# GRASS GIS Manual: v.clean

## NAME

v.clean

## 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 errorneous 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.

### 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 locations in degree. 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 locations in degree.

Snapped boundaries may need to be cleaned with break,rmdupl,rmsa.

### 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.

### 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 gives area size in map units, for latitude-longitude locations in square 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.

## 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 thresh=1

### 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 thresh=5,10,20,50

## SEE ALSO

v.info, v.build, v.digit, v.edit

## 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

Last changed: $Date: 2012-12-02 13:22:09 -0800 (Sun, 02 Dec 2012) $

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