
NAME
v.extract - Selects vector objects from an existing vector map and creates a new map containing only the selected objects. If 'list', 'file' and 'where' options are not specified, all features of given type and layer are extracted, categories are not changed in that case.
KEYWORDS
vector
SYNOPSIS
v.extract
v.extract help
v.extract [-dt] input=name output=name [type=string[,string,...]] [layer=integer] [new=integer] [list=range[,range,...]] [file=string] [where=sql_query] [--overwrite]
Flags:
- -d
- Dissolve common boundaries (default is no)
- -t
- Do not copy table (see also 'new' parameter)
- --overwrite
- Force overwrite of output files
Parameters:
- input=name
- Name of input vector map
- output=name
- Name for output vector map
- type=string[,string,...]
- 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.
- Default: 1
- new=integer
- Enter -1 to keep original category or a desired NEW category value. If new >= 0, table is not copied.
- Default: -1
- list=range[,range,...]
- Category ranges: e.g. 1,3-8,13
- file=string
- Text file with category numbers/number ranges
- where=sql_query
- WHERE conditions of SQL statement without 'where' keyword. (example: income < 1000 and inhab >= 10000)
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.
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
v.extract -d list=1,2,3,4 input=soils output=soil_groupa type=area new=1
produces a new vector area file soil_groupa containing 'area'
boundaries from soils with area category numbers of 1 thru 4; any
common boundaries are dissolved, and all areas of the new map will be
assigned category number 1.
Extract areas by category label with dissolving
v.extract -d list=Abc,Def1,12A,WWd input=soils output=soil_groupa type=area new=0
produces a new vector area file soil_groupa containing 'area'
boundaries from soils with area category labels of Abc,Def1, 12A,WWd;
these labels correspond to numbers 1 thru 4 of soils. Any common
boundaries are dissolved, all areas of the new map will be retain their
original category numbers 1 thru 4, in this case, since new was set to
0.
Extract areas to uniquely labeled area
v.extract input=soils output=soil_groupa type=area new=1 file=sample
produces a new vector area file soil_groupa containing 'area'
boundaries 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=1 \
where="(VEGTYPE = 'Wi') or (VEGTYPE = 'PS') or (PRIME_TYPE='Wi')"
produces a new vector map with category label 1 if the SQL statement is
fulfilled.
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
This produces a new vector map with common boundaries dissolved
where the reclassed attributes of adjacent (left/right) polygons 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
SEE ALSO
v.category,
v.reclass,
GRASS SQL interface
AUTHOR
R.L. Glenn, USDA, SCS, NHQ-CGIS
Last changed: $Date: 2006/06/13 23:23:56 $
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