On May 29, 2009, at 4:50 AM, Hamish wrote:
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
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