On Tue, Feb 13, 2018 at 9:02 AM, Paolo Cavallini <[email protected]> wrote: > > 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
This is the reason why neighboring polygons no longer match each other exactly. > > > 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. v.in.ogr suggests a range of snapping values specific for the data to be imported if something goes wrong. In this case snap=0.0001 helps. Markus M > > All the best, and thanks again. > > -- > Paolo Cavallini - www.faunalia.eu > QGIS & PostGIS courses: http://www.faunalia.eu/training.html > https://www.google.com/trends/explore?date=all&geo=IT&q=qgis,arcgis
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