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v.clean - Toolset to clean vector topology




v.clean help
v.clean [-b] input=name output=name [type=string[,string,...]] [error=name] tool=string[,string,...] [thresh=float[,float,...]] [--overwrite]


Do not rebuild and store the topology at the end
Force overwrite of output files


Name of input vector map
Name for output vector map
Options: point,line,boundary,centroid,area
Default: point,line,boundary,centroid,area
Name of output map where errors are written
Cleaning tool
Options: break,rmdupl,rmdangle,chdangle,rmbridge,chbridge,snap,rmdac,bpol,prune,rmarea,rmsa
break: break lines at each intersection
rmdupl: remove duplicate lines (pay attention to categories!)
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
snap: snap lines to vertex in threshold
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 attachement of centroid), first and last segment of the boundary is never changed
rmarea: remove small areas, the longest boundary with adjacent area is removed
rmsa: remove small angles between lines at nodes
Threshold in map units, one value for each tool (default: 0.0[,0.0,...])


v.clean allows the user to automatically fix topology of vector files.


The user does not have to run 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.

Removing small angles between lines at nodes

The rmsa tool only concerns angles which are so small that the calculated angle is 0. The user cannot change this threshold. The following figure should help to understand what the tool does.

What is a bridge?

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). v.clean can be used to optionally change the line type to fulfill the topology rules or to remove the bridge from the map:
    +-------------+             +-------------+   +-------------+
    |            P|  P: polygon |            P|   |            P|
    |    +---+    |  I: island  |    +---+    |   |    +---+    |
    |    | I |    |  B: bridge  |    | I |    |   |    | I |    |
    |    |   |    |  L: line    |    |   |    |   |    |   |    |
    |    +-+-+    |             |    +---+    |   |    +-.-+    |
    |      |      |             |             |   |      .      |
    |      | B    |             |             |   |      . L    |
    |      |      |             |             |   |      .      |
    +------+------+             +-------------+   +-------------+


Snap lines to vertex in threshold

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

Cleaning OGR imported data (Simple Feature data)

The import of areas with 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 'rmdupl' and 'bpol':
v.clean input=areamap output=areamap_clean tool=rmdupl,bpol 

Extracting intersection points of vector lines

v.clean input=lines1 output=lines2 err=points tool=break 
Intersection points are written to 'points' map.

Break lines

v.clean will break the lines where they cross, creating new node if needed. Example: -n out=crossed_lines format=standard << EOF
L 2
 0 5
 10 5
L 2
 5 0
 5 10

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


David Gerdes, U.S. Army Construction Engineering Research Laboratory
Radim Blazek, ITC-Irst, Trento, Italy

Last changed: $Date: 2006/09/29 21:22:55 $

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