How To: Convert a polygon shapefile to a coverage using SHAPEARC


Instructions provided describe how to convert a polygon shapefile into a coverage, using the ArcInfo Workstation command line. The attributes of the original shapefile features are maintained in the output coverage, either in a REGION subclass, or a polygon feature class.


  1. Set the coordinate precision to DOUBLE DOUBLE.

    Arc: precision double double

    Shapefiles store feature coordinates as double-precision floating point numbers. Create a double-precision coverage from the shapefile so maximum detail and coordinate accuracy are maintained.

  2. Run the SHAPEARC command and specify an out_subclass.

    Usage: SHAPEARC <in_shape_file> <out_cover> {out_subclass}
    Arc: shapearc inshape.shp tempcov temp

    Because polygon shapefiles allow for discontiguous and overlapping polygons, and may include 'donut holes', a region subclass {out_subclass} must be created in the output coverage to maintain attributes. Converting the shapefile to a coverage with a region subclass maintains all of the attributes in the region subclass attribute table.

  3. Warning:
    In Step 5, the CLEAN command is run. CLEAN must not be run on a coverage in geographic coordinates with units of decimal degrees.

    If the input shapefile has units in decimal degrees, the projection for the coverage to match the projection parameters of the shapefile must first be defined using the PROJECTDEFINE command.

    Arc: projectdefine cover tempcov

    After defining the projection, PROJECT the coverage to a projected coordinate system that uses units of feet or meters.

    PROJECT <COVER | FILE> <input> <output>
    Arc: project cover tempcov intercov

  4. The CLEAN command syntax allows the user to set the FUZZY and DANGLE tolerances to appropriate values. These tolerances are measured in coverage units. For a double precision coverage, a FUZZY tolerance of .001 avoids the possibility of damage to the data.

    We suggest using the TOLERANCE command to set the FUZZY tolerance value.

    TOLERANCE <cover>{ FUZZY | DANGLE | TIC_MATCH | EDIT | NODESNAP | WEED | GRAIN | SNAP} {tolerance_value}
    Arc: tolerance intercov fuzzy .001

  5. CLEAN the coverage to create polygon topology. Specify appropriate dangle and fuzzy tolerances if not specified in Step 4.

    Usage: CLEAN <in_cover> {out_cover} {dangle_length}
    {fuzzy_tolerance} {POLY | LINE}
    Arc: clean intercov cleancov # # poly

    It is necessary to clean the coverage to create proper polygon topology. Before executing CLEAN, the new coverage does not have a polygon feature class and only has preliminary region topology.

  6. If the input shapefile has no discontiguous or overlapping polygons, and there are no 'donut holes', REGIONPOLY may be executed to convert the region subclass to a polygon feature class with attributes.

    The out_table option in this command must be used, or the attributes are dropped from the final coverage.

    This option creates an INFO file which lists the relationship between each polygon and the regions of which it is part. When a polygon belongs to two or more region features, the output polygons in the coverage created by REGIONPOLY have the attributes of the associated region feature with the lowest internal number. The attributes of the other overlapping features are not maintained. To avoid this problem, edit the original shapefile to eliminate overlapping or discontiguous features. If overlaps exist and must be maintained, do not run the REGIONPOLY command.

    To determine if there are overlapping regions features, click the link entitled 'Check for overlapping regions' in the Related Information section below.
    Usage: regionpoly <in_cover> <out_cover> <in_subclass> {out_table}
    Arc: regionpoly cleancov finalcov temp junk.tab

    The REGIONPOLY command only creates labels in the output coverage if the out_table option is used. Use CREATELABELS, followed by BUILD for polygons to ensure proper attribute maintenance.

Related Information