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As you can see from the attached image, I have a shapefile of river network (line) and a point shapefile (this one represents the middle point of transects carried out in the area). I'm trying to select 300 m of the lines, both before and after the points. The aim is to finally have a 600 m linear buffer in which each point is in the center.

I've already tried with a 300 m buffer for each points, but that doesn't work because some river segments (inside the buffer) are longer/shorter than 600 m.

enter image description here

  • Which software you are using? – Moh Jun 30 '18 at 14:54
  • I'm using QGis 3.0.1 Girona. – Fabio Marcolin Jun 30 '18 at 16:03
  • Do you want to select 300 linear meters of rivers(for each side) from the points? – hugonbg Jun 30 '18 at 17:10
  • @hugonbg yes, 300 linear meters of river line for each side from the points – Fabio Marcolin Jun 30 '18 at 17:28
  • RivEX does this but this is a tool for ArcMap. – Hornbydd Jul 2 '18 at 13:39
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Try the "Service area (from layer)" algorithm in the processing toolbox. The "Vector layer representing network" will be your river network, and "Vector layer with start points" your point layer. The "Service area (lines)" output from that algorithm should match what you're after.

  • Thank you for your answer. I have tried but dosen't work as expected: it create points in existing nodes without taking into account the "distance" value that I have set (e.g. the distance from the start point is greater or less than 300 m). – Fabio Marcolin Jul 2 '18 at 10:17
  • Try upgrading to 3.0.3 or 3.2 – ndawson Jul 2 '18 at 21:09
  • Now works! Thank you, again. Just a small problem: where the rivers splits (before or after the middle point) it selects two branches of 300 m where instead of only the main river branch. It's not a big problem because I have split the lines shapefile and deleted the selected lines that I don't need. Just to let you know. – Fabio Marcolin Jul 5 '18 at 10:40
  • do you mean it also routes "upstream"? You could work around this by tagging your rivers with a "direction field" to restrict the routing to one way (downstream) – ndawson Jul 5 '18 at 10:53
  • I badly explained myself. I need also upstream routes but when the algorithm find a node splitting the river (either upstream or downstream) select 300m in both direction. I saw the "direction field" but it's difficult to set the parameter because the segment should be selected "following" the river rank value (1,2,3 etc.) of the starting point of the transect. – Fabio Marcolin Jul 5 '18 at 12:33
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you can try splitting the river into 300-meter segments, creating the point buffers, and then selecting the river segments by the location, where they overlap the buffers. You can also try dividing the river into segments of 100, 200 and 300 meters by going testing the overlap with the points buffers.

  • I already thought about this solution but didn't tried yet. Thank you – Fabio Marcolin Jul 3 '18 at 10:35

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