DESCRIPTION

v.overlay allows the user to overlay two vector area maps. The resulting output map has a merged attribute-table. The origin column-names have a prefix (a_ and b_) which results from the ainput- and binput-map.

NOTES

Currently only areas are supported for the operators or and xor! See also v.select. The operator defines what kind of operation will be done. Features are written to output, if the result of an operation 'ainput operator binput' is true.

Attributes of the tables from ainput and binput are joined into a new table linked to the output maps new cat-column.

EXAMPLE POLYGON TO POLYGON UNION

v.overlay ainput=lake binput=province output=lakeXprovince operator=or

EXAMPLE POLYGON TO POLYGON UNION

Polygon union of urban area and Census 2000 areas (North Carolina dataset): 
# input maps
d.vect urbanarea
d.vect census_wake2000

# union
v.overlay ain=census_wake2000 bin=urbanarea out=urban_census2000 operator=or

# show result, graphically zooming a subset
g.region n=230400 s=223800 w=655800 e=662400
d.erase
d.vect urban_census2000

# show merged attribute table
v.db.select urban_census2000 where="cat=108" -v
cat|108
a_cat|98
a_AREA|231001264
a_PERIMETE|67804.305
a_TRACT_|98
a_TRACT_ID|98
a_RINGS_OK|1
a_RINGS_NO|0
a_ID|98
a_FIPSSTCO|37183
a_TRT2000|054108
a_STFID|37183054108
a_TRACTID|541.08
a_TRACT|541.08
b_cat|55
b_OBJECTID|55
b_UA|73261
b_NAME|Raleigh
b_UA_TYPE|UA
GRASS v.overlay: polygon to polygon union (input 1) GRASS v.overlay: polygon to polygon union (input 2) GRASS v.overlay: polygon to polygon union (result)
v.overlay: Polygon union of urban area and Census 2000 areas (North Carolina dataset)

EXAMPLE LINE TO POLYGON CLIPPING

Using the North Carolina sample dataset, we clip the roads map to the area of city of Raleigh: 
g.region vect=zipcodes_wake

# extract Raleigh city
v.extract in=zipcodes_wake out=raleigh \
            where="ZIPNAME = 'RALEIGH'"

# clip road network to city polygon:
v.overlay ainput=roadsmajor atype=line binput=raleigh \
            out=roadsmajor_raleigh operator=and
GRASS v.overlay: Line to polygon clipping
v.overlay: Line to polygon clipping

SEE ALSO

v.db.connect, v.select, g.copy

AUTHORS

Radim Blazek, ITC-Irst, Trento, Italy

Last changed: $Date$