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

[DelazJ]

Hi Andy,
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management.

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Regards

2016-02-09 16:51 GMT+01:00 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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Re: [Qgis-user] [Qgis-community-team] QGIS - dissolve and/or buffer limit issues?

[Anita Graser]

>
> 2016-02-09 16:51 GMT+01:00 Andy Gillett :
>
>> *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.
>> ​
>>
>>
​In my experience, Difference is much faster than Dissolve. Create a big
polygon around the area containing the buffers. Then run Difference on the
polygon layer and the buffer layer, then run it a second time to get the
desired result.

Best wishes,
Anita​
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