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NAME

       v.overlay  - Overlays two vector maps offering clip, intersection, difference, symmetrical
       difference, union operators.

KEYWORDS

       vector, geometry, spatial query, clip, difference, intersection, union

SYNOPSIS

       v.overlay
       v.overlay --help
       v.overlay [-t]  ainput=name   [alayer=string]    [atype=string[,string,...]]   binput=name
       [blayer=string]         [btype=string[,string,...]]       operator=string      output=name
       [olayer=string[,string,...]]     [snap=float]     [--overwrite]    [--help]    [--verbose]
       [--quiet]  [--ui]

   Flags:
       -t
           Do not create attribute table

       --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:
       ainput=name [required]
           Name of input vector map (A)
           Or data source for direct OGR access

       alayer=string
           Layer number or name (vector map A)
           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

       atype=string[,string,...]
           Feature type (vector map A)
           Input feature type
           Options: line, area, auto
           Default: auto

       binput=name [required]
           Name of input vector map (B)
           Or data source for direct OGR access

       blayer=string
           Layer number or name (vector map B)
           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

       btype=string[,string,...]
           Feature type (vector map B)
           Input feature type
           Options: area
           Default: area

       operator=string [required]
           Operator defines features written to output vector map
           Feature  is  written  to output if the result of operation ’ainput operator binput’ is
           true. Input feature is considered to be true, if category of given layer is defined.
           Options: and, or, not, xor
           and: also known as ’intersection’ in GIS
           or: also known as ’union’ in GIS (only for atype=area)
           not: also known as ’difference’ (features from ainput not overlaid  by  features  from
           binput)
           xor: also known as ’symmetrical difference’ (features from either ainput or binput but
           not those from ainput overlaid by binput (only for atype=area)

       output=name [required]
           Name for output vector map

       olayer=string[,string,...]
           Output layer for new category, ainput and binput
           If 0 or not given, the category is not written
           Default: 1,0,0

       snap=float
           Snapping threshold for boundaries
           Disable snapping with snap <= 0
           Default: 1e-8

DESCRIPTION

       v.overlay allows the user to overlay two vector maps. Features in ainput can be  lines  or
       areas  and  are  cut  with  areas in binput. Simple clipping can be performed with the and
       operator.

       If areas in ainput are overlaid with areas in binput, it is sometimes  necessary  to  snap
       areas  of  binput  to those of ainput, otherwise areas can go missing or many sliver areas
       can be created. Snapping is enabled by default and can be disabled  by  setting  the  snap
       option  to  a  negative value. Recommended values are between 0.00000001 and 0.0001. Using
       larger values for snapping can have undesired side-effects, but may sometimes be necessary
       to  get  a clean output (see example below). In general, it is recommended to start with a
       small  snapping  threshold,  gradually  increasing  the  threshold  until  the  result  is
       reasonably  clean.  Snapping  modifies  only  boundaries  in  binput, which are snapped to
       boundaries in ainput. Boundaries in ainput are not modified.

NOTES

       Currently only areas in ainput 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.

       If the first number of the olayer option is greater than 0, then the resulting output  map
       has  a  merged attribute table in the given layer number. The original column names have a
       prefix (a_ and b_) corresponding to ainput and binput map.

       If the second number of the olayer option is greater than 0, then the categories of ainput
       in layer alayer are transferred to the output layer with the second number.

       If  the third number of the olayer option is greater than 0, then the categories of binput
       in layer blayer are transferred to the output layer with the third number.

       If atype=auto is given than v.overlay determines feature type for ainput  from  the  first
       found feature.

EXAMPLES

       Preparation of example data (North Carolina sample dataset):
       # Create an empty box for overlaying to ZIP code vector map
       v.mkgrid map=box grid=1,1 position=coor coordinates=584037,201970 box=50000,50000
       # set region to ZIP codes and box vector maps
       g.region vector=zipcodes_wake,box -p res=100 -a
       # enlarge region a bit for "white border" around map in monitor
       g.region n=n+1000 s=s-1000 w=w-1000 e=e+1000 -p
       d.mon wx0

   AND operator
       Clipping example (no attribute table is generated here):
       d.vect map=zipcodes_wake fill_color=0:128:0
       d.vect map=box fill_color=85:130:176
       v.overlay -t ainput=box binput=zipcodes_wake operator=and output=v_overlay_AND
       d.vect map=v_overlay_AND
       Figure: v.overlay with AND operator (selected polygons in grey color)

   OR operator
       Union example of areas:
       d.vect map=zipcodes_wake fill_color=0:128:0
       d.vect map=box fill_color=85:130:176
       v.overlay -t ainput=box binput=zipcodes_wake operator=or output=v_overlay_OR
       d.vect map=v_overlay_OR
       Figure: v.overlay with OR operator (selected polygons in grey color)

   XOR operator
       Symmetrical difference example:
       d.vect map=zipcodes_wake fill_color=0:128:0
       d.vect map=box fill_color=85:130:176
       v.overlay -t ainput=box binput=zipcodes_wake operator=xor output=v_overlay_XOR
       d.vect map=v_overlay_XOR
       Figure: v.overlay with XOR operator (selected polygons in grey color)

   NOT operator
       Difference example:
       d.vect map=zipcodes_wake fill_color=0:128:0
       d.vect map=box fill_color=85:130:176
       v.overlay -t ainput=box binput=zipcodes_wake operator=not output=v_overlay_NOT
       d.vect map=v_overlay_NOT
       Figure: v.overlay with NOT operator (selected polygon in grey color)

   Overlay operations: AND, OR, NOT, XOR
       ZIP code examples, based on North Carolina sample dataset:
       # creation of simple dataset
       v.extract input=zipcodes_wake output=poly1 where="cat = 42"
       v.extract input=urbanarea output=poly2 where="cat = 55"
       v.overlay ainput=poly1 binput=poly2 operator=and output=poly_1_2_and
       v.overlay ainput=poly1 binput=poly2 operator=or  output=poly_1_2_or
       v.overlay ainput=poly1 binput=poly2 operator=not output=poly_1_2_not
       v.overlay ainput=poly1 binput=poly2 operator=xor output=poly_1_2_xor
       Figure: v.overlay operations: original input polygons

       Figure: v.overlay results of AND, OR, NOT, XOR operations

   Polygons overlaid with polygons
       v.overlay ainput=lake binput=province output=lakeXprovince operator=or
       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
       Figure:  v.overlay:  Polygon  union  (right) of urban area (left) and Census 2000 (middle)
       areas (North Carolina dataset)

       As can be seen by the resulting large number of centroids on boundaries, the  urban  areas
       do not match exactly the Census 2000 areas. In this case a clean result can be obtained by
       snapping with a threshold of 0.1 m.

   Lines overlaid with polygons
       Using the North Carolina sample dataset, we clip the roads map to  the  area  of  city  of
       Raleigh, preserving road attributes in layer 1:
       g.region vector=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 olayer=0,1,0

       Figure: v.overlay: Line to polygon clipping

SEE ALSO

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

AUTHORS

       Radim Blazek, ITC-Irst, Trento, Italy
       Markus Metz

       Last changed: $Date: 2017-11-03 17:54:37 +0100 (Fri, 03 Nov 2017) $

SOURCE CODE

       Available at: v.overlay source code (history)

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       © 2003-2019 GRASS Development Team, GRASS GIS 7.6.1 Reference Manual