v.to.points creates points along input lines. The output is a vector with 2 layers.
Layer 1 holds the category and attributes of the input lines; all points created along the
same line have the same category, equal to the category of that line. In layer 2 each point
has it's unique category; other attributes stored in layer 2 are lcat - the
category of the input line and along - the distance from line's start.
The dmax parameter is the maximum limit but not an exact
distance. To place points with exact distance from the beginning
of the vector line the user should use
The type parameter is used to control which input vector
geometry types to convert into points.
Some caveats to consider about this parameter:
The -v flag is used to digitize points that fall on the line's vertices only.
dmax is ignored in this case.
Points and centroids can be considered as "lines" with only one
node. Consequently, the result of selecting point or
centroid as the type parameter is that all points/centroids
get written into the output vector. The original category numbers of
the input points/centroids get written to the 'lcat'
attribute in layer 2 of the output vector. All values for
along are zero in the output vector, as only point geometry
was used for input (there is no linear distance to calculate
along, as each point/centroid is the start and
end of its own "line".
Boundaries are treated as lines, with points getting interpolated
along the boundary perimeter according to dmax. If two
adjoining polygons share a topological boundary, the boundary only
gets converted to points once.
If the type parameter is set to area, the boundary of
each area is converted to points regardless of whether or not
there is a topological boundary between adjacent areas. In other
words, the common boundary of two adjoining areas, for example, gets
converted to points twice. The centroid is not converted to a point in
the output vector for type=area.
If the -i flag is used in conjunction with the -v flag,
v.to.points will digitize points on the line vertices, as
well as interpolate points between line vertices using dmax
as the maximum allowable spacing.
To get points created for the beginning, middle and end only, use
the -i switch and set dmax so that:
(length of input line / 2) <= dmax <= length of input line
So if dmax is between 0.5x and 1.0x the line length, you
will always get points created at exactly the beginning, middle and
end of the input line.
In this example, the 't_powerlines' vector lines map in the
location is used to create points along the input lines:
v.to.points in=t_powerlines out=t_powerlines_points dmax=120
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