## NAME

v.extract

## 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 10:42:51 +0100 (Tue, 08 Nov 2011) $

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