
    Each section has one or more data files associated with it.
    The file naming convention used is as follows:

     S00_XX.GSB

    Each file starts with the letter "S", followed by the section number
    (01 to 21) and an underscore (_).  The XX stands for the feature code
    (sometimes referred to as Overlay or Base Category).  Feature codes
    are:

    AB = Administrative Boundaries - (National Park, Monument, Wilderness
         Area, Grassland, Wildlife Refuge, Forest, Seashore, Lakeshore)
    CF = Cultural Features - (Airports and Alaska Pipeline)
    HP = Hypsography - (Continental Divide)
    PB = Political Boundaries - (National, State, County, Parish)
    RD = Roads and Trails - (Interstate, U.S. Highway, State Highway,
         Secondary Road, Unimproved Road, Tunnel Road, Auto Ferry)
    RR = Railroads
    ST = Streams and Rivers - (Canals, Ditches, Intercoastal Waterway)
    WB = Water Bodies - (Coastline, Lake, Reservoir, Island, Marsh/Swamp,
         Dry Lake, Alkali Flat, Glacier)

    If a section has more than one file for a feature, then an underscore
    (_) and a letter are used to uniquely name the files.  Examples:

     S01_WB_A.GSB
     S01_WB_B.GSB


    ADDITIONAL INFORMATION
    ----------------------
    The USGS maps were digitized at 0.001 inches.  This high resolution
    produces large files with many vertices.  The GSB files have
    been thinned to 10% of the original size, but some of the files are
    still quite large, especially in Alaska.  Large numbers of vertices
    increase screen redraw and print times.  When you try to move objects
    with a large number of vertices, you may get a message that MapViewer's
    Undo buffer has been exceeded.  Click the OK button to continue moving
    the objects.  It is possible to get a "Layer is Full" message when
    editing these detailed images.  When this happens, save the document to
    a [.GSM] file, close the document, open a new document and reload the
    [.GSM] file.  MapViewer compacts object memory when it saves to a
    [.GSM] file.

    USGS DLG files store political boundaries and coastlines in a way that
    requires editing if you are constructing county files.  The USGS
    political boundaries consist of the entire region of political
    authority, including land boundaries and areas that extend off the
    coast into the ocean.  For example, a county in Washington state has to
    be built from the political boundary file and the coastline file.
    The imported coastline is a series of curve objects.  Delete those
    coastline curves that aren't part of the county boundary, convert the
    county area to a curve, then manually splice the line segments together
    with MapViewer's Reshape tool.  The resultant county has considerably
    more detail.