[Qgis-user] Clipping tool takes a lot of time

[Jonas Sjögren]

Hi again,
unfortunately the processing is quite slow again with the difference
algorithm, about 2,5 hours. With a script running with 60 iterations the
total processing time will be about a week. No other processes are running
except for internet. The script sequence is
processing.runalg( 'qgis:difference', InputRoadNetwork, ClippingPolygon,
OutputRoadNetwork )
I have tried the 'gdalogr:ogr2ogr' clipping algorithm expressed in
http://darrencope.com/2011/03/31/clipping-large-shapefiles-using-ogr/ and
http://www.gdal.org/ogr2ogr.html
which I believe corresponds to the algorithm GDAL/OGR [32 geoalgorithms] =>
[OGR] Conversion => Convert format in the Processing Toolbox. Still I do
not know how to get the code right in 'Creation Options'. It should be
something like
[-clipsrc ClippingPolygon.shp]. Anyone who has an idea?

Best regards,
Jonas


2014-10-14 20:36 GMT+02:00 Jonas Sjögren <jonas.sjo at gmail.com>:

> Hi,
> to check that my data was ok I tried a clip tool in ArcMap where the same
> calcuation took exactly 10 seconds. After some modifications in my script
> and using the 'qgis:difference' algorithm, processing time was reduced to
> about 5 minutes, quite acceptable. Thank you for helping me sort this out.
> /Jonas
>
> 2014-10-14 7:47 GMT+02:00 Bernhard Ströbl <bernhard.stroebl at jena.de>:
>
>> Hm,
>>
>> goes pretty fast here, both with PostGIS and shp layers (some 5 lines, 1
>> polygon, both layers in the same CRS as the project, shp on local drive)
>> QGIS 2.4 64 bit on Win7
>>
>> Bernhard
>>
>> Am 14.10.2014 02:49, schrieb Mike Flannigan:
>>
>>
>>> I am also using QGIS 2.4 64b Chugiak on Windows 7.
>>> The same thing happens to me.  Even when I create
>>> a line layer with only 11 lines and try to clip 3
>>> of them with a polygon layer, the progress bar just
>>> goes back and forth for ever.
>>>
>>> I didn't run it for hours to see what would eventually
>>> happen.
>>>
>>> Mike
>>>
>>>
>>> I might be doing something wrong.
>>>
>>> On 10/13/2014 8:02 AM, qgis-user-request at lists.osgeo.org wrote:
>>>
>>>> Hi,
>>>> I am trying to clip a road network by a polygon in a script using the
>>>> algorithm 'qgis:clip'. The result seems to be quite ok but it takes an
>>>> incredible long time to produce, about 14 hours (!). The road network
>>>> shapefile consists of about 5000 features (not very many, I believe) and
>>>> the clipping shapefile consists of one single polygon. When using
>>>> Vector =>
>>>> Geoprocessing Tools => Clip from the QGIS menu, the time required is
>>>> about
>>>> the same. Does anyone have a clue why it takes so long time? Are there
>>>> other clip tools that I may try? I?ve been looking at the
>>>> 'gdalogr:ogr2ogr'
>>>> algorithm and its clipsrc function but not got it to work properly
>>>> yet. I?m
>>>> using QGIS 2.4 Chugiak on Windows 7 with 2.5 GHz processor. Thankful for
>>>> all hints that may help,
>>>> /Jonas
>>>>
>>>
>>>
>>
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