[Qgis-user] Incorrect Results in Buffer

[Carson Farmer]

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/