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NAME

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

KEYWORDS

       vector, extract, select, dissolve, random

SYNOPSIS

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

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

       -t
           Do not copy attributes (see also ’new’ parameter)

       -r
           Reverse selection

       --overwrite
           Allow output files to overwrite existing files

       --help
           Print usage summary

       --verbose
           Verbose module output

       --quiet
           Quiet module output

       --ui
           Force launching GUI dialog

   Parameters:
       input=name [required]
           Name of input vector map
           Or data source for direct OGR access

       layer=string
           Layer number or name (’-1’ for all layers)
           A single vector map  can  be  connected  to  multiple  database  tables.  This  number
           determines  which  table  to  use.  When used with direct OGR access this is the layer
           name.
           Default: 1

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

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

       where=sql_query
           WHERE conditions of SQL statement without ’where’ keyword
           Example: income < 1000 and population >= 10000

       output=name [required]
           Name for output vector map

       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
           Desired new category value (enter -1 to keep original categories)
           If new >= 0, attributes is not copied
           Default: -1

       dissolve_column=name
           Name of attribute column for dissolving areas
           Preserves category values

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  cats,  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 in the selected layer 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,  or  use
       layer=-1.

EXAMPLES

       The examples are intended for the North Carolina sample dataset:

   Extract areas by category number with dissolving #1:
       v.extract -d cats=1,2,3,4 input=soils_wake 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 cats=1-4 input=soils_wake 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_wake 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=lakes output=lakes_gaps where="FTYPE is NULL"

   Extract vector features which have the given field not empty:
       v.extract input=lakes output=lakes_ftype where="FTYPE not NULL"

   Reverse extracting (behaves like selective vector objects deleting):
       Remove meteorological stations from map which are located above 1000m:
       # check what to delete:
       v.db.select precip_30ynormals where="elev > 1000"
       # perform reverse selection
       v.extract -r input=precip_30ynormals output=precip_30ynormals_lowland \
         where="elev > 1000"
       # verify
       v.db.select precip_30ynormals_lowland

   Dissolving based on column attributes:
       # check column names:
       v.info -c zipcodes_wake
       # reclass based on desired column:
       v.reclass input=zipcodes_wake output=zipcodes_wake_recl_nam column=ZIPNAME
       # verify:
       v.info -c zipcodes_wake_recl_nam
       v.db.select zipcodes_wake_recl_nam
       # dissolve:
       v.extract -d input=zipcodes_wake_recl_nam output=zipcodes_wake_regions

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

   Extract 3 random areas from geology map
       v.extract input=geology output=random_geology type=area random=3
       This  creates  a 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: 2018-05-04 16:47:55 +0200 (Fri, 04 May 2018) $

SOURCE CODE

       Available at: v.extract source code (history)

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       © 2003-2019 GRASS Development Team, GRASS GIS 7.6.1 Reference Manual