v.clean - GRASS manual
Author: GRASS Development Team
Description: v.clean: Toolset for cleaning topology of vector map.
Keywords: vector, topology, geometry, snapping

## NAME

v.clean - Toolset for cleaning topology of vector map.

## KEYWORDS

vector, topology, geometry, snapping

## SYNOPSIS

v.clean
v.clean --help
v.clean [-bc] input=name [layer=string] [type=string[,string,...]] output=name [error=name] tool=string[,string,...] [threshold=float[,float,...]] [--overwrite] [--help] [--verbose] [--quiet] [--ui]

### Flags:

-b
  Do not build topology for the output vector
-c
  Combine tools with recommended follow-up tools
--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:

input=name [required]
  Name of input vector map
  Or data source for direct OGR access
layer=string
  Layer number or name ('-1' for all layers)
  A single vector map can be connected to multiple database tables. This number determines which table to use. When used with direct OGR access this is the layer name.
  Default: -1
type=string[,string,...]
  Input feature type
  Options: point, line, boundary, centroid, area, face, kernel
  Default: point,line,boundary,centroid,area,face,kernel
output=name [required]
  Name for output vector map
error=name
  Name of output map where errors are written
tool=string[,string,...] [required]
  Cleaning tool
  Options: break, snap, rmdangle, chdangle, rmbridge, chbridge, rmdupl, rmdac, bpol, prune, rmarea, rmline, rmsa
  break: break lines at each intersection
  snap: snap lines to vertex in threshold
  rmdangle: remove dangles, threshold ignored if < 0
  chdangle: change the type of boundary dangle to line, threshold ignored if < 0, input line type is ignored
  rmbridge: remove bridges connecting area and island or 2 islands
  chbridge: change the type of bridges connecting area and island or 2 islands from boundary to line
  rmdupl: remove duplicate geometry features (pay attention to categories!)
  rmdac: remove duplicate area centroids ('type' option ignored)
  bpol: break (topologically clean) polygons (imported from non topological format, like ShapeFile). Boundaries are broken on each point shared between 2 and more polygons where angles of segments are different
  prune: remove vertices in threshold from lines and boundaries, boundary is pruned only if topology is not damaged (new intersection, changed attachment of centroid), first and last segment of the boundary is never changed
  rmarea: remove small areas, the longest boundary with adjacent area is removed
  rmline: remove all lines or boundaries of zero length, threshold is ignored
  rmsa: remove small angles between lines at nodes
threshold=float[,float,...]
  One value for each tool; for threshold units, see each tool
  Default: 0.0[,0.0,...])

## DESCRIPTION

v.clean allows the user to automatically fix topology of vector maps. Several tools may be listed to be executed sequentially. In this case, also the threshold parameter requires several values to be listed accordingly. An error map is optionally written which stores the erroneous geometries.

### Break lines/boundaries

tool=break

The break tool breaks lines/boundaries at intersections and it also breaks lines/boundaries forming a collapsed loop. For example, 0.0;1.0;0.0 is broken at 1.0.

Threshold does not apply (it is ignored), use an arbitrary value (e.g., 0) if v.clean is run with several tools.

Hint: Breaking lines should be followed by removing duplicates, e.g. v.clean ... tool=break,rmdupl. If the -c flag is used with v.clean ... tool=break, duplicates are automatically removed.

### Remove duplicate geometry features

tool=rmdupl

The rmdupl tool removes geometry features with identical coordinates. Categories are merged. If a point and a centroid have identical coordinates, one of them will be removed if both points and centroids are selected with v.clean ... type=point,centroid. The same applies for lines and boundaries.

Threshold does not apply (it is ignored), use an arbitrary value (e.g., 0) if v.clean is run with several tools.

The rmdupl tool should be used after breaking lines and breaking polygons.

### Remove dangles or change boundary dangles to type line

tool=rmdangle and tool=chdangle

A line/boundary is considered to be a dangle if no other line of given type is on at least one end node. If a dangle is formed by several lines, such a string of lines is taken as one dangle and line lengths are summarized. The rmdangle tool deletes a dangle if the (combined) length is shorter than thresh or thresh < 0. If the combined length is larger than thresh, nothing is deleted.

