Hi Markus,

Il 12/02/2018 21:46, Markus Metz ha scritto:

> The original data in EPSG:3003 and EPSG:3857 are not identical. The
> original data in EPSG:3003 are topologically correct, while the original
> data in EPSG:3857 are not.

interesting. so reprojecting creates errors (tiny gaps, in fact). just
to document, the process was importing a shapefile into PostGIS, at the
same time reprojecting from 3003 to 3857

> Further on, the original data in EPSG:3857
> contain features that are not present in the original data in EPSG:3003
> which is quite strange.

forget about this, polygons added after import

> Reprojecting the original data in EPSG:3003 to EPSG:3857 within GRASS
> works fine, also with subsequent v.generalize.
> 
> That means that the original data in EPSG:3857 are some reprojected
> version of the original original data (which are these?). The
> reprojection step, apparently performed on polygons, not GRASS areas
> (how did you reproject?), introduced topological errors. Please use
> native GRASS v.in.ogr + v.proj to reproject polygons.

right; an easy alternative when working with QGIS Processing is to add a
snap param, which cures the small gaps.

All the best, and thanks again.

-- 
Paolo Cavallini - www.faunalia.eu
QGIS & PostGIS courses: http://www.faunalia.eu/training.html
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