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Note: This document is for an older version of GRASS GIS that will be discontinued soon. You should upgrade, and read the current manual page.

NAME - Builds polylines from lines or boundaries.


vector, topology, geometry, line, node, vertex

SYNOPSIS --help input=name output=name [cats=string] [type=string[,string,...]] [--overwrite] [--help] [--verbose] [--quiet] [--ui]


Allow output files to overwrite existing files
Print usage summary
Verbose module output
Quiet module output
Force launching GUI dialog


input=name [required]
Name of input vector map
Or data source for direct OGR access
output=name [required]
Name for output vector map
Category number mode
Options: no, first, multi, same
Default: no
no: Do not assign any category number to polyline
first: Assign category number of first line to polyline
multi: Assign multiple category numbers to polyline
same: Create polyline from lines with same categories
Input feature type
Options: line, boundary
Default: line,boundary

Table of contents

DESCRIPTION builds polylines from the lines or boundaries in a vector map.

A line is defined by one start node, one end node and any number of vertices between the start and end nodes. The shortest possible line consists of only two vertices where the coordinates of the start and end nodes are identical to those of the two vertices. picks a line and from its start node, walks back as long as exactly one other line of the same type is connected to this node. Line directions are reversed as required, i.e. it does not matter if the next line is connected to the current node by its start or end node. Once the start line of a polyline is identified, it walks forward and adds all vertices (in reverse order if needed) of connected lines to the start line, i.e. the start line and connecting lines are reversed as needed. That is, if a line is reversed depends on what node is initially picked for building polylines. If the direction of lines is important (it's not for boundaries to build areas), you have to manually change line directions with either v.edit or the wxGUI vector digitizer.

Polylines provide the most appropriate representation of curved lines when it is important that nodes serve to define topology rather than geometry. Curved lines are usually digitized as polylines, but these are sometimes broken into their constituent straight line segments during conversion from one data format to another. can be used to rebuild such broken polylines.

NOTES combines only lines of the same type to a new polyline, i.e. lines and boundaries are kept separate.

Category number(s) are assigned to a polyline based on cats parameter. correctly handles input vector maps containing lines, boundaries, centroids and points. Lines and boundaries will be converted to polylines. Areas are guaranteed to be preserved.


This program was originally written during Mark Lake's tenure of a Leverhulme Special Research Fellowship at University College London.

SEE ALSO,, v.edit, v.split


Mark Lake, Institute of Archaeology, University College London.
Major rewrite by Radim Blazek, October 2002
Category mode added by Martin Landa, FBK-irst, Trento, Italy, October 2007
Support for categories, attributes, and different line types by Markus Metz


Available at: source code (history)

Latest change: Thursday Feb 03 11:10:06 2022 in commit: 547ff44e6aecfb4c9cbf6a4717fc14e521bec0be

Note: This document is for an older version of GRASS GIS that will be discontinued soon. You should upgrade, and read the current manual page.

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