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I have thousands of points generated along river channels. I am trying to connect these points based on distance. Furthermore, I tried "Join attributes by nearest Points" and "Geometry by expression" in QGIS.

The method was mentioned here by @Babel. It works somehow. However, there are many gaps between the line segments and some lines are drawn repeatedly.

enter image description here

How can I use QGIS to fill the gaps and remove the repeated line segments?

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  • As far as I can see, this solution should work: gis.stackexchange.com/a/382246/88814
    – Babel
    Commented Dec 30, 2020 at 0:09
  • Please decide whether it’s ArcMap, QGIS or Python that you wish to ask about in this particular question. If it’s Python then please include a code snippet that illustrates what you have tried and where you are stuck.
    – PolyGeo ♦
    Commented Dec 30, 2020 at 0:13
  • As long as the question is closed, I can't add an answer, so please update your question (stating you're interested in a solution in QGIS) so that it can be reopened. Just to give a few ideas: 1. from the red lines, create start- and end points and use these to cretate a connecting line where you have gaps. 2. Repeated line segments: define a min/max value for the angle of your line to avoid this. 3. Can you use river date (e.g. OpenStreetMap)? You could snap your points to this line and try this: gis.stackexchange.com/a/382253/88814 (should work better with rivers than with streets).
    – Babel
    Commented Dec 30, 2020 at 11:08
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    I think part of the question (and resulting answer) is going to be "what do you want the output to look like"? --- Those straight line segments are probably not the exact course traveled. A moving average applied to each segment, followed by a curve fitting is probably more representative of the route taken; but you should be clear in your question if you want to preserve the error or beautify the output (at risk of introducing error).
    – Rob
    Commented Dec 30, 2020 at 17:23
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    Does this answer your question? Points from various sources to one line
    – Taras
    Commented Dec 31, 2020 at 18:38

3 Answers 3

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Let's suppose your points do not have an order attribute like an id or timestamp that would make it ease to connect the points accordingly (from 1 to 2 to 3 to 4 etc.). So I suppose your points are just randomly distributed all along the river with varying distances between them. I created such a points layer (see my first screenshot with the id as label) to demonstrate how to generate a connecting line based on location alone. There are five steps involved, including the Plugin Join multiple lines. Install it before you start from Menu Plugins / Manage and Install Plugins…. This plugin should do what you want to do, connecting the separate lines on your screenshot. Steps 1 to 3 generate such lines from a points layer.

  1. Menu Processing / Toolbox / Join attributes by nearest - set Maximum nearest neighbors to 1 only! Optionally, you can set a Maximum distance
  2. Menu Processing / Toolbox / Geometry by expression: select the output from above as input layer, for Output geometry type select line and as Geometry expression insert this expression: make_line ( make_point ( "feature_x" , "feature_y" ) , make_point ( "nearest_x" , "nearest_y" ) )
  3. Menu Processing / Toolbox / Delete duplicate geometries and set the output from 2 as input layer
  4. On the output layer from 3, select all features (I use Ctrl - A on my Windows machine, or click on the Select all icon in the Toolbar )
  5. Open/run the Plugin Join multiple lines (from Menu Vector): you get the connecting line you wanted, see screenshot:

enter image description here

enter image description here

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  • Thank you for your time! Please see my result after following your steps. The river have multi-channels and this may cause a mess of the connection. I have more than 30000 lines in the vector, if I select all and do the "Join multiple lines", QGIS becomes no response after running a while. photos.google.com/share/…
    – Bo Wang
    Commented Dec 31, 2020 at 16:29
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Find a river centerline dataset or digitize your own.

Then use the processing steps:

enter image description here

Result: enter image description here

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  • Thank you. The points are obtained from the google earth engine and I am trying to build river channel lines for global rivers. Therefore, I cannot digitize them manually. The existed global channel lines are simplified also.
    – Bo Wang
    Commented Jan 1, 2021 at 0:22
  • Then download them, for example: overpass-turbo.eu/s/5ts, then pan to your area and press Run
    – Bera
    Commented Jan 1, 2021 at 10:14
  • Thank you for the method. I tried it in the Model designer but the generated lines are in a mess. This is because the river has multiple channels separated by vegetated islands or channel bars in it. The centerlines downloaded from your supplied website are generalized into a single-channel which means there are no nearby extracted vertices for many points.
    – Bo Wang
    Commented Jan 5, 2021 at 23:35
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You can try "Nearest Points to Path" from ProcessX Plug-In. Default settings should do the job.

enter image description here

Disclaimer: I am the author of this Plug-In.

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