.TH v.extract 1grass "" "GRASS 6.4.2" "Grass User's Manual"
.SH NAME
\fI\fBv.extract\fR\fR  - Selects vector objects from an existing vector map and creates a new map containing only the selected objects.
.SH KEYWORDS
vector, extract
.SH SYNOPSIS
\fBv.extract\fR
.br
\fBv.extract help\fR
.br
\fBv.extract\fR [-\fBdtr\fR] \fBinput\fR=\fIname\fR \fBoutput\fR=\fIname\fR  [\fBtype\fR=\fIstring\fR[,\fIstring\fR,...]]   [\fBlayer\fR=\fIinteger\fR]   [\fBlist\fR=\fIrange\fR]   [\fBwhere\fR=\fIsql_query\fR]   [\fBfile\fR=\fIname\fR]   [\fBrandom\fR=\fIinteger\fR]   [\fBnew\fR=\fIinteger\fR]   [--\fBoverwrite\fR]  [--\fBverbose\fR]  [--\fBquiet\fR] 
.SS Flags:
.IP "\fB-d\fR" 4m
.br
Dissolve common boundaries (default is no)
.IP "\fB-t\fR" 4m
.br
Do not copy table (see also 'new' parameter)
.IP "\fB-r\fR" 4m
.br
Reverse selection
.IP "\fB--overwrite\fR" 4m
.br
Allow output files to overwrite existing files
.IP "\fB--verbose\fR" 4m
.br
Verbose module output
.IP "\fB--quiet\fR" 4m
.br
Quiet module output
.PP
.SS Parameters:
.IP "\fBinput\fR=\fIname\fR" 4m
.br
Name of input vector map
.IP "\fBoutput\fR=\fIname\fR" 4m
.br
Name for output vector map
.IP "\fBtype\fR=\fIstring[,\fIstring\fR,...]\fR" 4m
.br
Types to be extracted
.br
Feature type
.br
Options: \fIpoint,line,boundary,centroid,area,face\fR
.br
Default: \fIpoint,line,boundary,centroid,area,face\fR
.IP "\fBlayer\fR=\fIinteger\fR" 4m
.br
Layer number (if -1, all features in all layers of given type are extracted)
.br
A single vector map can be connected to multiple database tables. This number determines which table to use.
.br
Default: \fI1\fR
.IP "\fBlist\fR=\fIrange\fR" 4m
.br
Category values
.br
Example: 1,3,7-9,13
.IP "\fBwhere\fR=\fIsql_query\fR" 4m
.br
WHERE conditions of SQL statement without 'where' keyword
.br
Example: income = 10000
.IP "\fBfile\fR=\fIname\fR" 4m
.br
Input text file with category numbers/number ranges to be extracted
.br
If '-' given reads from standard input
.IP "\fBrandom\fR=\fIinteger\fR" 4m
.br
Number of random categories matching vector objects to extract
.br
Number must be smaller than unique cat count in layer
.IP "\fBnew\fR=\fIinteger\fR" 4m
.br
Enter -1 to keep original categories or the desired NEW category value
.br
If new >= 0, table is not copied
.br
Default: \fI-1\fR
.PP
.SH DESCRIPTION
\fIv.extract\fR 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.
.PP
If \fBlist\fR, \fBfile\fR, \fBrandom\fR or \fBwhere\fR options are not
specified, all features of given type and layer are
extracted. Categories are not changed in that case.
.SH 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 
\fIv.category\fR to add them.
.SH EXAMPLES
.SS Extract areas by category number with dissolving #1:
\fC
.DS
.br
v.extract -d list=1,2,3,4 input=soils output=soil_groupa type=area new=0
.br
.DE
\fR
.PP
produces a new vector \fBsoil_groupa\fR, containing those areas from vector
\fBsoils\fR which have category numbers \fB1 thru 4\fR; any common boundaries are
dissolved, and all areas in the new map will be assigned category number 0.
.SS Extract areas by category number with dissolving #2:
\fC
.DS
.br
v.extract -d list=1-4 input=soils output=soil_groupa type=area new=-1
.br
.DE
\fR
.PP
produces a new vector map \fBsoil_groupa\fR containing the areas from vector
\fBsoils\fR which have categories \fB1 thru 4\fR. Any common boundaries are
dissolved, all areas in the new map will retain their original category
numbers 1 thru 4, since \fBnew\fR was set to -1.
.SS Extract all areas and assign the same category to all:
\fC
.DS
.br
v.extract input=soils output=soil_groupa type=area new=1
.br
.DE
\fR
.PP
produces a new vector map \fBsoil_groupa\fR containing all areas from
\fBsoils\fR. No common boundaries are dissolved, all areas of the new
map will be assigned category number 1.
.SS Extract vectors with SQL:
\fC
.DS
.br
v.extract input=markveggy.shp output=markveggy.1 new=13 where="(VEGTYPE = 'Wi') or (VEGTYPE = 'PS') or (PRIME_TYPE='Wi')"
.br
.DE
\fR
.PP
produces a new vector map with category number 13 if the SQL statement is
fulfilled.
.SS Extract vector features which have the given field empty:
\fC
.DS
.br
v.extract input=forest output=forest_gaps where="CANOPY is NULL"
.br
.DE
\fR
.SS Extract vector features which have the given field not empty:
\fC
.DS
.br
v.extract input=forest output=forest_canopy where="CANOPY not NULL"
.br
.DE
\fR
.SS Reverse extracting (behaves like selective vector objects deleting):
Remove unreferenced stations from the GlobalSOD database:
\fC
.DS
.br
# check what to delete:
.br
v.db.select gsod_stationlist where="latitude < -91"
.br
.br
# perform reverse selection
.br
v.extract -r gsod_stationlist out=gsod_stationlist_clean where="latitude < -91"
.br
.br
v.db.select gsod_stationlist_clean
.br
.DE
\fR
.SS Dissolving based on column attributes:
\fC
.DS
.br
# check column names:
.br
v.info -c polbnda_italy
.br
.br
# reclass based on desired column:
.br
v.reclass polbnda_italy out=polbnda_italy_recl_nam col=vmap_nam
.br
.br
# verify:
.br
v.info -c polbnda_italy_recl_nam
.br
v.db.select polbnda_italy_recl_nam
.br
.br
# dissolve:
.br
v.extract -d polbnda_italy_recl_nam out=pol_italy_regions
.br
.DE
\fR
.PP
produces a new vector map with common boundaries dissolved where the reclassed
attributes of adjacent (left/right) areas are identical.
.SS Remove islands from polygon map
\fC
.DS
.br
v.extract in=map_with_islands out=maps_without_islands list=1-99999
.br
# and/or
.br
v.extract -d in=map_with_islands out=maps_without_islands
.br
.DE
\fR
.SS Extract 3 random areas from geology map
.PP
\fC
.DS
.br
v.extract input=geology output=random_geology type=area random=3
.br
.DE
\fR
will create new map with three random categories matching areas.
Note that there may be more than one feature with the same category.
.SH SEE ALSO
\fI
v.category,
v.dissolve,
v.reclass,
GRASS SQL interface
\fR
.SH AUTHORS
R.L. Glenn, USDA, SCS, NHQ-CGIS
.br
GRASS 6 port by Radim Blazek
.PP
\fILast changed: $Date: 2011-11-08 10:42:51 +0100 (Tue, 08 Nov 2011) $\fR
.PP
Full index
.PP
© 2003-2011 GRASS Development Team