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v.extract(1) Grass User's Manual v.extract(1)

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

vector, extract, select, dissolve, random

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] [--overwrite] [-- help] [--verbose] [--quiet] [--ui]

-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

input=name [required]
 
Name of input vector map
 
Or data source for direct OGR access
layer=string
 
Layer number or name
 
Vector features can have category values in different layers. This number determines which layer 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

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.

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.

The examples are intended for the North Carolina sample dataset:

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.

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.

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.

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.

v.extract input=lakes output=lakes_gaps where="FTYPE is NULL"

v.extract input=lakes output=lakes_ftype where="FTYPE not NULL"

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

# 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.

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.

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

R.L. Glenn, USDA, SCS, NHQ-CGIS
 
GRASS 6 port by Radim Blazek
Last changed: $Date: 2016-03-16 21:17:59 +0100 (Wed, 16 Mar 2016) $

Available at: v.extract source code (history)
Main index | Vector index | Topics index | Keywords index | Graphical index | Full index
© 2003-2018 GRASS Development Team, GRASS GIS 7.4.3 Reference Manual
GRASS 7.4.3

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