On 15-01-14 19:28, Lynkos wrote: > Thanks Paolo for the rapid response. It does look as though my problem > is related. Any guidance on how I can clean up the original shapefile? > Sarah.
Hi Sarah, I was curious what was happening here, and was questioning if it was a QGIS fault, or a GEOS (the underlying lib which takes care of the merging) one. I loaded your shape in Postgis as table 'merge', and merged those geoms via: insert into merge (id_0, id, geom) VALUES (999, 999, (select ST_Multi(ST_UNION (geom)) from merge where id_0 in (24,25,29)) ) but this looked exactly the same as your merged shape... (postgis also uses GEOS :-) )... So the problem is indeed: http://lists.osgeo.org/pipermail/qgis-developer/2014-January/030119.html looking into the docs of geos, there was talking about a buffer. Which made me think about: what about adding (and later removing a small buffer). So just before the merge, add a small buffer (1): insert into merge (id_0, id, geom) VALUES (999, 999, (select ST_Multi(ST_UNION (st_buffer(geom,1))) from merge where id_0 in (24,25,29)) ) which then did NOT sow the artifacts anymore, but indeed is one meter too big. So now a negative buffer (-1) over that one: insert into merge (id_0, id, geom) VALUES (9999, 9999, (select ST_Multi(st_buffer(geom,-1)) from merge where id_0 = 999)) And I think you have what you want... http://www.flickr.com/photos/97361298@N07/12031619434/ http://www.flickr.com/photos/97361298@N07/12031256205/ Off course this can give other artifacts sometimes (because you are actually merging the buffered geometries), but I think it is better then a normal merge... It's pretty easy to do this in postgis, but can probably also been done via processing? Regards, Richard _______________________________________________ Qgis-user mailing list Qgis-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/qgis-user