3
<em>v.build.polylines</em> builds polylines from the lines or
4
boundaries in a vector map.
7
A line is defined by one start node, one end node and any number of
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vertices between the start and end node. The shortest possible line
9
consists of only two vertices where the coordinates of the start and end
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node are identical to those of the two vertices.
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<em>v.build.polyline</em> picks a line and from its start node, walks
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back as long as exactly one other line of the same type is connected to
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this node. Line directions are reversed as required, i.e. it does not
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matter if the next line is connected to the current node by its start or
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end node. Once the start line of a polyline is identified, it walks
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forward and adds all vertices (in reverse order if needed) of
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connected lines to the start line, i.e. the start line and connecting
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lines are reversed as needed. That is, if a line is reversed depends
21
on what node is initially picked for building polylines. If the
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direction of lines is important (it's not for boundaries to build
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areas), you have to manually change line directions with
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either <em><a href="v.edit.html">v.edit</a></em>, <em><a href="v.digit.html">v.digit</a></em>,
25
or the <em><a href="wxGUI.Vector_Digitizer.html">wxGUI vector
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Polylines provide the most appropriate representation of curved lines when it
30
is important that nodes serve to define topology rather than geometry. Curved
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lines are usually digitized as polylines, but these are sometimes broken into
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their constituent straight line segments during conversion from one data
33
format to another. <em>v.build.polylines</em> can be used to rebuild such broken
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<em>v.build.polylines</em> combines only lines of the same type to
39
a new polyline, i.e. lines and boundaries are kept separate.
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Category number(s) are assigned to a polyline based on <b>cats</b> parameter.
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<li><b>cats=no</b> - No category number is assigned to a
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polyline. Also attributes tables linked to the input vector map are
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not copied to the output vector map.</li>
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<li><b>cats=first</b> - Assign to a polyline category number of the
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first line. All linked attributes tables are copied to the output
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<li><b>cats=multi</b> - If the lines that make up a polyline have
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different category numbers then <em>v.build.polylines</em> will
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set the multiple catetory numbers to a polyline. Also all linked
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attributes tables are copied to the output vector map.</li>
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<em>v.build.polylines</em> correctly handles <b>input</b> vector maps
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containing lines, boundaries, centroids and points. Lines and
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boundaries will be converted to polylines. Areas
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are guaranteed to be preserved.
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<!-- Points will remain points provided that <b>type</b> has been set
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to `source'. It is possible to convert lines and area edges to points
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or vice versa, but this is rarely useful. [type no longer exists,
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other behaviour still true??] -->
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<h2>ACKNOWLEDGEMENTS</h2>
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This program was originally written during Mark Lake's tenure of a
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Leverhulme Special Research Fellowship at University College London.
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<a href="v.build.html">v.build</a>,
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<a href="v.in.ascii.html">v.in.ascii</a>,
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<a href="v.split.html">v.split</a>,
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<a href="v.edit.html">v.edit</a>
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Mark Lake, Institute of Archaeology, University College London.<br>
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Major rewrite by Radim Blazek, October 2002<br>
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Category mode added by Martin Landa, FBK-irst, Trento, Italy, October 2007<br>
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Support for categories, attributes, and different line types by Markus Metz
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<i>Last changed: $Date: 2014-02-10 18:24:20 +0100 (Mon, 10 Feb 2014) $</i>