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I am doing an analysis of a road network. The attribute table holds information on the road name, type (e.g. A road, motorway etc). I would like to group all the minor roads as a single field. I have tried the dissolve function in QGIS, selecting all the minor roads and dissolving the selection. This process has been running for over 3 hours and is only on 7%! Forums suggest other people have found this tool incredibly slow.

I ran the Line dissolve from the processing toolbox which took about a minute top and saved the local roads as a new layer. I'm hoping I can overlay this somehow onto the original layer as a work around to the dissolve function not working properly. I believe the function I would need in ArcGIS would be 'Update' but I can't find a similar tool in QGIS.

Any suggestions?

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  • One method I found to have helped when dissolving fields from large datasets is to save the selected features into a new layer and then run the dissolve tool on this layer.
    – Joseph
    Commented Aug 11, 2017 at 12:04
  • Thank you Joseph, I have already done this (albeit in a slightly different process), I just need to know how to get this dissolved layer back into the original roads layer, overriding the undissolved local roads.
    – user13641
    Commented Aug 11, 2017 at 12:11
  • Can you create a copy of your original roads layer, select all the local roads and delete them, then merge that with your dissolved local roads?
    – John
    Commented Aug 11, 2017 at 12:52
  • Eventually you'll hit the limit of shapefiles and QGIS and need to seek options for processing this data like PostGIS, which I think would handle this quite easily...
    – Inactivated Account
    Commented Aug 11, 2017 at 15:17

2 Answers 2

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I would suggest this:

  1. Use Geoprocessing Tools (Vector-> Geoprocessing tools-> Symmetrical Difference) to extract part of the Original file that is different than the selected minor roads.

  2. Merge new created layer and the selected roads layer together This should give you an update like layer that includes the dissolved features + the original non-dissolved features.

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I would suggest this:

  1. Use spatial query plugin to select features from original layer that intersect dissolve layer features
  2. Open attribute table of original layer and reverse selection
  3. Export only selected features out to a new layer from the original layer
  4. Merge new exported layer and the dissolve layer together

This should give you an update like layer that includes the dissolved features + the original non-dissolved features.

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