Threshold has to be given as maximum line/boundary length in map units; for latitude-longitude projects in degrees. Dangles shorter than thresh are removed sequentially. All dangles will be removed if thresh < 0.

With thresh < 0, only closed loops and lines connecting loops will remain. This is useful to remove all incorrect boundaries after other cleaning operations with thres is < 0. Areas can then be successfully built.

To preferentially remove shortest dangles first, a first pass with a small thresh value can be followed by subsequent passes with higher thresh values. This can be done as one v.clean job by listing the tool several times and by defining a list of increasing thresh values.

The chdangle tool is similar to the rmdangle tool, but works only on boundaries and changes dangling boundaries to lines instead of removing them.

### Remove or change bridges connecting an area and an island or two islands

tool=rmbridge and tool=chbridge

A bridge is an area type connection of an island (polygon in a polygon) to the outer polygon. This is topologically incorrect (but OGC Simple Features allow it). The rmbridge tool removes bridges and the chbridge tool changes bridges to type line:

        +-------------+             +-------------+   +-------------+
        |            P|  P: polygon |            P|   |            P|
        |    +---+    |  I: island  |    +---+    |   |    +---+    |
        |    | I |    |  B: bridge  |    | I |    |   |    | I |    |
        |    |   |    |  L: line    |    |   |    |   |    |   |    |
        |    +-+-+    |             |    +---+    |   |    +-.-+    |
        |      |      |             |             |   |      .      |
        |      | B    |             |             |   |      . L    |
        |      |      |             |             |   |      .      |
        +------+------+             +-------------+   +-------------+

Islands and areas must be already clean, i.e. without dangles or small angles, e.g. v.clean ... type=boundary tool=rmdangle,rmsa,break,rmdupl,rmbridge thresh=-1,0,0,0,0.

Threshold does not apply (it is ignored), use an arbitrary value (e.g., 0) if v.clean is run with several tools.

### Snap lines to vertex in threshold

tool=snap

The snap tool snaps vertices to another vertex not farther away than thresh. If there is no other vertex within thresh, no snapping will be done. The type option can have a strong influence on the result. A too large threshold and type=boundary can severely damage area topology, beyond repair.

Threshold gives maximum distance to another vertex in map units, for latitude-longitude projects in degrees.

Snapped boundaries may need to be cleaned with break,rmdupl,rmsa. If the -c flag is used with v.clean tool=snap, the sequence of break,rmdupl,rmsa is automatically repeated after snapping until no more small angles a left. Additional cleaning with e.g. tool=rmdanglemay be necessary.

### Remove duplicate area centroids

tool=rmdac

The rmdac tool removes duplicate area centroids that can result from deleting boundaries.

Threshold does not apply (it is ignored), use an arbitrary value (e.g., 0) if v.clean is run with several tools.

### Break (topologically clean) areas (imported from a non topological format like ShapeFile)

tool=bpol

The bpol tool breaks boundaries on each point shared between 2 and more areas where angles of boundary segments are different and on all boundary nodes (start and end points of each boundary). The bpol tool behaves similar to break for boundaries, but does not break collapsed loops. The bpol tool is faster than the break tool but needs more memory.

Threshold does not apply (it is ignored), use an arbitrary value (e.g., 0) if v.clean is run with several tools.

The bpol tool should be followed by rmdupl. If the -c flag is used with v.clean ... tool=bpol, duplicates are automatically removed.

### Remove vertices in threshold from lines and boundaries

tool=prune

The prune tool simplifies lines and boundaries by removing vertices according to threshold. This tool preserves area topology, areas are never deleted and centroid attachment is never changed. v.generalize offers much more functionality for line simplification but does not preserve area topology.

### Remove small areas

tool=rmarea

The rmarea tool removes all areas <= thresh. The longest boundary with an adjacent area is removed or all boundaries if there is no adjacent area. Area categories are not combined when a small area is merged with a larger area.

Threshold must always be in square meters, also for latitude-longitude projects or projects with units other than meters.

