Attribute:
  Attribute_Label: COA_ID
  Attribute_Definition:
    A unique number (integer) used to identify each of the
    geologic map units as originally described in the
    source literature (the maps or databases that were
    compiled to create the NR_GEO spatial database). All
    polygons were assigned a COA_ID (Compound Object
    Archive Identifier) value; only arc features
    representing geologic map units (those arcs with
    LINECODE greater than 2000 in the NR_GEO.AAT file) were
    assigned a value for COA_ID. A total of 2,137 geologic
    map units were described in the 43 maps used to create
    the NR_GEO compilation. If the same geologic feature
    occurs on more than one map, it was assigned a
    different value of COA_ID on each map. For example, the
    map unit with NAME = Eagle Sandstone has four values
    for COA_ID in the NR_GEO.PAT file: 305 (for features
    derived from the Gallatin National Forest source map),
    1689 (for features derived from the Livingston source
    map), 3022 (for features derived from the Big Timber
    source map), and 4358 (for features derived from the
    Red Lodge source map). COA_ID is the key field for the
    NR_GEO.MU (map unit), NR_GEO.LITH (lithologic unit),
    and NR_GEO.UN (unconsolidated unit) ArcInfo look-up
    tables. In addition, the NR_GEO_MUD.TXT and NR_GEO_
    MAPUNITS.PDF files are indexed by COA_ID values. The
    map units are briefly described in the NR_GEO.MU look-up
    table. Full descriptions of the units are provided
    in the NR_GEO_MAPUNITS.PDF file (provided in this data
    release).