[Qgis-user] how to use v.clean.advanced?

[Bernd Vogelgesang]

Hi Uwe,

Am 25.02.2016, 09:44 Uhr, schrieb Uwe Fischer <gisfisch at t-online.de>:


>
> Hello list,
>
>
> I have a line shapefile (call it input.shp, derived from polygons) that  
> i'd like to turn it into clean lines (output.shp) using >the processing  
> toolbox (PT). Clean lines means: no duplicates, broken at intersections.  
> Dangles and pseudo nodes are >uninteresting here.
>
>
> In my understanding, it's necessary to perform the following steps: 1.  
> snap vertices together within small threshold, 2. >break lines, 2.  
> remove duplicates.
>
>
> After entering my values and options in the v.clean.advanced input mask  
> of the PT, some problems and questions come >up (I tried QGIS 2.10 and  
> 2.12 on Windows):
>
>
> ·         the PT input mask does not accept more than one value in the  
> input field for threshold. In fatal contrast to that, in >the help text  
> it reads that there have to be multiple threshold values entered if  
> multiple cleaning tools are used. In >my case, the box doesn't accept a  
> comma separator nor an additional numeric value.
>
>
> ·         the system puts together a command string (found in PT  
> history) containing values I never did enter and I don't >want to use:
>
>
> processing.runalg("grass:v.clean.advanced","D:/>input.shp","snap,break,rmdupl",0.01,"655282.346386,662305.603469,5871019.54139,5877018.65557",-1,0.0001,"D:/>output.shp",None)
>
>

>
> ·         please note the above values before „D:/output.shp“: they  
> smell like default values for snapping and small area >threshold, but I  
> never did choose those values! Where the hell do they come from?
>
>
The values you are wondering about derive from the default values for  
snapping and remove small areas (does not apply for lines anyway) under  
the hidden "Advanced parameters" field in the mask. -1 for snapping means  
no snapping, 0,000100 for removing small areas will most likely remove no  
area at all cause its too small (in case of polygons)

>
> ·         the result is poor: lines are not broken and dups are not  
> removed.
>
>
>
> Does anybody know what went wrong?
>
>
I think the tool does not work as expected, cause you are not able to  
insert anything else than one number in threshold. Furthermore on a german  
OS, even switched to english language, the decimal separator in all tools  
is the comma and not a point. The comma gets changed to point for the  
algo, while the comma should be the separator for the threshold values. I  
think this can not work. How should the algo distinguish between a decimal  
input and a separator?

Would be nice to know if this happens only on computers where the comma is  
the decimal separator, or if the threshold field does not accept multiple  
values in any scenario.
>
>
So best to do would be to skip the v.clean.advanced and to do it step by  
step with v.clean, while you can spare the snap part cause you can already  
set this in the "Advanced options" (While I never found out when this will  
be executed: before the main action or afterwards or even in between???  
Documentation for grass is ... thin and only nerd-proof)


>
> Best regards, Uwe
>
>
Cheers
Bernd


-- 
Bernd Vogelgesang
Siedlerstraße 2
91083 Baiersdorf/Igelsdorf
Tel: 09133-825374
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