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v.extract - Selects vector features from an existing vector map and creates a new vector map containing
Contents
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.
Examples
The examples are intended for the North Carolina sample dataset:
Extractareasbycategorynumberwithdissolving#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 1thru4; any common boundaries are dissolved, and all areas in the new map will be assigned category
number 0.
Extractareasbycategorynumberwithdissolving#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 1thru4. 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.
Extractallareasandassignthesamecategorytoall:
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.
ExtractvectorswithSQL:
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.
Extractvectorfeatureswhichhavethegivenfieldempty:
v.extract input=lakes output=lakes_gaps where="FTYPE is NULL"
Extractvectorfeatureswhichhavethegivenfieldnotempty:
v.extract input=lakes output=lakes_ftype where="FTYPE not NULL"
Reverseextracting(behaveslikeselectivevectorobjectsdeleting):
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
Dissolvingbasedoncolumnattributes:
# 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.
Extract3randomareasfromgeologymap
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.
Keywords
vector, extract, select, dissolve, random
Name
v.extract - Selects vector features from an existing vector map and creates a new vector map containing
only the selected features.
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.
See Also
v.category,v.dissolve,v.reclass,GRASSSQLinterface
Source Code
Available at: v.extract source code (history)
Accessed: Friday Apr 04 01:20:16 2025
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© 2003-2025 GRASS Development Team, GRASS GIS 8.4.1 Reference Manual
GRASS 8.4.1 v.extract(1grass)
Synopsis
v.extractv.extract--helpv.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: 1type=string[,string,...]
Types to be extracted
Input feature type
Options: point,line,boundary,centroid,area,face
Default: point,line,boundary,centroid,area,facecats=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: -1dissolve_column=name
Name of attribute column for dissolving areas
Preserves category values
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