[Qgis-community-team] QGIS - dissolve and/or buffer limit issues?

[Andy Gillett]

Dear List

I have just started using QGIS for some spatial processing work (using Dissolve & Buffer) and encountering some problems with run-time and hanging. Probably down to my relative inexperience! So would be very grateful for some feedback or suggestions. I have spent a fair bit of time searching various information sites such as GIS Stack Exchange but with no luck yet.

Objective
I am trying to Dissolve approx. 500,000 polygons which represent a 250m buffer around many line segments (e.g. electrical cable network or could be a road network). I have 5 buffer distances to create in total. The 500,000 polygon shp file is a sub-set (approx. 1/10th) of the entire dataset requiring processing in this way.

Issues / Questions
Have tried to use the Dissolve (polygon) on the 250m Buffer polygon dataset or Dissolve (line) function on the line network dataset in QGIS but it's very slow or doesn't work on the above (unrealistic run-time ... overnight still running).
Is there a limit on the number of polygons or lines that QGIS can handle for some of the operations (Dissolve, Buffer, etc)?
Is the desktop QGIS Dissolve function just plain slow?
Do I just keep reducing the polygon sub-set dataset size until I find a workable solution?
Would the ArcGIS Dissolve function be quicker/handle the 500,000 polygon dataset above?

Looking on GIS Stack Exchange I came across SAGA and have successfully used the Dissolve (line) function (lightening quick) - thinking I could then Buffer to 250m, etc in QGIS or in SAGA - but for both (QGIS & SAGA) of those options leaves the operation just crawling along very slowly or just not working, it is hard to tell.

The Dissolve (polygon) function on the 250m Buffer dataset in SAGA appears to work initially very fast and gets to circa 80+% on the activity bar and then just stops and appears to go nowhere (i.e. runs out of memory I suppose). Another search on e-bulletin board suggested there may be a problem with the Dissolve (polygon) function in the current version of SAGA v.2.1.2?

I have had a quick look at GRASS but I am not really sure what I am doing (i.e. not such an intuitive user interface) and wonder if the non-working function(s)/issues in SAGA would also apply?



Andy Gillett
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