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This is probably an extended question from my another question (Increase spatial query performance (st_difference))

This is the background is this. I have a road link and a tunnel link. From the road link, I need to erase the tunnel link out. With builtin GIS geoprocessing, I can use these tools Erase tool in ArcMap, Difference toon in QGIS.

I used ArcMap erase tool, and it seemed to work fine. with QGIS tool, some areas were fine but another areas did not work well.

I assume that major issue was the topology of two links were not the same. (It does look like but it was not).

I wonder why ArcGIS returned the answer that I want but QGIS/PostGIS didn't. Also, I would like to have exact same answer of ArcGIS erase tool in QGIS (though I know it is difference but I wonder if there are any tricks...)

Here is one of the screenshots of ugly areas. considering that the scale is negative 9832 (1: -9832) the topology error? is not viewable with ArcMap (I suppose) enter image description here

  • it is possible that ArcGIS uses a looser tolerance for geometrical equality per default than QGIS, e.g. it implicitly snaps vertices while erasing. with that answer I gave you in your other question, it's easy to expand the tunnel geometries with a tiny buffer to make sure those nasty lines will intersect. there are also other ways, like ST_Snap or, possibly easier, ST_SnapToGrid. screwed up topologies can be a huge pain, though... – ThingumaBob Aug 3 '18 at 11:04
  • regardless of the software, zooming more than 1:1 will produce some visual misalignments that are not necessarily true – JGH Aug 3 '18 at 12:11

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