v.build.polylines - GRASS GIS manual
Author: GRASS Development Team
Description: v.build.polylines: Builds polylines from lines or boundaries.
Keywords: vector, topology, geometry, line, node, vertex

## NAME

v.build.polylines - Builds polylines from lines or boundaries.

## KEYWORDS

vector, topology, geometry, line, node, vertex

## SYNOPSIS

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

### Flags:

--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
output=name [required]
  Name for output vector map
cats=string
  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
type=string[,string,...]
  Input feature type
  Options: line, boundary
  Default: line,boundary

## DESCRIPTION

v.build.polylines 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.

v.build.polylines 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. v.build.polylines can be used to rebuild such broken polylines.

## NOTES

v.build.polylines 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.

- cats=no - No category number is assigned to a polyline. Also attributes tables linked to the input vector map are not copied to the output vector map.
- cats=first - Assign to a polyline category number of the first line. All linked attributes tables are copied to the output vector map without filtering, but the categories are processed according to the cats option.
- cats=multi - If the lines that make up a polyline have different category numbers then v.build.polylines will set the multiple category numbers to a polyline. Also all linked attributes tables are copied to the output vector map.
- cats=same - Assigned lines to a polyline have same category numbers in all layers. Linked attributes tables are copied to the output vector map.

v.build.polylines 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.

## ACKNOWLEDGEMENTS

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

## SEE ALSO

v.build, v.in.ascii, v.edit, v.split

## AUTHORS

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

## SOURCE CODE

Available at: v.build.polylines source code (history)

Accessed: Thursday Jun 04 03:31:17 2026

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