v.cleangrass - Online in the Cloud

PROGRAM:

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,...]
Threshold in map units, one value for 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 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. 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 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. 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 locations or
locations 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

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

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