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I want to split a network shapefile by cost isolines.

I have two layers

  • a point layer (with 297 records)
  • a road network layer

Every time I run the GRASS v.net.iso module in QGIS 3.2.2 on the road network layer, the output will be an empty shapefile with an empty attribute table.

Can anyone tell me how to use the v.net.iso tool correctly?

I couldn't find a tutorial online for the v.net.iso tool.

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2 Answers 2

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You have to prepare the network first.

Use the v.clean from GRASS with the break option to create the intersections.

Then you can run the v.net.iso.

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Note: Executed on QGIS 2.18

In my case, I can suggest several approaches that helped me to overcome an empty output after the execution of v.net.iso.

  1. A really trivial and simple one. However, substituting a point layer with an absolutely new shapefile did the work. Even the previous one was correctly snapped. Additionally, the snapping of a new layer to vertexes is necessary. I replaced it after I have "cleaned" my polylines.

  2. As was already mentioned by @muri, the implementation of v.clean geoalgorithm makes a tremendous impact.

    I made a small graphical model which helped to refine the road network, see the image below. The core logic: bpol > rmdupl > snap > break > rmdupl

Graphical_model

I built this process based on v.clean manual recommendations.

The bpol tool should be followed by rmdupl. If the -c flag is used with v.clean ... tool=bpol, duplicates are automatically removed.

The rmdupl tool should be used after breaking lines and breaking polygons.

Snapped boundaries may need to be cleaned with break,rmdupl,rmsa. If the -c flag is used with v.clean tool=snap, the sequence of break,rmdupl,rmsa is automatically repeated after snapping until no more small angles a left. Additional cleaning with e.g. tool=rmdanglemay be necessary.

Hint: Breaking lines should be followed by removing duplicates, e.g. v.clean ... tool=break,rmdupl. If the -c flag is used with v.clean > ...tool=break, duplicates are automatically removed.

My data was basically the same, two shapefiles:

  • one with points, created manually
  • second with polylines, "osm_roades", an OpenStreetMap data.

References:

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