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I have a shapefile of city streets. I tried to run the Road Graph tool on them but the select box does not populate with the layer. I ran the Topology Checker on the layer and found that there are 2,000 pseudo nodes. I then ran the Dissolve tool on them and reduced the number to about 700 (these are pseudo nodes that are not in straight line segments in the layer).

I have also tried importing to GRASS, which fixes topology errors, but the layer results do not display on the map. Don't know why, tried several times.

Any suggestions for getting rid of all of these pseudo nodes in QGIS? I'm using QGIS 2.4, Chugiak.

This link: http://www.ian-ko.com/et/ETUserGuide/dictionary.htm

...describes pseudo nodes as

The definition of Pseudo node is "Pseudo nodes occur where a single line connects with itself or where only two Polylines intersect."

In QGIS, they look like points on the lines between legitimate intersecting nodes.

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    Could you share a good link which explains what a pseudonode is? – user30184 Sep 7 '14 at 17:36
  • Same problem as here gis.stackexchange.com/questions/90031/…? – underdark Sep 7 '14 at 21:59
  • I think the problem with the layer not showing up in the road graph plugin box happens when the layer has invalid geometries. I have got the plugin to work when the geometries are valid. – Jim Sep 7 '14 at 22:50
  • I understood that some pseudonodes are same then nodes with cardinality=2 (pass-through) in this Spatialite document gaia-gis.it/spatialite-2.3.1/spatialite-network-2.3.1.html. Such nodes have not made big trouble for me but nodes connecting a line to itself have. For example turning circles in OSM are often digitized as loops. In Spatialite I haven't tried to correct them but I have just deleted all the edges which has node_from=node_to. QGIS probably has different problems than Spatialite. – user30184 Sep 8 '14 at 11:13
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Some options for pseudo-node removal with QGIS version 2.18:

  1. QGIS Dissolve will remove pseudo-nodes in a line data set. It creates multi-lines. By running QGIS Multipart to Singleparts, one gets back to "ordinary" lines with no pseudo-nodes, except for islands (lines that connect to themselves). The QGIS Dissolve tool does not scale well and is incredibly slow for larger datasets.

  2. GRASS v.build.polylines removes pseudo-nodes (except for islands). It is much faster than QGIS Dissolve. I have not been able to get it to work in QGIS Processing, but it works fine in the GRASS plugin.

SAGA (2.3.1) Line Dissolve is also faster than QGIS Dissolve, but does not remove pseudo-nodes - it only seems to collect the lines into multi-lines.

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