NAME

       v.extract   -  Selects  vector  objects  from an existing vector map and creates a new map
       containing only the selected objects.

KEYWORDS

       vector, extract

SYNOPSIS

       v.extract
       v.extract help
       v.extract  [-dtr]  input=name  output=name   [type=string[,string,...]]    [layer=integer]
       [list=range]      [where=sql_query]     [file=name]     [random=integer]     [new=integer]
       [--overwrite]  [--verbose]  [--quiet]

   Flags:
       -d
           Dissolve common boundaries (default is no)

       -t
           Do not copy table (see also 'new' parameter)

       -r
           Reverse selection

       --overwrite
           Allow output files to overwrite existing files

       --verbose
           Verbose module output

       --quiet
           Quiet module output

   Parameters:
       input=name
           Name of input vector map

       output=name
           Name for output vector map

       type=string[,string,...]
           Types to be extracted
           Feature type
           Options: point,line,boundary,centroid,area,face
           Default: point,line,boundary,centroid,area,face

       layer=integer
           Layer number (if -1, all features in all layers of given type are extracted)
           A single vector map  can  be  connected  to  multiple  database  tables.  This  number
           determines which table to use.
           Default: 1

       list=range
           Category values
           Example: 1,3,7-9,13

       where=sql_query
           WHERE conditions of SQL statement without 'where' keyword
           Example: income = 10000

       file=name
           Input text file with category numbers/number ranges to be extracted
           If '-' given reads from standard input

       random=integer
           Number of random categories matching vector objects to extract
           Number must be smaller than unique cat count in layer

       new=integer
           Enter -1 to keep original categories or the desired NEW category value
           If new >= 0, table is not copied
           Default: -1

DESCRIPTION

       v.extract allows a user to select vector objects from an existing vector map and creates a
       new map containing only the selected objects.  Database tables can  be  queried  with  SQL
       statements,  if a connection is established.  Dissolving (optional) is based on the output
       categories. If 2 adjacent areas have the same output category, the boundary is removed.

       If list, file, random or where options are not specified, all features of given  type  and
       layer are extracted. Categories are not changed in that case.

NOTES

       Only  features  with  a  category  number  will  be  extracted.  So if you want to extract
       boundaries (which are usually without category, as that information is  normally  held  in
       the area's centroid) you must first use v.category to add them.

EXAMPLES

   Extract areas by category number with dissolving #1:

       v.extract -d list=1,2,3,4 input=soils output=soil_groupa type=area new=0

       produces  a  new  vector  soil_groupa, containing those areas from vector soils which have
       category numbers 1 thru 4; any common boundaries are dissolved, and all areas in  the  new
       map will be assigned category number 0.

   Extract areas by category number with dissolving #2:

       v.extract -d list=1-4 input=soils output=soil_groupa type=area new=-1

       produces  a  new  vector map soil_groupa containing the areas from vector soils which have
       categories 1 thru 4. Any common boundaries are dissolved, all areas in the  new  map  will
       retain their original category numbers 1 thru 4, since new was set to -1.

   Extract all areas and assign the same category to all:

       v.extract input=soils output=soil_groupa type=area new=1

       produces  a  new  vector  map  soil_groupa  containing  all  areas  from  soils. No common
       boundaries are dissolved, all areas of the new map will be assigned category number 1.

   Extract vectors with SQL:

       v.extract  input=markveggy.shp  output=markveggy.1  new=13  where="(VEGTYPE  =  'Wi')   or
       (VEGTYPE = 'PS') or (PRIME_TYPE='Wi')"

       produces a new vector map with category number 13 if the SQL statement is fulfilled.

   Extract vector features which have the given field empty:

       v.extract input=forest output=forest_gaps where="CANOPY is NULL"

   Extract vector features which have the given field not empty:

       v.extract input=forest output=forest_canopy where="CANOPY not NULL"

   Reverse extracting (behaves like selective vector objects deleting):
       Remove unreferenced stations from the GlobalSOD database:
       # check what to delete:
       v.db.select gsod_stationlist where="latitude < -91"
       # perform reverse selection
       v.extract -r gsod_stationlist out=gsod_stationlist_clean where="latitude < -91"
       v.db.select gsod_stationlist_clean

   Dissolving based on column attributes:

       # check column names:
       v.info -c polbnda_italy
       # reclass based on desired column:
       v.reclass polbnda_italy out=polbnda_italy_recl_nam col=vmap_nam
       # verify:
       v.info -c polbnda_italy_recl_nam
       v.db.select polbnda_italy_recl_nam
       # dissolve:
       v.extract -d polbnda_italy_recl_nam out=pol_italy_regions

       produces  a new vector map with common boundaries dissolved where the reclassed attributes
       of adjacent (left/right) areas are identical.

   Remove islands from polygon map

       v.extract in=map_with_islands out=maps_without_islands list=1-99999
       # and/or
       v.extract -d in=map_with_islands out=maps_without_islands

   Extract 3 random areas from geology map

       v.extract input=geology output=random_geology type=area random=3
        will create new map with three random categories matching areas.  Note that there may  be
       more than one feature with the same category.

SEE ALSO

        v.category, v.dissolve, v.reclass, GRASS SQL interface

AUTHORS

       R.L. Glenn, USDA, SCS, NHQ-CGIS
       GRASS 6 port by Radim Blazek

       Last changed: $Date: 2011-11-08 01:42:51 -0800 (Tue, 08 Nov 2011) $

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