### Remove all lines or boundaries of zero length

tool=rmline

The rmline tool removes all lines or boundaries of zero length that may have resulted from other cleaning operations. Zero length boundaries are redundant and do not influence area topology.

Threshold does not apply (it is ignored), use an arbitrary value (e.g., 0) if v.clean is run with several tools.

### Remove small angles between lines at nodes

tool=rmsa

The rmsa tool only concerns angles which are so small that the calculated angle is 0. The following figure should help demonstrate what the tool does.

Threshold does not apply, use dummy value if v.clean is run with several tools.

tool=rmsa

The rmsa tool should be followed by break,rmdupl. The rmsa tool followed by break,rmdupl may need to be run more than once to remove all small angles. If the -c flag is used with v.clean ... tool=rmsa, the sequence of rmsa,break,rmdupl is automatically repeated until no more small angles a left.

## NOTES

The user does not have to run v.build on the output vector, unless the -b flag was used. The -b flag affects only the output vector - topology is always built for error vector.

## EXAMPLES

### Snap lines to vertex in threshold

    v.clean input=testmap output=cleanmap tool=snap threshold=1

### Inspecting the topological errors visually

Both v.build and v.clean can collect the topological errors into a vector map:

    v.build -e map=imported error=build_errors
    v.clean -c input=imported output=clean error=cleaning_errors tool=snap,rmdangle,rmbridge,chbridge,bpol,prune threshold=5

The vector maps can be visualized together with the original data by the following set of display commands:

    d.vect map=imported color=26:26:26 fill_color=77:77:77 width=5
    d.vect map=build_errors color=255:33:36 fill_color=none width=5 icon=basic/point size=30
    d.vect map=cleaning_errors color=255:33:36 fill_color=none width=5 icon=basic/point size=30

Figure: Topological errors detected in the original data (left) and cleaned data (right)

### Cleaning OGR imported data (Simple Feature data) such as SHAPE file

The import of areas with v.in.ogr -c (no cleaning) requires a subsequent run of v.clean to update the map to a topologically valid structure (removal of duplicate collinear lines etc). The tools used for that are bpol and rmdupl:

    v.clean input=areamap output=areamap_clean tool=bpol,rmdupl type=boundary

### Extracting intersection points of vector lines

    v.clean input=lines1 output=lines2 err=points tool=break type=line

Intersection points are written to 'points' map.

### Break lines

v.clean will break the lines where they cross, creating new node if needed. Example:

    v.in.ascii -n out=crossed_lines format=standard << EOF
    L 2
     0 5
     10 5
    L 2
     5 0
     5 10
    EOF

    v.clean in=crossed_lines out=crossed_lines_brk \
            error=intersection tool=break type=line

### Remove all lines of zero length

    v.out.ascii zero format=standard
    L  2 1
     -819832.09065589 -987825.2187231
     -806227.28362601 -971104.80702988
     1     1
    L  2 1
     -799165.24638913 -972974.16982788
     -799165.24638913 -972974.16982788
     1     2

    v.clean input=zero output=zero_clean tool=rmline type=line

    v.out.ascii zero_clean format=standard
    L  2 1
     -819832.09065589 -987825.2187231
     -806227.28362601 -971104.80702988
     1     1

v.clean type=boundary would remove nothing.

### Repeatedly remove dangling lines up to 50m length

    v.clean input=testmap output=cleanmap type=line \
            tool=rmdangle,rmdangle,rmdangle,rmdangle threshold=5,10,20,50

## SEE ALSO

v.info, v.build, g.gui.vdigit, v.edit, v.fill.holes, v.generalize

## AUTHORS

David Gerdes, U.S. Army Construction Engineering Research Laboratory
Radim Blazek, ITC-irst, Trento, Italy
Martin Landa, FBK-irst (formerly ITC-irst), Trento, Italy

## SOURCE CODE

Available at: v.clean source code (history)

Latest change: Friday Apr 17 01:04:26 2026 in commit: 5a31d549cc0150239894bd2b9e67250af87d38b5

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© 2003-2026 GRASS Development Team, GRASS 8.6.0dev Reference Manual