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I have a road network in the form of a line, that I need split up into sections, if a polygon intercepts.

This is fairly straight forward, but I have several layers of polygons where I need the full extent of the line, if it is continuous across several polygon layers.

I've tried to illustrate this with a picture, what I need is basically all the lines marked with Red on the picture, to be a separate new line.

As the picture shows, sometimes only a green polygon intersects, but other times it's a green polygon followed by a purple, and then I need it to be a full line across the several polygons. What is the easiest way to achieve this in QGIS?

enter image description here

7

I would probably do the following:

  1. Merge all polygon layers:

    Vector > Data Management Tools > Merge vector layers
    
  2. Dissolve your merged layer:

    Vector > Geoprocessing Tools > Dissolve
    
  3. Clip your line layer with the merged polygon layer:

    Vector > Geoprocessing Tools > Clip
    
  4. Find the symmetrical difference of your clipped line layer:

    Vector > Geoprocessing Tools > Symmetrical difference
    
  5. Delete duplicate geometries of your symmetrically differenced line layer:

    Processing Toolbox > QGIS geoalgorithms > Vector general tools > Delete duplicate geometries
    

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