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<em>v.build.polylines</em> builds polylines from the lines or
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boundaries in a vector map. <!-- ???? and copies the attribute
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and category files from the original file. -->
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boundaries in a vector map.
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A line is a single straight line segment defined by one start node, one end
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node and no other nodes. A polyline is also defined by one start node, one end
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node and no other nodes, but is made of two or more consecutive straight line
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segments. The connections between the constituent line segments of a polyline
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do not appear as nodes in the vector map.
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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 is connected to this node. Line
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directions are reversed as required, i.e. it does not matter if the
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next line is connected to the current node by its start or end
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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
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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>,
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or the <em><a href="wxGUI.Vector_Digitizing_Tool.html">wxGUI vector
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Polylines provide the most appropriate representation of curved lines when it
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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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If the lines that make up a polyline are of different types, then
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<i>v.build.polylines</i> will set the type from the first constituent line.
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<i>v.build.polylines </i> will issue a warning unless the flag <b>-q</b> has
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been set. It is possible to keep a list of all such warnings by redirecting
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standard output to a file.
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<em>v.build.polylines</em> will set the type from the first constituent line.
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Category number(s) are assigned to a polyline based on <b>cats</b> parameter.
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<li>cats=no - No category number is assigned to a polyline. Also
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attributes tables linked to the input vector map are not copied to
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the output vector map.</li>
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<li>cats=first - Assign to a polyline category number of the first
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line. All linked attributes tables are copied to the output vector map.</li>
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<li>cats=multi - If the lines that make up a polyline have different category numbers
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then <em>v.build.polylines</em> will set the multiple catetory numbers to a polyline.
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Also all linked attributes tables are copied to the output vector map.</li>
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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 of the desired type. Areas
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are only guaranteed to be preserved if the constituent lines of the
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polylines that define them are all boundaries in the input vector map.
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<!-- Points will remain points provided that <b>type</b> has been set to `source'.
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It is possible to convert lines and area edges to points or vice versa, but this
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is rarely useful. [type no longer exists, other behaviour still true??] -->
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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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<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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vector/v.build.polylines/description.html
