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

AUTHOR

R.L. Glenn, USDA, SCS, NHQ-CGIS

Last changed: $Date$