[GRASS-user] Forcing polyline nodes through a point

[Hamish]

ok,  try   'v.to.points -n in=lines out=line_nodes'

.... then 'v.distance from=gps to=line_nodes' + v.patch + v.clean ...


Hamish



> > Dwight wrote:
> >> I am looking for a way to force the polylines
> (from GPS
> >> tracks) to terminate at the points (from averaged
> >> waypoints). One possibility would be to snap each
> terminal
> >> node to the nearest point (within a threshold),
> with the
> >> point coordinates taking precedence.
> > 
> > 
> > Hi Dwight,
> > 
> > try 'v.distance output=' + v.patch + v.clean.
> > 
> > 
> > Hamish
> 
> Thanks, Hamish. That is very close to what I want, but I am
> having some difficulty closing the gap.
> 
> In the simplest circumstance, the part of the line closest
> to the point is the node on the end, so your suggested
> procedure extends the line to the point. However, the
> original node is still a node (rather than a vertex), so
> there is a simple line segment attached to the end of a
> polyline segment. I was hoping to find a way to end up with
> a single polyline so that any kink generated at the end will
> be straightened out using v.generalize
> method=sliding_averaging.
> 
> Any ideas on how to accomplish this? Can two-line nodes be
> converted to a vertex within a threshold distance of a
> reference point or if one of the connected line segments is
> shorter than a threshold?
> 
> Under other circumstances, the part of the line closest to
> the point is somewhere along the line segment rather than at
> the end, resulting in a "Y" configuration. I thought I could
> use v.clean to break the original polyline where the
> v.distance segment intersects, which would generate a dangle
> that could then be removed with v.clean. However, I had
> trouble with each of these steps. Here is what I did, using
> a point near each terminus of a line.
> 
> v.distance -p from=terminus to=trail from_type=point
> to_type=line output=trail_connections dmax=20 upload=dist
> column=dist
> v.patch input=trail,trail_connections output=trail_patched
> v.clean input=trail_patched output=trail_break type=line
> tool=break thresh=1
> v.clean input=trail_break output=trail_rmdangle type=line
> tool=rmdangle thresh=20
> 
> v.distance created  lines that were about 6 and 13 ft
> long that intersected the trail line at about 8 and 14 ft
> away from each terminus, respectively (creating a "Y" on
> each end).
> 
> v.clean tool=break clipped the trail at the second
> intersection, but not the first (confirmed using v.digit).
> What causes this difference in behavior?
> 
> v.clean tool=rmdangle removed both of the line segments
> generated by v.distance, but did not remove what appears to
> be a 14 ft dangle created by the v.clean tool=break step.
> Why was the 14 ft dangle not removed (in v.digit it has a
> green X on one end and a red X on the other, and it is
> shorter than the threshold).
> 
> There are enough problems here that I am beginning to think
> that I don't understand what should be happening. Can
> someone straighten me out?
> 
> Thanks, -Dwight
> 
>