GRASS GIS manual: v.build.polylines

NAME

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

KEYWORDS

vector, geometry, topology

SYNOPSIS

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

Flags:

--overwrite: Allow output files to overwrite existing files
--verbose: Verbose module output
--quiet: Quiet module output

Parameters:

input=name: Name of input vector map; Data source for direct OGR access
output=name: Name for output vector map
cats=string: Category number mode; Options: no,first,multi; 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

DESCRIPTION

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

A line is a single straight line segment defined by one start node, one end node and no other nodes. A polyline is also defined by one start node, one end node and no other nodes, but is made of two or more consecutive straight line segments. The connections between the constituent line segments of a polyline do not appear as nodes in the vector map.

v.build.polyline picks a line and from its start node, walks back as long as exactly one other line 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

If the lines that make up a polyline are of different types, then v.build.polylines will set the type from the first constituent line.

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.
cats=multi - If the lines that make up a polyline have different category numbers then v.build.polylines will set the multiple catetory numbers to a polyline. Also all 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 of the desired type. Areas are only guaranteed to be preserved if the constituent lines of the polylines that define them are all boundaries in the input vector map.

ACKNOWLEDGEMENTS

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

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

Last changed: $Date: 2010-10-20 04:08:29 -0700 (Wed, 20 Oct 2010) $

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