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 [-
q]
input=
name
output=
name [
cats=
string] [--
overwrite]
[--
verbose] [--
quiet]
Flags:¶
- -q
-
Unused
- --overwrite
-
Allow output files to overwrite existing files
- --verbose
-
Verbose module output
- --quiet
-
Quiet module output
Parameters:¶
- input=name
-
Name of input vector map
- 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.digit, 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.
SEE ALSO¶
v.build, v.in.ascii, v.split, v.edit
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: 2011-12-11 21:06:55 +0100 (Sun, 11 Dec 2011) $
Full index
© 2003-2011 GRASS Development Team