v.build.polylines(1grass) GRASS GIS User's Manual v.build.polylines(1grass)
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 re-
versed 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 man-
ually 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 dur-
ing 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 poly-
line, 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 differ-
ent category numbers then v.build.polylines will set the multi-
ple 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 con-
verted to polylines. Areas are guaranteed to be preserved.
ACKNOWLEDGEMENTS
This program was originally written during Mark Lake’s tenure of a Lev-
erhulme 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: unknown
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