[Qgis-user] Incorrect Results in Buffer

[Frederick Löbig]

Hi Carson,
indeed it does. As mentioned in my previous mail, i've solved the problem by simply exporting the lines into a new projection (in this case: british national grid). Although it is possible doing it this way, i really would apreciate if there would be a much smarter behaviour for this in qGIS in the future ;)
Thanks,Freddy


Frederick Löbig
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> Date: Fri, 4 Mar 2011 18:49:32 +0000
> Subject: Re: [Qgis-user] Incorrect Results in Buffer
> From: carson.farmer at gmail.com
> To: frederick.loebig at grunsnid.de
> CC: frederick.loebig at live.de; qgis-user at lists.osgeo.org
> 
> Hi Frederick,
> 
> > The situation: I have my project SRS set to EPSG:900913 (web mercator). As a
> > layer I have a line-layer also with EPSG:900913 as a projection. The
> > project-wide units are meters. I now want to add a 250m buffer around the
> > lines so i click vector->geoprocessing tools->Buffer(s), select my
> > line-layer, type in 250 for 250 meters, check dissolve buffer results ans
> > set a filename. Then the buffer is created, but unfortunately, when I export
> > the buffer as kml and check the buffer width in Google Earth it is around
> > 150-160 meters, so far off my 250meters. I also checked the distance with
> > the qGIS measurement-tool - same result.
> By 'same result' here, do you mean it measures 150-160 metres in QGIS as well?
> > The lines are located in London. I have tried to use simple WGS84 and the
> > british nationale grind, but none of them worked.
> What does this mean? No output was created, or they produced similarly
> 'wrong' results?
> > This has to do something with differente base-ellipsoids I think, but I did
> > not get to the bottom of the problem.
> > Could anybody please help me with this?
> Well one possible issue could be due to the fact that fTools (i.e. the
> vector menu), does not take into account the project CRS _or_ 'on the
> fly projection'. In fact, it only considers the CRS of the unput
> layer. This being the case, if the units for your input layer aren't
> in metres, then neither will your output. There are several reasons
> for why fTools does this, though it's possible that this will be
> changed in the future.
> 
> Does that shed any light on the error?
> 
> Carson
> 
> -- 
> Carson J. Q. Farmer
> ISSP Doctoral Fellow
> National Centre for Geocomputation
> National University of Ireland, Maynooth,
> http://www.carsonfarmer.com/
 		 	   		  
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